> **FIRE Phase II: Cirrus Implementation Plan**
>
> Prepared by the Cirrus Drafting Panel and the FIRE Project Office
>
> David O\'C. Starr, Chairman

  --------------- --------------------------- ---------------------------
                  Thomas P. Ackerman          Anthony Slingo

                  Steven A. Ackerman          William L. Smith

                  William Cotton              Graeme L. Stephens

                  Stephen K. Cox              Charles H. Whitlock

                  Andrew J. Heymsfield        Bruce A. Wielicki

                  David S. McDougal           Donald P. Wylie

                  Kenneth Sassen              
  --------------- --------------------------- ---------------------------

> August 1990
>
> EXECUTIVE SUMMARY
>
> FIRE (the First ISCCP Regional Experiment) is an ongoing multi‑agency
> program designed to promote the development of improved cloud and
> radiation parameterizations for use in climate models, and to provide
> for assessment and improvement of International Satellite Cloud
> Climatology Program (ISCCP) products. The first five years of FIRE and
> ISCCP have yielded significant progress toward these objectives.
> FIRE\'s accomplishments to date are summarized below. ISCCP has
> designed a methodology for monitoring cloudiness from space; this
> methodology has been employed to collect a near‑global cloud
> climatology since 1983. This data set is now being used by scientists
> studying cloud and climate questions. For the future, the relationship
> between clouds and climate has been identified as the top research
> priority for the U.S. Global Change Research Program. FIRE offers one
> of the best multidisciplinary strategies for addressing this research
> program.
>
> FIRE is a multi‑agency effort, enjoying support from the National
> Aeronautics and Space Administration (NASA), the National Science
> Foundation (NSF), the Office of Naval Research (ONR), the Department
> of Energy (DOE), the National Oceanic and Atmospheric Administration
> (NOAA), the United States Air Force (USAF), and other elements of the
> Department of Defense (DOD). Although FIRE is principally a United
> States national project, it benefits from important contributions by
> scientists from the United Kingdom and France.
>
> The FIRE Science Team (FST) was organized in 1984 to define and
> implement a coordinated research effort to investigate relationships
> between cloud systems and climate and to verify and improve cloud
> monitoring techniques from satellite platforms (Bretherton et al.,
> 1983). The FST is composed of scientists with modeling, experimental,
> and analysis skills for a diverse range of disciplines. FIRE has
> undertaken tasks that are essential to understanding cloud‑climate
> relationships. The strategy of FIRE has been to combine modeling
> activities with satellite, airborne, and surface observations to study
> two types of climatically important cloud systems: cirrus and marine
> stratocumulus. This strategy consists of three principal thrusts:
>
> *• A modeling program that encompasses GCMs, process models,
> large‑eddy models, and radiative transfer models;*
>
> *• a cirrus observing program that conducted a field campaign in 1986
> and plans a follow on campaign for late 1991; and*
>
> *• a marine boundary‑layer cloud observing program that conducted a
> field campaign in 1987 and plans a follow‑on campaign (ASTEX ‑
> Atlantic Stratocumulus Transition Experiment) in the summer of 1992.*
>
> One of the reasons for the success of FIRE to date is the collection
> of a diverse group of scientific talent working toward an important,
> common goal. The FST contains all of the elements critical to making
> advances in the understanding of the relationships between clouds and
> climate. It is a diverse group of observationalists, analysts, and
> modelers from varied discipline acting with the support and
> encouragement of visionary administrators. They have melded themselves
> into a powerful scientific engine working on critical, unresolved
> issues in cloud‑climate research, working productively together to a
> remarkable and perhaps unprecedented extent. Although it has become
> all too common to find that a program ostensibly organized to aid in
> the development of improved climate models attracts the participation
> of few climate modelers, FIRE has proven to be a heartening exception.
> It has enjoyed the continuous and growing support and participation of
> climate modelers from NASA/GSFC, NASA/GISS, Colorado State University,
> NCAR, the University of Illinois, Geophysical Fluids Dynamics
> Laboratory, and the United Kingdom Meteorological Office.
>
> Some of the results of FIRE I research activities are documented in
> two conference publications resulting from a series of research
> presentations at FIRE annual science meetings held in 1988 arid 1989
> (McDougal and Wagner, 199(); McDougal, 1990). Additional analyses of
> the FIRE I data will continue in the future. Some key scientific
> findings are summarized below:

*• Cirrus clouds are multi‑layered systems with pronounced vertical
structure. They are generated through both top‑down (particle
sedimentation from a cloud‑top generating layer) and bottom‑up
(convective growth from a cloud‑hale generating layer) processes.*

*• Strong linkages exist between synoptic‑scale and mesoscale structure
of cirrus clouds.*

*• Small ice particles (less than 20 microns) and ice crystal morphology
may be very important in determining the radiative properties of cirrus
layers. At present the small particles cannot be directly observed;
their existence must be inferred.*

*• ISCCP retrievals of cirrus and marine stratocumulus cloud amounts are
in reasonable* agreement *with independent analyses using the full suite
of available satellite data.*

*• Remote sensing of cloud microphysical properties may be feasible.
Effective cloud drop size can be inferred for marine stratocumulus
clouds and, possibly, for cirrus clouds. Cloud particle phase can also*
*be deduced.*

*• Complex thermodynamic structures, such as moisture layers, exist
above the marine boundary‑layer.*

*• The marine boundary layer is frequently decoupled into
dynamically‑distinct surface and cloud layer circulations.*

*• Cloud reflectivity is affected by both liquid water content and cloud
particle size, independently. Parameterizations of cloud optical
properties must take loot/\' parameters into account.*

*• Aerosols from troth continental sources and ship plumes can strongly
modify boundary layer cloud particle size, and can significantly affect
the albedo of the cloud systems.*

> From its inception, FIRE has been designed to be conducted in two
> phases. FIRE Phase I (1984‑1989) was designed to address fundamental
> questions concerning the maintenance of cirrus and marine
> stratocumulus cloud systems. Based on the results of Phase I,
> including lessons learned in conducting intensive field programs, FIRE
> Phase II (1989‑1994) will focus on more detailed questions concerning
> the formation, maintenance, and dissipation of these cloud systems.
> Specifically, the scientific objectives of FIRE Phase II, as outlined
> in the FIRE Phase II Research Plan (1989), are to:

*• Expand our basic knowledge of how clouds and cloud systems interact
with their environment and the climate;*

*• Identify, quantify, and simulate the processes instrumental in the
evolution of large‑scale cloud systems;*

*• Quantify the capabilities of current models for simulating
large‑scale cloud systems and the radiative properties of these systems,
and improve cloudiness and radiation parameterizations used in GCMs;*

*• Improve the capability of current models to simulate the large‑scale
cloud .systems and thereby make the models more reliable*

*• Assess and improve the reliability of currently used cloud/radiation
monitoring systems from .space and from the ground.*

*• ,Assess the capability of future cloud/radiation monitoring systems,
such as the Earth Observing System (EOS).*

> The key to increasing our knowledge of climatically important cloud
> systems rests in the close coupling of future multi‑scale modeling
> investigations with intensive data‑gathering field programs. Modeling
> is a fundamental component of EIRE Phase II. Models assist in the
> design
>
> of the field experiments, while observations provide initial
> conditions, physical process modules, and verification data for the
> models. Models provide a means to study nonlinear, interdependent
> processes; observations provide essential but only \'snapshot\'
> independent views of important parameters. Large‑scale models provide
> the necessary link to understanding the coupling between
> cloud‑radiation processes and climate; observations represent the
> means of monitoring the climate and validating the models.
>
> A second key component of FIRE Phase II is a cirrus observational
> program scheduled for late fall, 1991 in the lower, midwestern U.S. A
> central question around which this investigation is designed is:
>
> ***Are the cirrus cloud systems found in association with the
> polar/midlatitude and subtropical jet streams similar with respect to
> physical and radiative properties and evolution?***
>
> The location takes advantage of a newly deployed wind profiler network
> and offers the opportunity to extend FIRE cirrus investigations into
> the realm of the subtropical jet stream. The advent of new observation
> systems makes it possible to observe these upper tropospheric cloud
> systems in detail not possible only a few years ago.
>
> Several specific questions which will be addressed using combined
> observational and modeling approaches are:

*•* What is the role of small‑scale circulations in the evolution of
midlatitude cirrus cloud systems?

*•* Do the radiative and microphysical properties of cirrus clouds
differ between subtropical and polar/midlatitude jet stream systems?

*•* What roles do small ice particles and ice crystal morphology play in
determining the radiative properties of cirrus?

*•* Are currently used cirrus cloud models capable of rendering
realistic life‑cycle simulations of cirrus cloud systems?

*•* What are the ranges of natural variability in upper tropospheric
water vapor within and surrounding cirrus cloud systems?

In recognition of the true multi‑scale control of cirrus cloud systems,
the Phase II cirrus intensive field observations (IFO) will consist of a
set of nested observations/platforms. The large-scale environment will
be defined from NWS rawinsonde data extending from the west coast of the
U.S. to the Mississippi river complemented by NWS gridded data and
products and satellite data. The regional environs in the vicinity of
the experiment will be monitored by special rawinsonde stations, the NWS
wind profiler network, and satellites. The smaller scale will be
intensely observed using special wind profiler installations,
sophisticated surface-based lidar, radar and radiometry, and remote and
*in situ* sensing from aircraft. Active participation by GCMs and
mesoscale models will provide, for the first time, a methodology to
investigate the links between the large-scale environment and the
intensive, small-scale measurements.

A third key component of FIRE Phase II is a major field experiment that
is planned for the summer of 1992 in the eastern North Atlantic. The
ASTEX (the Atlantic Stratocumulus Transition Experiment) observational
program will be conducted in June 1992 from a base in the Azores and
will consist of aircraft, island-based, ship-based, buoy, and satellite
platforms. The surface-based sensors will include a number of sensing
systems, such as Doppler cloud radar, wind profiler, sodar, passive
microwave, rawinsonde, tethered balloon, and radiation instruments. This
program has been designed to address a number of scientific questions
that revolve around the following central, very complex question:

> ***What are the consequences for the atmosphere and ocean of the
> prevalent* *boundary-layer cloud type and amount, and how are the
> cloud type and* *amount selected?***

We have chosen to conduct ASTEX over the subtropical eastern North
Atlantic Ocean during summer because in that region at that time of year
there is a high probability of encountering a wide variety of
boundary-layer cloud, regimes that are easily accessible to observations
from aircraft and island platforms, including the important broken cloud
regimes. In contrast, the first boundary-layer cloud field campaign
conducted by FIRE encountered almost uniformly overcast conditions off
the coast of Southern California.

Several physical mechanisms have been proposed to account for the
observed variations of cloud type and amount. These include:

• Cloud top entrainment instability,

• Diurnal decoupling of the cloud layer from the boundary layer, and
subsequent cloud break-up due to solar warming and entrainment drying,

• Patchy drizzle leading to a similar decoupling scenario,

> • Mesoscale circulations, and
>
> • Episodic strong subsidence lowering the inversion below the LCL.
>
> Each of these mechanisms has been investigated in a variety of
> already‑published but we currently lack the data to put the various
> ideas to the test.
>
> A concurrent oceanographic study of oceanic subduction, SARI (the
> Subduction Accelerated Research Initiative), will be carried out in
> the Azores region. SARI and ASTEX will be highly complementary as SARI
> needs meteorological data and ASTEX needs oceanographic data; also
> ship, buoy, and island platforms may be shared and coordinated.
> Satellite data will play an especially important role in ASTEX because
> of the limited availability of ship, buoy, and island platforms.
>
> A major goal of ASTEX is to provide data that can be used to drive and
> validate climate models. A key strategy is the measurement of the
> budgets of mass, thermodynamic energy, and moisture, for an Eulerian
> \'grid column\' located in the ASTEX observing region. Some of these
> observations are needed as inputs to climate model codes; good
> examples are the large‑scale pressure gradient force, the large‑scale
> vertical motion field, and the tendencies of temperature and moisture
> due to horizontal advection. A single‑column version of a climate
> model can be used to simulate observed ASTEX cases, provided that such
> external forcings, which no single column model can determine, are
> provided as input. Obviously, additional ASTEX observations can be
> compared directly with results produced by a single‑column climate
> model; examples are the cloudiness, the convective and radiative
> fluxes, and the vertical profiles of temperature and water vapor. In
> some cases, such observations can be used to test, decisively, the
> physical assumptions underlying the parameterizations used in a
> climate model. For example, models typically parameterize the
> entrainment rate in terms of the convective flux profiles; ASTEX will
> provide observations of both the flux profiles and the entrainment
> rate, allowing direct tests of such closure assumptions.
>
> In addition, an Extended Time Observations (ETO) activity will consist
> of coordinated satellite observations, meteorological analyses, and
> data from a limited number of special ground sites with lidar and/or
> radiation measurements throughout the year. These data will provide a
> means of extending the results derived in the more detailed IFO
> intercomparison studies to larger time and space scales. The ETO
> program will directly support the ISCCP and GCM validation efforts.
>
> FIRE Phase I has graphically demonstrated that cloud‑radiation effects
> permeate many different climate and meteorological phenomena. Because
> of this, FIRE actively seeks out collaborations with other programs
> studying important meteorological and climate problems. In
>
> particular, TOGA/COARE, CERES, SPECTRE, ECLIPS, SARI, and atmospheric
> chemistry have been specifically targeted for close collaboration with
> FIRE.
>
> This document, along with the companion FIRE Phase II: ASTEX
> Implementation Plan, outlines a strategy for addressing the
> outstanding problems in the critically important research area of
> cloud‑climate relationships. These two plans represent natural
> extensions of previous FIRE research. They contain contributions from
> many FST members, and thus represent a contemporary and authoritative
> view of the future direction for cloud‑radiation climate research.
> Lastly, appendix C describes the scientific organization, data
> management activities, field experiment management, and key milestones
> that will be used to implement the research activities for the cirrus
> and ASTEX components of FIRE II.
>
> Additional copies of this document are available from:
>
> David S. McDougal
>
> FIRE Project Manager
>
> Mail Stop 483
>
> NASA Langley Research Center
>
> Hampton, VA 23665‑5225
>
> TABLE OF CONTENTS
>
> EXECUTIVE SUMMARY i
>
> 1.0 INTRODUCTION 1
>
> 2.0 BACKGROUND 3
>
> 3.0 SCIENTIFIC OBJECTIVES AND STRATEGIES 11
>
> 3.1 Modeling Cirrus Cloud Development 12
>
> 3.2 Characterizing Development of Cirrus Clouds 15
>
> 3.2.1 Structure of Cirrus Cloud Fields 15
>
> 3.2.2 Origin and Maintenance of Mesoscale Cirrus Cloud Features 16
>
> 3.2.3 Vertical Motions at the Mesoscale and Convective Scale 16
>
> 3.2.4 Water Budget of the Upper Troposphere 17
>
> 3.2.5 Radiative Budget of Cirrus Clouds 18
>
> 3.2.6 Microphysical Development of Cirrus Clouds 18
>
> 3.2.7 Turbulence and Convection Processes in Cirrus Clouds 19
>
> 3.3 Relating Cirrus Cloud Radiative and Physical Properties 20
>
> 3.4 Capabilities and Limitations of Satellite Cirrus Cloud Retrievals
> 22
>
> 3.5 Radiative Impacts of Cirrus Clouds 24
>
> 3.5.1 Monitoring the Radiative Impact of Cirrus 24
>
> 3.5.2 Evaluating the Radiative Impact of Cirrus Using GCMs 26
>
> 3.6 Surface‑based Remote Sensing of Cirrus Clouds 26
>
> 4.0 CIRRUS INTENSIVE FIELD OBSERVATIONS (CIRRUS IFO‑II) 29
>
> 4.1 Synopsis 29
>
> 4.2 Schedule and Location 31
>
> 4.3 Observations 36
>
> 4.3.1 Satellite Observations 36
>
> 4.3.2 Surface Radiation and Remote Sensing Observations 39
>
> 4.3.2.1 Active Sensing Systems 40
>
> 4.3.2.2 Radiance and Radiative Flux Observations 44
>
> 4.3.3 Aircraft Observations 47
>
> 4.3.3.1 Aircraft Platforms and Instrumentation 48
>
> 4.3.3.2 Aircraft Sampling Strategies 52
>
> 4.3.3.3 Aircraft Missions 58
>
> 4.3.4 Rawinsonde and Profiler Observations 64
>
> 4.4 Operational Forecasting 70
>
> 5.0 EXTENDED TIME OBSERVATIONS 75
>
> 5.1 Satellite Validation Studies 75
>
> 5.2 Comparisons of GCM and Satellite‑Derived Cirrus Cloud Statistics
> 76
>
> 5.3 The University of Utah ETO
> Project\...\...\...\...\...\...\...\..... 77
>
> 5.4 University of Wisconsin Cirrus Remote Sensing Pilot Experiment 78
>
> 5.5 Pennsylvania State University Cirrus Remote Sensing Pilot
> Experiment 80
>
> 5.6 NOAA/WPL Cloud Lidar and Radar Exploratory Test (CLARET) 82
>
> 6.0 COLLABORATIVE EXPERIMENTS 85
>
> 6.1 Spectral Radiance Experiment (SPECTRE) 85
>
> 6.2 Experimental Cloud Lidar Pilot Study (ECLIPS) 86
>
> 6.3 TOGA/COARE Cirrus Research 87
>
> 6.4 STORM Cirrus Research 88
>
> 6.5 Atmospheric Radiation Measurements Program (ARM) 88
>
> 7.0 REFERENCES 91
>
> APPENDIX A DESCRIPTION OF AIRCRAFT AND INSTRUMENTATION FOR
>
> CIRRUS IFO‑II A-1
>
> APPENDIX B LISTING OF NWS STATIONS AND CODES B‑1
>
> APPENDIX C FIRE SCIENTIFIC ORGANIZATION C-1
>
> APPENDIX D STANDARD DATA FORMATS FOR FIRE D‑1
>
> APPENDIX E NCDS USERS ACCESS INFORMATION E‑1
>
> APPENDIX F ACRONYMS F-1
>
> 1.0 INTRODUCTION
>
> Uncertainties about the abilities of current climate models to
> realistically simulate large-scale cloud systems and their effects on
> climate are directly linked to a critical lack of quantitative
> observations of cloud systems. ISCCP (Schiffer and Rossow, 1983) and
> FIRE (Cox et al., 1987) are research programs designed to provide
> unprecedented observations that will lead to improved understanding of
> the physical processes that govern cloud systems and determine their
> influence on the earth\'s energy and hydrologic cycles. ISCCP is
> providing global cloud data sets necessary to evaluate the performance
> of general circulation models (GCMs) used to simulate the global
> climate system. FIRE is directed toward testing and improving cloud
> parameterization schemes used in GCMs and increasing knowledge of the
> physical processes that contribute to the development of cloud systems
> and their climatic effects. FIRE supports ISCCP by quantifying
> capabilities for defining cloud properties from satellite
> observations.
>
> FIRE: is focused on two cloud types, cirrus and marine stratocumulus
> clouds, that have important roles in the climate system by virtue of
> their extensive areal coverage, persistence and radiative effects.
> Cirrus and marine stratocumulus clouds may be quite sensitive to
> changes in the climate system with the capability to significantly
> interact with and modulate those changes (feedback). FIRE research
> over the last five years (Phase I) has already led to major
> improvements in our understanding of the role of these clouds in the
> global climate system. FIRE has shown the strong influence of clouds
> on climate and how these effects are produced. Perhaps more
> importantly, FIRE investigations have enabled us to ask the right
> questions for the next phase of productive research.
>
> This document describes a research program (FIRE Phase II) designed to
> address outstanding problems in understanding cirrus clouds and their
> roles in the climate system.
>
> The goals of FIRE Phase II cirrus research are:
>
> • To improve quantitative understanding of the processes responsible
> for the formation and maintenance of extended cirrus cloud systems in
> the middle latitudes, with emphasis on the roles and interactions of
> processes acting on scales ranging from the microscale to the synoptic
> scale and on characterizing the physical properties of these clouds.
>
> • To improve the ability to utilize and interpret remote sensing
> observations for characterizing the properties of cirrus clouds,
> especially satellite‑based radiometric observations for descriptions
> of global cloudiness (e.g., ISCCP); and including surface and airborne
> lidar, radar and passive radiometric observations.
>
> Section 2 gives a brief background of cirrus cloud research
> emphasizing results from Phase I. Section 3 gives the scientific
> objectives research strategies and observational requirements for FIRE
> Phase II cirrus research. In comparison to Phase I this research
> program represents a more mature scientific endeavor. Phase I research
> was often exploratory since comparatively little was known about
> cirrus up to that time. The results have served to focus the Phase I1
> science objectives on specific critical questions that need to be
> addressed and to spur efforts to develop new instrumentation sampling
> strategies and analysis methodologies that are required to answer
> those questions. Section 4 describes The FIRE Phase II Cirrus
> Intensive Field Observations (Cirrus IFO‑II) campaign that is a
> centerpiece of FIRE Phase II and will provide many of the needed
> observations. The design of Cirrus IFO‑II reflects many of the lessons
> and experiences from the Phase I Intensive Field Observations (Starr
> 1987a) with a number of significant enhancements predicated on the
> results from Phase I. FIRE has significantly strengthened its
> activities in the area of large‑scale modeling for Phase II. These
> efforts will be critically important in realizing the underlying goal
> of improved understanding of the global climate system and climate
> change using GCM simulations as a primary tool. The data requirements
> of these model‑based investigations have significantly influenced the
> design of Cirrus IFO‑II. Section 5 describes observations and analyses
> that will be made over extended time periods to provide a basis for
> extending the results from the intensive field observations to the
> global scale. In addition pilot studies to test certain critical
> capabilities in preparation for IFO‑II are described there. In
> particular these include efforts to fully exploit the capabilities of
> multiwavelength active and passive remote sensing techniques to
> investigate cirrus clouds e.g. CLARET. In section 6 various
> collaborative experiments are described including the extension of
> FIRE cirrus research efforts to encompass tropical cirrus clouds which
> play an important role in the global climate system.
>
> Although much of this document is devoted to observational aspects and
> strategies the analysis of these observations and their incorporation
> into a variety of models is absolutely essential to achieving the
> scientific objectives of FIRE. The R = M3 strategy of an integrated
> program of *measurements, monitoring* and *modeling* to generate
> scientific *results,* as described in the FIRE Phase II Research Plan
> (1989) is central to the implementation described herein. Indeed the
> identification of specific critical problem areas in our understanding
> of cirrus clouds that resulted from Phase I research derived primarily
> from the efforts to model the observed processes and to use the
> observations to variously initialize and verify those models. It is
> through models that the results will ultimately be extended to the
> global scale of the climate system. The strong coupling of modeling
> and observational efforts and the unique experience in integrating
> diverse data sets for this purpose are fundamental strengths of the
> FIRE Science Team (FST) that have grown through these activities.
>
> 2.0 BACKGROUND
>
> In spite of their often tenuous nature and innocuous appearance,
> cirrus clouds exert a pronounced influence on climate. This fact has
> been increasingly recognized in recent years as general circulation
> and climate modelers have attempted to include the effects of
> cloudiness in their models. This has come as no surprise to radiation
> experimentalists and theorists (or weather forecasters), who have long
> been aware of the remarkable impact that cirrus clouds have on the
> planetary, atmospheric and surface radiation budgets. The
> climatological importance of cirrus clouds derives from the fact that
> cirrus clouds cover extensive areas of the earth (Warren et al., 1986;
> Warren et al., 1988; Wylie and Menzel, 1989) and are located high in
> the troposphere where temperatures are cold. Even though these clouds
> do not interact as efficiently with the incident radiation fields as
> the other more opaque cloud types, they exert a very strong influence
> on the upwelling and downwelling infrared radiation by virtue of their
> cold temperature. The radiative effects of cirrus clouds can be quite
> variable given the high variability of their radiative (and physical)
> properties. Either warming or cooling can occur depending on the cloud
> radiative properties and temperature (height) and its thermal contrast
> with the surface (e.g., Manabe and Strickler, 1964; Cox, 1971).
>
> Two recent investigations using general circulation models illustrate
> the potential importance of cirrus in the global climate system.
> Randall et al., (1989) showed dramatic impacts of upper tropospheric
> clouds on large‑scale circulation in the tropics with attendant
> effects on precipitation and water vapor amounts. Ramaswamy and
> Ramanathan (1989) have drawn renewed attention to the potential
> radiative heating effects of cirrus cloud systems, which may explain
> inconsistencies found between previous GCM simulations and observed
> upper tropospheric temperature structure in the tropics and
> subtropics. These radiative effects likely affect the intensity of the
> general circulation, in particular the subtropical jet stream, similar
> to what was shown by Ramanathan et al., (1983) for polar regions when
> changes were made in the treatment of cirrus cloud radiative
> properties. However it is very evident that the parametric treatments
> of clouds in GCMs remains quite uncertain at this time. For example,
> in a equivalent climate change scenario comparison of fourteen GCMs,
> Cess et al., (1989) found relatively good agreement between the models
> in terms of global temperature change in response to an imposed sea
> surface temperature change when clear sky conditions were assumed for
> the radiative computations. In contrast, the models exhibited much
> greater differences when the radiative effects of clouds were
> included. The effect of cloud feedback was comparable in magnitude to
> that due the imposed forcing (sea surface temperature change) in some
> models, but was found to be an amplifying factor (positive feedback)
> in certain models while, in others, it was found to be a damping
> factor (negative feedback) counterbalancing the effects of the imposed
> forcing.
>
> Prior to FIRE, comparatively little was known about cirrus clouds. The
> microphysics oriented studies of Heymsfield and Knollenberg (1972),
> Hobbs et al., (1975) and Heymsfield (1975a, 1975b) reported some of
> the first quantitative observations of cirrus clouds. Although
> Heymsfield emphasized microphysical aspects, he did begin to formulate
> a conceptual model of how a certain class of cirrus (cirrus uncinus)
> propagates. More recently, Starr and Cox (1985a, 1985b) reported a
> modeling study that provides an integrated analysis of the dynamic,
> thermodynamic, microphysical and radiative processes acting in cirrus
> cloud layers. Their results clearly show the dominance of
> microphysical composition in determining bulk cloud properties via the
> ice particle sedimentation process. Starr (1987a, 1987b) also
> quantified the importance of cloud temperature (height) as a factor
> controlling cloud development through its relationship to ambient
> water vapor amounts. These findings are consistent with the general
> features of the microphysical parameterization (average vertical
> profile) suggested by Heymsfield and Platt (1984) and the related
> radiative parameterization of Platt and Harshvardhan (1988). Analysis
> of FIRE IFO‑I observations (middle latitudes) and GATE data (tropical
> maritime) by Smith et al., (1990b) indicate that the general
> relationship between cirrus cloud temperature and extinction suggested
> by Platt and Harshvardhan (1988) may hold rather universally!\] Starr
> and Cox (1985b) also demonstrated the importance of radiative
> processes in maintaining cirrus clouds and the fine balance that
> exists between large‑scale forcing (vertical motion) and cloud
> response in the upper troposphere. This model suggested the conceptual
> design for the intensive field observation program implemented during
> FIRE Phase I (Starr, 1987a). It is worth noting that the microphysical
> observations of Heymsfield (1977) and others and the microphysical and
> radiative observations of Paltridge and Platt (1981), Griffith et al.,
> (1980) and others were very instrumental in development of the Starr
> model. This illustrates a basic tenet of FIRE: quantitative
> observations do lead to significant advances in understanding.
>
> The Starr model continues to be improved (Sassen et al., 1988). Lilly
> (1988) has recently reported a new cirrus anvil cloud model and Zhang
> et al., (1989) have also reported a new cirrus cloud model. Addition
> of a cirrus cloud capability to a mesoscale model has also been
> reported by Flatau et al., (1989), Westphal and Toon (1989) and
> others. Heymsfield and Donner (1990) have reported a new parametric
> treatment of cirrus cloud formation that will soon be implemented in
> the NCAR GCM. This parameterization is a direct result of FIRE Phase I
> findings and represents a new level of sophistication in the
> parameterization of large‑scale cirrus cloud development.
>
> FIRE Phase I research has yielded several very important results that
> will guide cirrus cloud research in the years ahead. First, there is
> little doubt that large‑scale dynamical forcing is a dominant factor
> in the generation of extensive cirrus cloud systems in the middle
> latitudes (Starr and Wylie, 1988; Durran and Weber, 1988; Westphal and
> Toon, 1989; Nicholls et al., 1990; and Starr and Wylie, 199()). This
> is a very important result that lends support to the feasibility of
> cirrus parameterization in large‑scale models. A second important FIRE
> result is the documentation of prevalent and persistent mesoscale
> organization within extended cirrus cloud systems (Sassen et al.,
> 1989; Starr and Wylie, 1990). This finding has direct implications for
> the development and maintenance of cirrus clouds and for the design of
> observational programs and modeling studies. The Phase I IFO also
> revealed the pervasive layered structure of cirrus where cirrus cloud
> systems were often comprised of multiple cloud layers (Sassen et al.,
> 1990b). An implication of this finding is that the bulk variations in
> cirrus cloud fields observed from satellites (or the surface) may
> result as much from variations in the number of cloud layers as from
> variations in the depth or density of individual cloud layers (Starr
> and Wylie, 1990). It also indicates that the vertical resolution
> required in large‑scale models may need to be significantly finer
> (\~0.5 km or less) than presently used if this characteristic
> structure is to be captured in the simulations.
>
> Another important result from FIRE Phase I is the finding that
> comparisons of microphysical cloud properties observed *in‑situ* with
> those deduced from radiative observations using detailed radiative
> transfer models are in substantial disagreement (Wielicki et al.,
> 1990; Ackerman et al., 1990; Spinhirne and Hart, 1990; Smith et al.,
> 1990b, Kinne et al., 1990, and Hammer et al., 1990). A number of
> potential explanations have been put forth. The most common is that
> substantial numbers of very small ice crystals were present. These
> particles are presumed to be smaller than the minimum detectable
> particle size using state‑of‑the‑art instrumentation (minimum size of
> \~25 to 50 μm depending on the aircraft) at that time (Heymsfield et
> al., 1990). An illustration of the particle number density size
> distribution needed to \'explain\' the radiative observations is shown
> in comparison to the observed size distribution in figure 1. Although
> the observations reported by Heymsfield et al., (1990) provide some
> support for this interpretation, uncertainties in the proper radiative
> treatment of complex ice particle shapes and in the refractive index
> of ice may also be partly responsible for the discrepancies Wielicki
> et al., 1990; Kinne et al., 1990, and Sassen et al., 1990c). Moreover,
> there are questions as to how a significant number of much larger
> crystals could have developed in the presence of so many small
> particles (fig. 1). FIRE has also stimulated new efforts to understand
> the microphysical development of cirrus and the potential role of
> aerosol particles as nucleating agents in this process (Sassen and
> Dodd, 1988; Sassen and Dodd, 1989; and Heymsfield and Sabin, 1989).
>
> Nonetheless, one conclusion from FIRE Phase I is that satellite‑based
> remote sensing of cirrus cloud microphysical properties may be
> feasible. Although the radiometrically inferred effective particle
> size (reff) was found to be much smaller than that deduced from the
> *in situ* microphysical measurements, a usefully broad range of reff
> was observed (5 to 40 um) where larger reff were associated with
> cirrus clouds comprised of larger ice particles (Spinhirne and Hart,
> 1990). The EOS will routinely provide the required multispectral
> radiance observations by the end of this decade.

FIRE Phase I observations have proven valuable in evaluating cirrus
cloud retrieval techniques, which utilize observations from the current
operational satellites. Minnis et al..

![](media/image1.wmf){width="7.402777777777778in"
height="9.777777777777779in"}

\(1990\) have derived improvements for bi-spectral techniques, such as
used by ISCCP, while Wylie and Menzel (1989) and Arking (1989) have
quantified the error characteristics of multispectral methods
(CO2-slicing and AVHRR, respectively). Based on high-spectral resolution
observations, Ackerman et al.(1990) have developed a promising new
three-channel infrared technique (8, 11, and 12 gm). Addition of an 8 μm
channel has been proposed for future AVHRR sensors.

Two other very important lessons have been learned from analysis of the
Phase I observations and have a strong bearing of the design of the FIRE
Phase II observational program. First, accurate knowledge of cirrus
microphysical properties is critical for a wide range of applications.
However, the high small-scale variability of microphysical properties
observed within cirrus, as illustrated in figure 2, suggests that *in
situ* sampling be maximized in order that reliable estimates of average
microphysical structure can be made over the scale of mesoscale cloud
features. In addition, any improvements in our capability to accurately
monitor cirrus cloud microphysical properties using remote sensing
techniques are expected to be very beneficial for cirrus cloud research.

Second, integration of diverse observations from multiple sensors and
platforms appreciably increases the information content in comparison to
that contained in the separate data sets. This is especially well-shown
in Wielicki et al., (1990), Sassen et al., (1989) and Spinhirne and Hart
(1990). For example, Spinhirne and Hart (1990) combine coincident high
resolution lidar and passive multispectral radiometric observations from
a high altitude aircraft (NASA ER-2) and ancillary observations to
deduce the vertical profile of relative source function for upwelling
infrared radiance at the top of the atmosphere (fig. 3). This profile
clearly illustrates the radiative dominance of a lower cirrus cloud
layer in comparison to the cirrus layer(s) located just below the
tropopause. It also suggests that the upwelling radiance seen from
satellites originates predominantly from the lower portion of the
underlying cirrus layer. Thus, the effective radiative cloud height is
much lower than the actual cloud top height and lower than the mid-cloud
height as also shown by Grund and Eloranta (1990) and Minnis et al.,
(1990). A proper interpretation of cloud heights derived from satellite
observations clearly depends on a proper understanding of characteristic
vertical structure of cirrus as represented in depictions like that
shown in figure 3. For example, Minnis et al., (1990) have implemented a
modification to account for this effect in their bi-spectral cloud
retrieval algorithm used for analyzing satellite observations and note
the added possibility of inferring cirrus cloud geometrical thickness
using this method. Advances such as this are facilitated by the
acquisition of coincident observations of multiple parameters and the
integration of those observations using models.

![](media/image2.wmf){width="7.319444444444445in"
height="9.708333333333334in"}

![](media/image3.wmf){width="7.352777777777778in"
height="5.442361111111111in"}

Figure - 3 - The average vertical profile of backscatter cross section
(visible) and source function for a flight line on October 28, 1986
during the FIRE Cirrus IFO-I. The backscatter cross section is deduced
from lidar observations and is used to define the cloud density profile
for the calculations. The source function defines the relative
contribution from each altitude interval to the observed upward beam
radiance at 10.84 μm**.** Figure from Spinhirne and Hart (1990).

3.0 SCIENTIFIC OBJECTIVES AND STRATEGIES

> The key FIRE Phase II science objectives are to:
>
> 1\) Incorporate FIRE Phase I data and Phase II data (when available)
> into models of varying scale and complexity for the purpose of
> developing and testing cirrus parameterizations and assessing
> capabilities to reliably simulate cirrus development on short and long
> time scales.
>
> 2\) Characterize the physical, thermodynamical, and dynamical
> development of cirrus clouds on
>
> • the synoptic scale,
>
> • the mesoscale,
>
> • the convective/turbulent scale, and
>
> • the microscale.
>
> 3\) Characterize relationships among various cirrus cloud optical
> properties, including,
>
> • cloud optical depths in the visible, near infrared, and infrared,
> and
>
> • cloud scattering phase functions;

and the corresponding cloud physical properties, including,

> • particle size, number density, phase and habit, and
>
> • cloud height, temperature, and thickness.
>
> 4\) Explicitly quantify the capabilities and limitations of methods to
> derive physical and optical cirrus cloud properties from satellite
> observations, especially ISCCP and future techniques for producing
> global cloud climatologies in the EOS era.
>
> *5)* Quantify the impact of cirrus clouds on the surface, atmosphere,
> and top-of-atmosphere radiation budgets.
>
> 6\) Improve the capability to utilize surface-based, active and
> passive remote sensing observations for quantitative studies of cirrus
> clouds.
>
> The rationale and strategy for achieving each of these objectives are
> described below.

3.1 Modeling Cirrus Cloud Development

Numerical modeling is a fundamental component of FIRE research into the
physics and dynamics of cirrus clouds and the impact of cirrus on the
global climate. Current techniques used to model cirrus clouds in GCMs
are extremely crude. Nonetheless, GCM modeling studies have demonstrated
the potential importance of changes in the distribution and composition
of high clouds to global climate change due to greenhouse gases or
external forcing (section 2). Greater reliability and credibility of
GCM-based predictions of climate change requires development of
physically-based cirrus cloud parameterizations that have been
experimentally verified. Because GCMs diagnose the ice and liquid water
contents, cloud heights and amounts, and cloud radiative properties of
cirrus from the resolved large-scale fields, each of these must be
measured as accurately as possible and with as much temporal resolution
as possible in constructing suitable verification data sets for testing
cirrus cloud parameterizations.

Many processes acting to determine the physical properties and
distribution of cirrus clouds operate on scales smaller than those
resolved in GCMs (section 2). Extended cirrus systems are typically
organized into distinct mesoscale regions of enhanced cloudiness which
are often comprised of numerous small scale convective-appearing
cellular structures. The small-scale circulations strongly influence the
microphysical development of the cloud (Starr and Cox, 19851)). The
resultant sizes and habits of the constituent cloud particles determine,
to a large extent, the net downward vertical flux of cloud water via
gravitational settling. In turn, it is the balance between the rates of
ice water generation and depletion, primarily via ice particle fall­out,
that determines the overall cloud water content and radiative properties
as well as the vertical development of the cloud layer and feedback to
the larger scale (Starr, 1987a). Clearly, development of a
physically-based GCM cirrus cloud parameterization requires knowledge of
these processes.

Consequently, FIRE cirrus research includes a strong effort to model
cirrus cloud development on smaller scales. Models are used to
investigate the basic physics and dynamics governing cirrus cloud
formation, evolution, and structure. These include regional mesoscale
models nested within larger scale models, very high resolution mesoscale
cloud models, and models of fundamental turbulence interactions and
microphysical growth. Detailed modeling studies have significantly
increased our understanding of cirrus clouds (section 2) and have served
to focus the observational objectives for Phase II. In Phase II, these
models will serve as surrogates for high resolution data sets to assist
in the formulation and testing of cirrus parameterization schemes.

As in the case of GCM parameterizations, the credibility of these models
must also be established which requires that suitable initialization and
verification data sets be obtained. Accurate measurements of
thermodynamic and dynamic conditions at the appropriate time and space
scales in conjunction with appropriately detailed and quantitative
physical descriptions of the corresponding clouds are essential. In
particular, accurate descriptions of vertical motions, the water vapor
budget, cloud structure, and cloud composition (especially the
populations of small particles) are essential if the uncertainties are
to be reduced to an acceptable level. The data sets must adequately
resolve the fundamental scales at which the physical processes are
organized. Observations of the aerosol particles involved in the
nucleation process and of the characteristics of turbulence are also
needed.

There is also a need to assess the predictability of cirrus clouds on
large and small scales. Do the large-scale fields of temperature,
moisture, and winds contain sufficient information to diagnose the
locations, heights, water con tents, and radiative proper-ties of cirrus
clouds? To what extent is cirrus predictability enhanced by
incorporating the temporal evolution of the large-scale fields in a
model-based data assimilation? To what extent is cirrus predictability
enhanced by incorporating special mesoscale data sets into mesoscale
models nested within the large-scale models? Is the absence of routine,
quality upper tropospheric observations of moisture a major impediment
to improved diagnosis of cirrus cloudiness? To what extent is this
compensated by incorporating time-dependent data into model-based data
assimilations? These questions can only be answered by obtaining high
quality data sets of cirrus cloud structure and evolution along with
simultaneous measurements of the large-scale fields.

Forecast experiments using FIRE Phase I data sets (e.g., Westphal and
Toon, 1989 and 1990; Nicholls et al., 1990) have served to define key
issues and observational requirements for Phase II. In Phase II,
forecast experiments with both mesoscale and global-scale models will be
used to test current and new cirrus parameterization schemes and to
examine the predictability of cirrus. Specific models that will be used
include the CSU/RAMS and PSU/NCAR mesoscale models and the UKMO global
model. Large Eddy Simulations (LES) experiments using nested grids will
be performed with the mesoscale models and will draw heavily on results
from the highly detailed models. Large-scale data sets with increased
temporal resolution over a wide area are required for improved
definition of the development of the dynamic and thermodynamic
environment of extended cirrus cloud systems, especially the
distribution of upper tropospheric water vapor. This is particularly
true for the LES experiments that serve as the bridge between the very
detailed models and the global-scale models and provide the framework
for investigating scale interactions that are central to the problem of
relating cloud response and feedback to large-scale control.

The strategy for obtaining the data sets (Phase II) required for
initializing numerical models and verifying models results over a range
of scales, developing and testing parameterizations of cirrus clouds,
and evaluating the predictability of cirrus is to:

> • Obtain data sets with improved resolution of the large-scale fields
>
> 1\) by enhancing the frequency of rawinsonde launches over a wide
> area,
>
> 2\) by enhancing the horizontal resolution of rawinsonde data over a
> mesoscale region through the addition of several special stations, and
>
> 3\) by including new nearly continuous wind data from a mesoscale
> network of NWS wind profilers over this same region.
>
> • Incorporate the above data into a Four Dimensional Data Assimilation
> (FDDA) using regional mesoscale models to produce dynamically
> consistent gridded analyses on the time and space scales required for
> testing parameterizations. This approach makes optimal use of data
> obtained from irregularly spaced and asynchronous observing systems.
>
> • Obtain quantitative physical descriptions of the cirrus cloud
> fields. *These data* *sets must be as complete and accurate as
> possible at each scale.* For the [wide]{.underline}
> [area]{.underline}, the required descriptions will be derived from
> satellite observations and include time-dependent, horizontal
> distributions of.-
>
> \- top-of-atmosphere radiative fluxes,
>
> \- cloud heights and amounts, and
>
> \- cloud radiative properties (broadband infrared emittance and
> visible albedo).
>
> Over the [core mesoscale observing area]{.underline}, the descriptions
> should additionally include time-dependent, vertical (and horizontal)
> distributions of-.
>
> \- cloud ice and liquid water contents, and
>
> \- cloud particle number density size distributions and habits.
>
> In addition, the following [mesoscale]{.underline} measurements are
> highly desired:
>
> \- upper tropospheric water vapor content,
>
> \- numbers, types, and composition of ice nuclei and cloud
> condensation nuclei, and
>
> \- dynamic structure (convective/turbulence) within cirrus clouds.

At this latter scale, the descriptions will be derived from *in situ*
(aircraft and sondes) and remote sensing (satellite, airborne and
ground-based) observations. The integration of these observations into a
coherent description is a formidable task (section 3.2).

> 3.2 Characterizing Development of Cirrus Clouds
>
> Production of data sets suitable for initialization and verification
> of cirrus cloud development models requires careful analysis and
> integration of diverse observations. Even in the case of FDDA analyses
> using regional mesoscale models, independent analysis of the input
> data base and comparison to the model-produced fields is essential to
> establish the integrity of those large-scale fields. As evidenced by
> the results from FIRE Phase I, there is much to be learned simply from
> analyzing the data. While the data requirements for characterizing
> cirrus cloud development are equivalent to those described in section
> 3. 1 . there are a number of areas where analysis of these data are
> particularly critical and/or problematic. These areas are briefly
> highlighted here.
>
> 3.2.1 Structure of Cirrus Cloud Fields
>
> Quantitative descriptions of the time-dependent, horizontal, and
> vertical structure of the observed cirrus cloud fields are required.
> These descriptions must include accurate distributions of cloud
> height, areal coverage, and cloud radiative properties over a large
> region encompassing the domain of regional mesoscale models.
> Additional parameters such as cloud thickness and the bulk cloud
> microphysical properties (vertically integrated ice water content and
> mean particle size) are highly desirable. Characterization of the
> vertical distribution of cloud parameters is also desirable (e.g.,
> delineating multilayered structure). Satellite-based observations
> provide the only means to produce such analyses. However, achievement
> of the level of detail and accuracy needed to *meaningfully constrain*
> regional mesoscale models requires significant improvements in current
> algorithms used to process satellite data. Such improvements will only
> result from detailed comparisons of satellite observations with
> quantitative \'ground truth\' descriptions derived from surface-based
> (remote sensing) and airborne (remote sensing and *in situ)*
> observations.

To be useful for application to cloud process models, which are
appreciably more detailed but consider a much smaller domain than
regional mesoscale models (and may even be nested within a larger scale
model), the cloud physical descriptions required for meaningful
comparison to model simulations must be correspondingly more detailed.
The time-dependent, vertical structure of cloud water content, particle
size distribution, and particle phase are needed over a relatively small
mesoscale region (\~20 - 50 km). Both Eulerian and Lagrangian
descriptions are highly valuable. In addition, the structure of these
fields must be characterized in terms of the magnitude and spatial
scales of variability, including that of the cloud radiative properties.
Although *in situ* observations (aircraft) can provide the basic
required microphysical and radiative measurements, the sampling
limitations of aircraft platforms do result in uncertainties associated
with spatial coverage and representativeness. Integration of those
observations with satellite, airborne, and surface-based remote sensing
observations can potentially provide a significantly more complete
characterization.

3.2.2 Origin and Maintenance of Mesoscale Cirrus Cloud Features

Extended cirrus cloud systems exhibit a significant degree of persistent
mesoscale organization. This may reflect corresponding structure in the
ambient dynamic and thermodynamic fields (Starr and Wylie, 1990).
Mechanisms potentially responsible for generating such structure include
the effects of up-scale energy transfer in two-dimensional turbulence,
inertial instability due to strong horizontal wind shear, deep
tropospheric gravity waves associated with ageostrophic flow in the jet
stream, and longitudinal roll circulations associated with jet flow. At
the present time, it is unknown to what extent these or other mechanisms
operate in organizing cirrus or to what extent they maintain the
observed structure. It is also unknown to what extent the radiative
(section 3.2.5) and thermodynamic (latent heating and cooling - section
3.2.4) effects of cirrus clouds act to maintain this structure. In order
to understand the origin and maintenance of mesoscale cirrus cloud
features, the time-dependent structure of the cloud fields (section
3.2.1) must be known in conjunction with the corresponding dynamical and
thermodynamical fields. Given that high wind speeds are typical at
cirrus altitude, significant enhancements to the routine synoptic upper
air observing schedules will be required to resolve these fast moving
and relatively small-scale dynamical features. Fortunately, a new
capability exists to actually resolve upper tropospheric wind fields on
a near-continuous basis (-10 min values) at a relatively fine scale
(-200 km) using the NOAA/NWS wind profiler network that is presently
under construction. Acquisition of thermodynamic information at
increased frequency (supplemental rawinsondes) will facilitate analysis
of these mesoscale features.

3.2.3 Vertical Motions at the Mesoscale and Convective Scale

Observations of vertical motions in cirrus clouds has always been very
problematic and have significantly retarded progress in quantitatively
testing cloud development models since model simulations are very
sensitive to the prescribed large-scale forcing. In particular, accurate
knowledge of vertical motions on the scale of individual mesoscale cloud
features (20 - 100 km) is essential. Recent technological and
methodological advances have provided various ways to observe or
estimate vertical motions in cirrus clouds at this scale. However, the
quality and representativeness of these data (and analysis methods) have
yet to be definitively established. Thus, it is clear that the required
approach is to apply *every available means* for characterizing vertical
motions in cirrus clouds. Only in this way can the present uncertainties
in independent analysis of data from individual systems be reduced to an
acceptable level.

Data from a mesoscale network of wind profilers would enable calculation
of the upper tropospheric vertical motion field at the required space
and time scales using kinematic techniques (derived from divergence
analysis of observed horizontal winds). Similar techniques can also be
applied to scanning Doppler lidar (CO2) and Doppler radar (mm)
observations and to aircraft-observed horizontal wind profiles (Gultepe
and Heymsfield, 1990) to generate profiles of vertical velocity at a
scale of about 20 km. Kinematic and adiabatic techniques can be used for
deriving vertical motions from rawinsonde observations at a larger
scale. Vertical motion fields are also derived from FDDA (section 3. 1).

Conventional *in situ* measurements from aircraft (inertial and gust
probe) provide a means to characterize convective scale (\~1 km) motions
which are needed for initialization of microphysical models and
verification of high-resolution mesoscale cloud models. Alternate
strategies and methods should be utilized to the extent possible in
order to provide the broadest possible observational basis. For example,
direct inference based on Doppler velocity spectra obtained using
C02-lidar or millimeter radar (Eberhard et al., 1990; Kropfli et al.,
1990) or wind profiler measurements taken in a vertically-pointing
configuration is possible. Such capabilities are highly desirable. In
addition, radar tracking of chaff released in a cloud and *in situ*
sampling and analysis of inert chemical tracers, such as sulfur
hexafloride, can be used to assess transports within a cloud. Each of
these techniques will be tried.

3.2.4 Water Budget of the Upper Troposphere

A key factor influencing cirrus cloud development and also manifesting
the effects of cloud processes is the distribution and redistribution,
respectively, of upper tropospheric water vapor. However, useful
observations of water vapor content at cirrus altitudes have not been
available, even at the synoptic scale. This severely constrains our
ability to quantitatively test cirrus cloud parameterizations since
current treatments are typically formulated in terms of relative
humidity. Although imagery from the GOES 6.7 μm water vapor channel has
long been very useful to forecasters, it has proven very difficult to
quantify and incorporate into operational or research models since the
detected signal is also strongly coupled to atmospheric temperature
structure and the vertical location of water vapor (Blackwell, et al.,
1988). Similarly, information on tropospheric water vapor derived from
the three HIRS water vapor channels on the operational NOAA polar
orbiting satellites has also seen little use in operational forecast
models. Recent advances in aircraft instrumentation (cryogenic
frost-point hygrometer) now provide the means to accurately observe
atmospheric water vapor contents at cold temperatures. Raman lidar has
been used to remotely sense water vapor contents to altitudes of 6 km at
night (Melfi et al., 1989). On-going improvements (a more powerful lidar
and more sensitive detectors) promise to extend the useful Raman lidar
operating range into the upper troposphere at night and permit daylight
observations. Longer integration times (30 minutes) can also be used to
enhance the sensitivity at these levels (presently, 2-minute values are
derived). These technologies should be utilized to the fullest extent
possible to provide the required observations. In addition, the
possibility of using these new observations to improve our ability to
quantify the satellite observations should be fully explored.

Characterizing the effects of cirrus cloud latent heating and cooling
and the *rate* of water vapor incorporation into cirrus clouds provides
an essential constraint on model-based studies and our understanding of
cirrus cloud development. Model evaluation in terms of system throughput
is most meaningful. Knowledge of the upper tropospheric water vapor
budget is a key ingredient in this methodology.

3.2.5 Radiative Budget of Cirrus Clouds

Radiative processes significantly affect the development of cirrus
clouds. Net radiative cooling or heating can enhance or suppress the
overall rate of production of condensate by altering the relative
humidity within the cloud and adjacent layers. Vertical gradients in
radiative heating can lead to stabilization or convective overturning of
a cirrus cloud. - Horizontal gradients in net radiative heating can
regulate the intensity of convective or forced circulations.., The
intensity and structure of internal cloud circulations and dynamic
entrainment at cloud boundaries has a major influence on the
microphysical development of the cloud and its overall composition and
properties (Starr and Cox, 1985b). Thus, efforts to model cirrus cloud
development using detailed cloud models must necessarily incorporate the
effects of radiative processes. The radiative component of these models
are uncertain, especially in the treatment of solar absorption. An
accurate characterization of the radiative energy budget of cirrus
clouds is essential both for development and verification of suitable
parameterizations and models.

It is important that *broadband* radiative properties be resolved in a
*consistent* manner for the infrared, near infrared, and visible
spectral regions. The objective and requirements here are somewhat
different than in sections 3.3 and 3.4 where a high degree of
coincidence is required to resolve fundamental issues regarding the
transfer of radiation in cirrus. For the application here, a statistical
approach is desired in which the means and variances of the observed
radiative fluxes (rather than radiances) and cloud water content can be
used to derive representative bulk relationships enabling reproduction
of the gross features of the observed cloud radiative budget. Such
relationships can then be incorporated into detailed cloud models to
simulate the structure of radiative forcing in cirrus clouds.

3.2.6 Microphysical Development of Cirrus Clouds

Cirrus clouds are microphysically complex consisting of a broad range of
particle sizes and particle habits (e.g., Heymsfield et al., 1990). Even
descriptions of the crystal habits are difficult in that combined
polycrystalline forms, including aggregates, are prevalent in many
cases. The microphysical composition of the clouds is the dominant
factor determining the cloud radiative properties. In addition, the
water and energy transports associated with the gravitational settling
of larger crystals is a dominant factor regulating overall cloud
properties and exchanges with surrounding air. FIRE Phase I has led to
significant improvements in our fundamental understanding of how cirrus
particle populations evolve. These studies have relied on observations
obtained in previous field experiments (FIRE Cirrus IFO-I and pre-FIRE
data) and incorporated these data into theoretical microphysical growth
models. While much has been learned, these studies have raised a number
of basic questions that need to be answered. For example, does air-mass
origin and composition have a pronounced effect on the microphysical
composition of cirrus clouds through the effects of the contained
populations of condensation and ice nuclei? How effective are cirrus at
cleansing the upper troposphere of particulates? What is the effect of
this cleansing on subsequent cloud development in the same air mass?
Furthermore, there are fundamental uncertainties with respect to the
nucleation mechanisms actually operating. A well-supported explanation
of how significant populations of the very large ice crystals typically
observed (order 100s μm) can develop in the presence of large quantities
of small crystals, especially if the small crystals are as prevalent as
indicated by remote sensing observations (section 2), within
geometrically thin cloud generating layers would significantly enhance
our basic understanding of how cirrus clouds work.

Observations of the ambient cloud condensation nuclei (CCN) and ice
nuclei concentrations and compositions in conjunction with detailed
observations of the entire ice crystal populations are essential for
resolving these issues. Such observations should attempt to define these
parameters on the scale required for microphysical growth models,
usually that of an individual convective updraft (-l km). In addition to
optical probes and collection devices on aircraft, balloon-borne ice
crystal replication sondes provide an additional means to sample ice
crystal populations and ice water contents.

3.2.7 Turbulence and Convection Processes in Cirrus Clouds

Turbulent heat and moisture fluxes play an important role in the
evolution of cloud layers. Knowledge of the characteristic scales and
intensity of these processes would provide another constraint on the
performance of cloud development models and cirrus cloud
parameterizations. In addition, results from FIRE Phase I (Smith et al.,
1990b; Flatau, et al., 1990) and ICE (Quante et al., 1990a and 1990b)
indicate that turbulent energy cascades may be an important factor
influencing the organization of cirrus cloud fields. Although
measurement of turbulence and turbulent fluxes at cirrus (or any)
altitude is very difficult, the potential applicability of these data
warrants the effort. Even relatively simple information on the
characteristic scales of cloud organization and scale dependence of
variance of dynamic and thermodynamic parameters would be quite useful.

Analysis of turbulence characteristics typically employ spectral
techniques. These analyses require high-frequency sampling (20 Hz) using
aircraft platforms. Adequate sample length is also required for
meaningful results (flight legs more than six times the length of the
analyzed scale). In addition, characterization of turbulence intensities
using remote observations (e.g., wind profilers) may be possible and
should be exploited.

3.3 Relating Cirrus Cloud Radiative and Physical Properties

Characterizing relationships between cirrus cloud radiative properties
in various spectral regions and cloud microphysical properties supports
each of the other key science objectives. While the determination of
cloud microphysical parameters in a GCM necessarily proceeds from the
resolved large-scale fields of temperature, moisture, and winds, the
radiative effects of cirrus clouds on the resolved fields are computed
based on the radiative properties associated with those microphysical
properties. These radiative effects represent the most important impact
of the clouds on the simulated climate. To first order, it is the cloud
radiative properties that are the primary desired output of a GCM cirrus
cloud parameterization. At the scale of individual cirrus clouds,
radiative processes also directly influence the development of the cloud
to a significant degree by modulating the scales and intensities of
convective and turbulent circulations and transports as well as the
overall production rate of condensed cloud water. Thus, the cloud
radiative; properties help determine the cloud physical properties
which, in turn, determine the radiative properties. Conversely,
monitoring of cirrus cloudiness and its radiative impact on the surface
and atmospheric energy budgets, whether from space or from the earth's
surface, requires that the detected radiances be interpreted in terms of
more useful quantitative measures of cloud physical properties. This is
particularly important given the general lack of opacity and great
natural variability of cirrus radiative and physical properties in space
and time.

The radiative properties of cirrus clouds critically depend on two
poorly known fundamental properties: the effective radiative particle
size and scattering phase function of the particle population. Both
depend on the sizes, shapes, orientations and, possibly, the composition
of the particles. Larger particles affect the longer wavelength infrared
radiation more strongly than the smaller particles, while smaller
particles, if present in sufficient concentrations, dominate the
transfer of solar radiation. Results from FIRE Phase I clearly
demonstrate the uncertainties in present knowledge (section 2). Model
computations of effective particle size based on observed multispectral
radiances (reflected and emitted) were inconsistent with the observed
particle size spectra. Similarly, computations of cloud bidirectional
reflectance patterns based on the microphysical observations and present
best-estimates of the phase function for citrus crystals were
inconsistent with the corresponding radiative observations. The
differences are very significant and have been attributed, variously, to
the presence of substantial numbers of undetected small ice crystals
(the called \'small particle radiative anomaly\'), the presence of
significant numbers of small liquid phase particles, inadequate
knowledge of the phase functions of actual cirrus particle populations,
and uncertainties in the refractive index of ice.

Relationships between the radiative and physical properties of cirrus
clouds can only be determined if adequate observations of the incident
and emergent radiative fields are available in conjunction with
coincident measurements of cloud composition. The FIRE Phase I data sets
are inadequate for resolving these basic uncertainties primarily due to
deficiencies in observing capabilities. Motivated by these findings, new
instruments have been developed or are in the process of being
developed. Improvements in our ability to observe the microphysical
composition of cirrus clouds, especially the numbers of small particles,
and enhancements in our ability to resolve the associated radiative
fields, including the possibility of direct measurements of scattering
phase functions, will yield significant improvements in our knowledge of
and ability to model fundamental relationships between cirrus cloud
optical properties and microphysical properties.

> The corresponding data requirements for Phase II are:
>
> • Accurate measurements of the upwelling and downwelling radiance
> fields, including
>
> \- observations at visible, near infrared, and infrared wavelengths,
> and
>
> \- observations from various viewing angles, especially of reflected
> solar radiation.
>
> • Measurements of the cirrus cloud microphysical composition,
> including vertical distributions of
>
> \- particle number density size distribution to minimum of 10μm, and
>
> \- particle phase, habits, and morphology.
>
> • Observations of the vertical distribution of cloud optical depth and
> temperature.
>
> • Direct measurement of ice crystal scattering phase functions, if
> possible

Obtaining these data requires *insitu* observations from aircraft and
balloon-borne ice particle replicators to define the microphysical
composition (and possibly the scattering phase function) and remote
observations from satellite, aircraft, and ground-based sensors to
define the radiative fields and cloud optical depths.

> Three key observational requirements are that:
>
> 1\) the measurements be as coincident as operationally feasible,
>
> 2\) some data sets be obtained over a water background to reduce the
> effects of surface reflectance, and
>
> 3\) the radiance observations be as accurate as possible.

This last requirement necessitates a concentrated effort to calibrate
and intercompare all radiometric sensors. To the extent possible, all
instruments should be referenced to a common standard.

This diverse set of observations will be incorporated into a hierarchy
of radiative transfer models as initial conditions and verification data
sets. These models range from present radiance and simplified scattering
models to experimental multi-dimensional models and explicit treatments
of the effects of crystal morphology on individual scattering events. In
this way, self-consistent relationships between the microphysical and
radiative properties of cirrus can be derived.

3.4 Capabilities and Limitations of Satellite Cirrus Cloud Retrievals

Satellite observations provide a means to quantify global cloudiness on
a climatological basis. In comparison to surface-based observing
systems, satellite platforms provide the advantage of uniform and
representative sampling and truly global coverage. Since cloud
parameters are derived from observed radiances, consistency is insured
with respect to the associated cloud radiative impact on the observed
radiative budget of the earth-atmosphere system (top-of-atmosphere
budget). Satellite observations permit monitoring of the physical and
radiative properties of cirrus clouds over extended time on a global
basis enabling *resolution of* chances in an *internal* component of the
climate system that will potentially play an important role in
modulating any global climate change. Moreover, the credibility of
models of the global climate system (GCMs) will be significantly
enhanced if the realism of crucial internal components, such as the
treatment of cirrus clouds, can be verified against suitable
observations of the present climate, e.g., the annual cycle in the
global distribution of cirrus clouds.

Cirrus present a particularly difficult target for remote detection from
space (Rossow et al., 1985) because of their general lack of opacity and
great natural variability of radiative and physical properties in space
and time. Due to the tendency for low cloud albedo, they may be
difficult to distinguish from the surface background at visible
wavelengths. Although much more evident in the infrared observations,
derivation of cloud height (temperature) and cloud emittance is
problematic given the intrinsic coupling of these parameters with
respect to the observed radiances in window regions. Inference of cloud
physical properties from satellite-observed radiances requires
assumptions about cloud structure and composition. In essence, a
radiative-physical cirrus cloud model is invoked in any retrieval
procedure in order to make the desired inference. An objective of FIRE
is to *test the underlying assumptions in these models. It* is only
through a rigorous examination of the validity and representativeness of
these assumptions and models that improvements in satellite-based cloud
observation capabilities will result.

> Model assumptions of particular concern include:
>
> • the relationship between visible reflectance and infrared emittance,
> the
>
> • detection thresholds in terms of visible and infrared cloud optical
> depths, and
>
> • the bidirectional reflectance/scattering phase function models for
> cirrus.
>
> Additional aspects that need to be well-understood are the effects of:
>
> • inhomogeneous cloud structure in the horizontal, and
>
> • situations of multiple cloud layers, particularly cirrus overlying
> altostratus or stratus.

Explicit quantitative knowledge of the capabilities and limitations of
present methodologies will enhance the value of the derived cloud
observations by providing a measure of the applicability of and
confidence in the cloud data. This will facilitate the optimal use of
ISCCP and other satellite-derived cloud data by the modeling community.
It will also provide a basis to improve current techniques using
existing satellite observing systems. Moreover, dramatic increases in
observing capabilities (satellite instrumentation) and data processing
capacity are planned (EOS). FIRE seeks to provide a basis for the design
and development of future cloud retrieval methodologies that will take
full advantage EOS as a global cloud observing system.

Optimally, the cirrus cloud properties that are desired from present and
future methodologies and that will be considered in FIRE include:

> • cloud height, thickness, and amount,
>
> • cloud optical depth in the visible, near infrared, and infrared
> spectral regions, and
>
> • cloud composition in terms of some integrated measure of the sizes
> of cloud particles and the cloud water content.

The data requirements to support the required investigations are nearly
identical to those given in section 3.3. The key elements are for high
spatial resolution, multispectral observations at multiple viewing
angles in conjunction with coincident observations of cloud composition
and fundamental radiative properties as well as thermodynamic structure.
These data can only be obtained by simultaneous measurements using
satellite, aircraft, and surface-based instruments. Accuracy and
representativeness are essential. Cross-calibration of all radiometric
sensors is critical (satellites, aircraft and surface). In addition, a
high resolution, quantitative, satellite-independent characterization of
the horizontal and vertical structure of the cloud fields over an area
larger than the resolution of operational satellite sensors (-30 km for
sounding channels) is needed. Scanning cloud lidar or radar provide this
capability.

Some data sets must be acquired over a water background to provide
suitable simplest cases for treating cloud reflectances. In addition,
data sets corresponding to land surfaces, to nighttime conditions (a
cold background), and to snow-covered backgrounds should also be
obtained to provide a means to evaluate and contrast model performance
in more demanding situations. In order to insure the representativeness
of the data and derived models for a wide range of naturally occurring
conditions, routine Comparisons of satellite-derived cloud parameters to
\'ground truth\' cloud observations (lidar and passive radiometric
observations) will be conducted at a number of sites over the course of
Phase II (section 5). Though these data sets will not be as
comprehensive as the intensive observations, they will provide coverage
of a variety of climatological situations (solar elevation, surface
conditions, and cloud conditions) and will benefit from efforts to
improve the utilization of surface-based remote sensing for observing
cirrus clouds (section 3.6).

3.5 Radiative Impacts of Cirrus Clouds

The importance of cirrus clouds in modulating the radiative energy
budget of the earth\'s surface and atmosphere is a prime motivation for
FIRE cirrus research. FIRE research directed at quantifying the
radiative impact of cirrus clouds on the surface, top-of-atmosphere
(TOA), and atmospheric radiation budgets follows two general approaches.

3.5.1 Monitoring the Radiative Impact of Cirrus

> Cirrus clouds constitute an important component of global climate data
> sets such as the cloud climatology being produced by ISCCP and the TOA
> radiation budget climatology being produced by ERBE. Efforts to
> develop techniques for monitoring the global *surface* radiation
> budget from space, both in the solar and infrared spectral regions,
> must necessarily account for the effects of cirrus clouds. In
> conjunction with efforts to quantify the limitations and capabilities
> of methods to derive cloud properties from satellite observations
> (section 3.4), FIRE seeks to better understand relationships between
> cloud parameters and the TOA and surface radiative energy budgets. The
> strategy is to obtain data sets suitable for investigating
> relationships between cirrus cloudiness and the surface TOA radiative
> budgets. The required data sets includes:
>
> • downwelling radiative fluxes at the surface,
>
> • upwelling radiative fluxes at the top-of-the-atmosphere,
>
> • cloud heights, temperatures, and amounts, and
>
> • bulk cloud radiative properties.
>
> It is important that these quantities be *broadband* and that the
> infrared, near infrared, and visible spectral regions be resolved in a
> *consistent* manner. Moreover, the data must be as *simultaneous* as
> possible in time and space. In addition, *rawinsonde observations* of
> atmospheric temperature and humidity structure are also required since
> atmospheric temperature and moisture also impact the surface and TOA
> radiation budgets.
>
> As in the case of methods to derive cirrus cloud properties from
> satellite observations (section 3.4), continuing efforts to collect
> data representative of a wide range of situations over *extended time*
> periods will be made (section 5). The acquisition and analysis of
> these data will supplement the intensive field observations and will
> provide a broader data base for these studies.
>
> FIRE also encompasses efforts to evaluate models of atmospheric
> (gaseous) radiative transfer in the infrared spectral region. The need
> for testing our capabilities in this area have been clearly
> established by the results of ICRCCM (Intercomparison of Radiation
> Codes in Climate Models) where an unacceptably high level of variance
> was found among infrared radiative transfer models. The reasons for
> the disparities have proven difficult to isolate but given the
> importance of infrared radiative processes in climate change
> scenarios, especially exchanges due to water vapor, renewed efforts to
> verify *such models* with observations are warranted. SPECTRE
> (Spectral Radiance Experiment) is a collaborative experiment (section
> 6. 1) to be conducted in conjunction with FIRE intensive field
> observations. Although SPECTRE emphasizes clear sky situations,
> observations under cirrus cloud conditions will be obtained and
> analyzed. These observations should prove valuable in assessing the
> radiative impact of cirrus. SPECTRE adds the additional requirement
> for observations of-.
>
> • spectrally-resolved downwelling infrared radiance at the surface,
>
> • lower to mid-tropospheric water vapor and temperature structure on a
> near continuous basis with high accuracy and high vertical resolution,
> and
>
> • vertical profiles of ozone concentration and aerosol optical depth.

Remote sensing techniques will be required for water vapor (Raman lidar)
and for temperature (RASS; Radio-Acoustic Sounding System) since
rawinsonde data will not be adequate.

3.5.2 Evaluating the Radiative Impact of Cirrus Using GCMs

Large-scale atmospheric models provide an alternate means of quantifying
the radiative and climatic impact of cirrus clouds. Although present GCM
cirrus cloud parameterizations are extremely crude and uncertain,
investigations using these models have provided important insights into
the role of cirrus in the climate system (section 2). As such, GCM
studies can serve to focus research on specific critical questions. For
example, inadequate knowledge of solar absorption in tropical cirrus
clouds has been identified as a probable cause of persistent problems in
reproducing upper tropospheric temperatures in tropical and subtropical
regions with resultant impact on the strength of upper tropospheric
general circulation and climate (Ramaswamy and Ramanathan, 1989). GCM
studies on the radiative and climatic impacts of cirrus clouds will
continue during FIRE Phase II, Efforts will be made to intercompare the
simulated TOA and surface radiation budgets and cloud parameters with
global climatologies such as ERBE and ISCCP. New cirrus
parameterizations will be implemented as they are developed and verified
during FIRE Phase II (section 3. 1).

3.6 Surface-based Remote Sensing of Cirrus Clouds

Routine ground-based remote sensing observations offer considerable
promise for characterizing the properties of cirrus clouds and their
environment over extended time periods. Active sensing technologies not
presently available from satellite platforms can be applied including
visible and infrared lidars, short-wavelength cloud radars, and
long-wavelength wind radars (profilers). These observations provide
\'ground-truth\' corroboration for properties derived from passive
sensing techniques, especially retrievals from global satellite-based
systems. A wide variety of passive remote sensing technologies can also
be utilized with considerably more spectral coverage and resolution than
presently available from satellite platforms. Although, in most
instances, the physical interpretation of remote sensing observations is
significantly more uncertain than for in situ measurements, the benefits
in terms of reduced costs and increased coverage in comparison to
aircraft or balloon-borne observations are appreciable. Moreover, these
techniques provide the possibility for needed quantitative observations
of the climatologically important, high-altitude, tropical cirrus
systems that are largely inaccessible to research aircraft (section
6.2).

A primary advantage of active remote sensing of cirrus is the capability
to accurately measure, with unparalleled precision and resolution, the
temporal evolution of the height-dependent structure of the clouds. The
fundamental parameter measured is the vertical profile of backscattered
intensity at the probing wavelength. Although this parameter has
intrinsic value, inference of more widely applicable cloud parameters is
highly desired, including profiles of cloud optical depth, particle
phase and habit, and cloud water content. The Doppler velocity spectrum
(radar and infrared lidar) and polarization of the returned signal are
also measured by some systems. The Doppler velocity spectrum provides a
means of estimating fall speeds of the ice crystals which are highly
dependent on crystal mass (and habit). These may be used to derive the
net vertical flux of ice mass which is a dominant component of the cloud
water budget. Polarization provides a measure of the phase and habit of
the cloud particles. In addition, high spectral resolution lidar offers
a much more direct measurement of the vertical structure of cloud
optical depth than achievable with other systems where appreciable
uncertainty is associated with the assumptions required to make this
inference.

Although current lidar and radar probing techniques offer the promise of
reasonably quantitative analysis of cloud composition, the analysis
methods are presently based on rather scant preliminary evidence in this
developing field. Since the interaction of radiation with cloud
particles depends both on the size of the particles and the wavelength
of the radiation and since a wide range of particle sizes typically
occur in cirrus, systems probing at different wavelengths may respond
quite differently to the same population of cloud particles (Intrieri et
al., 1990). This indicates that the aggregate information derivable from
coincident remote sensing measurements using a variety of these systems
may be increased in a highly synergistic manner. This possibility has
only begun to be explored (e.g., Sassen et al., 1989; Eberhard et al.,
1989 and I 990).

Similar possibilities exist in the combination of lidar and radar
observations with passive high spectral resolution measurements in
visible, near infrared, and infrared window regions, especially with
respect to determination of cloud optical properties (Grund et al.,
1990). When combined with surface-based remote sensing observations of
ambient winds. (wind profiler), moisture (microwave profiler), and
thermodynamic structure (infrared and microwave profilers), the
possibility exists for a rather comprehensive surface-based observing
system. Given the demanding observational requirements described in the
preceding sections, the potential of this approach needs to be fully
explored.

The strategy for improving utilization of remote sensing observations
for quantitative characterization of cirrus clouds and their environment
during FIRE Phase II is *to intercompare* *coincident observations from
a wide variety of active and passive systems together with in situ*
*observations of cloud physical composition and atmospheric state.*
Although maximizing the quantitative information from each system is an
obvious objective, results from FIRE Phase I indicate that the
capabilities of individual systems are limited and that *coincident
observations* *are highly desirable.* Thus, the emphasis of Phase II
research will be on defining the potential synergistic increase in
quantitative information from combined systems encompassing:

> active remote sensing systems, including
>
> \- dual polarization visible wavelength lidar,
>
> \- short-wavelength Doppler radar,
>
> \- high spectral resolution visible wavelength lidar,
>
> \- dual polarization, Doppler, infrared lidar (CO2); and
>
> • passive remote sensing systems, including
>
> \- high spectral resolution, infrared, and near infrared
> spectrometers, - microwave radiometers in water vapor absorption
> region,
>
> \- narrow-beam radiometers in visible, near infrared, and infrared
> window regions.

In addition to plans for intensive field observations (section 4), pilot
studies (extended time) have been initiated (sections 5.3 - 5.6) and
will continue during Phase H. It should also be noted that utilization
of remote sensing observations of cirrus clouds from aircraft and
satellites will also benefit from these activities.

4.0 CIRRUS INTENSIVE FIELD OBSERVATIONS (CIRRUS IFO-II)

4.1 Synopsis

A second intensive midlatitude cirrus field experiment (FIRE Cirrus
IFO-II) is required to meet the objectives of FIRE Phase II (section 3).
Findings from the first experiment (IFO-I) have led to the development
of new instrumentation and observing capabilities for the purpose of
resolving some of the most critical unresolved questions (section 2).
Advantage will also be taken of other new observing capabilities that
will markedly increase the overall utility of the data sets for use in
understanding cirrus cloud development. FIRE has grown to include active
participation by mesoscale (regional) and large-scale (GCM) modelers in
the area of cirrus cloud parameterization and climatic effects. The
plans for FIRE Cirrus IFO-II have been strongly influenced by the data
requirements of these models. Experiences from IFO-I have also played a
major role in defining the measurements that will be needed and in
redesigning the sampling strategies that will be used. This experiment
will address the problem of cirrus cloud development and impacts over a
wide range of scales through the integration of models and observations.
FIRE Cirrus IFO-II will provide a unique observational basis for
improving the quantitative utilization of remote sensing observations
from satellites and from the surface for investigating and monitoring
cirrus clouds. The observational elements are summarized below and
described in more detail in succeeding sections. It must be emphasized
that the true value of the observations will derive from their use in
evaluating and improving models and observing systems (data analysis
algorithms) which can be used to extend the knowledge gained into the
context of the global climate system.

The Cirrus IFO-II will be conducted in southern Kansas from November 13
to December 7, 1991 (section 4.2). The primary subjects are the extended
large-scale cirrus systems associated with the subtropical and
midlatitude jet streams. Cirrus are expected to occur about 60% of the
time during this period. The location coincides with the inner ring of
the mesoscale NWS wind profiler network (200-km spacing) presently under
construction by NOAA (section 4.3.4). The availability of this data was
the primary reason for selecting this site. Two additional FIRE wind
profiler sites will be deployed for the purpose of defining dynamic
phenomena on a smaller scale (50-km) at which cirrus often exhibit
significant and persistent organization. A substantial increase in the
collection of rawinsonde data is proposed for Citrus IFO-II (section
4.3.4). Three cases where six-hourly soundings will be taken for a
36-hour period over much of the continental United States are planned
(at least 43 stations). During the final twelve hours, three-hourly
soundings will be made over a more limited region of the central U.S.
(15 stations). These data are absolutely essential for evaluating and
improving large-scale models of cirrus cloud development. Supplemental
soundings (1800 UTC) from the inner 15 stations are also required on
most other experiment days (10 additional days). Five CLASS-type
rawinsonde stations will be used to support remote sensing studies and
to resolve thermodynamic structure over an area defined by three NWS
wind profiler stations.

Satellite data (section 4.3.1) will be collected at full spatial,
temporal and spectral (all channels) resolution from all geosynchronous
and polar orbiting meteorological satellites viewing the region from 25N
to 5ON latitude and 75W to 125W longitude. Data from other satellites
will also be obtained for this region, including Landsat. The ISCCP
analysis products (CX) will be acquired as part of the Cirrus IFO-II
data sets. These data are integral to achieving the experiment
objectives.

Four surface-based remote sensing sites will be fielded (section 4.3.2).
The stations will be closer to each other than in IFO-I (50 km versus
135 km) to better resolve the dominant scales of cloud organization. A
greater diversity of the most sophisticated cirrus cloud remote sensing
instrumentation will be deployed including polarization lidar, scanning
cloud lidar, high spectral resolution lidar, infrared (CO2) Doppler
lidars, short-wavelength (mm) Doppler radars, high spectral resolution
infrared interferometer spectrometers, and other radiometric sensors. A
much greater effort to monitor ambient meteorological conditions using
active and passive sensors is also planned. The strategy is to obtain
coincident observations from as broad a range of complementary observing
systems as possible. This will facilitate analyses that combine data
from multiple systems. In this way, the aggregate information content
will be maximized in that the limitations and uncertainties of
individual systems may be compensated by others whose characteristics
are quite different. Exploring this potential for synergism is a main
theme for Cirrus IFO-II. A complete suite of observing systems will only
be possible at one location (the Hub) since some of these systems are
unique. The instrument complement at the second site will also be
extensive while the third site will be quite limited. A fourth site near
the Hub location is also planned to accommodate the operational
requirements of the volume imaging (scanning) lidar (VIL, see section
4.3,2). This is different than the IFO-I strategy of evenly spreading
resources over the sites (Starr, 1987a). It should also be noted that
the scanning systems will provide some coverage over the other sites
given the closer spacing adopted for Cirrus IFO-II. In addition, the
closer site spacing will facilitate greater coincidence with airborne
remote sensing and in situ observations since the site separation
distance is compatible with the scales of typical aircraft sampling
patterns.

Five aircraft are proposed for the experiment, including two remote
sensing platforms and three in situ platforms (section 4.2.3). The
high-altitude NASA ER-2 will provide a \'satellite view\' with
significantly greater observing capability than presently available from
satellites, including cloud lidar for unambiguous detection of cloud top
and internal cloud structure. A low-altitude remote sensing aircraft (UW
C-131) will provide coincident observations that are highly
complementary to the top-down view from the ER-2, including up-looking
lidar and short-wavelength radar. Three *in situ* platforms are highly
desirable given that cirrus cloud systems are often quite complex
occurring in multiple layers over appreciable depths of up to 5 km
(Starr and Wylie, 1990) and typically exhibit great spatial variability
in physical structure, especially in microphysical structure (Heymsfield
et a\]., 1990). Cloud microphysical structure represents the key linkage
between dynamical and radiative processes. Knowledge of this
relationship is fundamental for understanding cirrus cloud development
and for evaluating and improving parameterizations of cirrus clouds and
their effects for use in large-scale models (GCMs). *Accurate knowledge
of cloud microphysical properties is essential for achieving the* *FIRE
Phase II scientific objectives* (section 3). Three *in situ* platforms
(NCAR Saberliner, NCAR King Air, and UND Citation) will significantly
improve the sampling of microphysical cloud properties and allow a
greater degree of flexibility in mission planning and execution in
comparison to the Cirrus IFO-I (two platforms).

A number of key improvements have been made in aircraft instrumentation
in comparison to IFO-I. Ice particle number density size distributions
will be observed down to 10 μm particle sizes. The lack of knowledge of
the concentrations of particles smaller than 25 to 50 μm represents a
major uncertainty in the analysis of the IFO-I observations with
important ramifications in the areas of cirrus cloud radiative
interactions and remote sensing possibilities (Wielicki et al., 1990;
Ackerman et al., 1990). The capacity for collecting actual crystals has
also been significantly enhanced. These observations are essential for
assessing ice crystal habit and morphology which impacts the processing
of data from airborne optical microphysical probes and is an important
factor in determining cloud development and radiative properties. These
instruments have been developed and installed on the NCAR King Air and
Sabreliner aircraft. In addition, accurate observations of ambient water
vapor concentrations in and around cirrus clouds will be possible using
a new cryogenic frost point hygrometer on the NCAR Sabreliner. These
observations are critical for understanding cloud development and
testing cloud models. The data obtained should help to resolve a number
of pivotal uncertainties in our knowledge of cirrus clouds.

4.2 Schedule and Location

The FIRE Phase II Cirrus Intensive Field Observations campaign (Cirrus
IFO-II) will commence on November 13, 1991 and conclude 25 days later on
December 7, 1991. Deployment and check-out of surface-based
instrumentation and support facilities will occur during the week prior
to November 13. Tear-down and packing will occur during the week after
December 7.

The experiment will be conducted principally in southern or eastern
Kansas. The primary motivation for selection of this location is the
anticipated availability of nearly continuous observations from a
mesoscale network of surface-based wind profilers presently under
construction by NOAA/ERL (fig. 10, section 4.3.4). These data will serve
to greatly enhance the value of both the synoptic scale observations and
the *in situ* and remotely sensed cloud observations for developing,
testing, and validating models of cirrus cloud development on scales
ranging from very high resolution cloud models to regional mesoscale
models and, ultimately, to cirrus parameterizations in large-scale
forecast models and GCMs. In particular, the data will permit derivation
of upper tropospheric vertical motions at scales comparable to the
mesoscale cirrus cloud features that are such a prevalent characteristic
of extended large-scale cirrus cloud systems.

The selection of the time period for the field campaign was motivated by
a desire to minimize the occurrence of cirrus in association with deep
convective systems and intense extratropical cyclones where obscuration
by lower level clouds is a serious impediment to surface-based remote
sensing and where the occurrence of intense or widespread precipitation
greatly complicates analysis and modeling on the synoptic scale and the
mesoscale. In addition, although some situations of an underlying
snow-covered surface are desired, conducting the experiment wholly over
a snow background would clearly compromise the objectives in the area of
satellite-based remote sensing. The fall transition season most fully
meets these requirements. Selection of the specific time period was
based on the cloud climatology of Warren et al., (1986) and Hahn et al.,
(1984) and on a careful review of three years of daily GOES imagery. The
primary target is the extended cirrus sheets associated with the
subtropical jet stream that frequent this location during this time
period.

Based on a 3:2 weighting of the fall (SON) and winter (DJF) season
values contained in Warren et al., (1986), cirriform clouds are expected
to occur **58%** of the time with an average fractional cloud cover of
50% when they occur. Thus, about 15 days of cirrus are expected over the
25 days of field observations. Interannual variability of monthly mean
cirrus frequency of occurrence is less than 5% (10 year std. dev.).
Corresponding frequencies of occurrence (and average fractional coverage
when they occur) for other cloud types are: stratus-stratocumulus, 28%
(81%); altostratus-altocumulus, 23% (49%); nimbostratus, 7% (99%);
cumulus, 3% (26%); cumulonimbus, 1% (53%); and obscuring fog, 1%. Clear
conditions occur 31% of the time (-8 of 25 days). When cirrus occur, no
other cloud forms are present **62%** of the time while
stratus-stratocumulus and altostratus-altocumulus are also present 19%
and 26% of the time, respectively (Hahn et al., 1984).

The specific location of surface remote sensing sites is critically
contingent on the ability to conduct aircraft operations in and around
cirrus clouds over and within a 100 km radius of the sites at altitudes
ranging from about 20,000 to 42,000\' (-6 to 13 km). Flight legs of from
20 km to more than 100 km in length will be flown at various attitudes
(profiling) over this area (section 4.3.3). In addition, relatively
rapid spiral descents of 10,000\' or more will be flown. Since our
airspace requirements conflict with altitudes of commercial jet traffic
and military aircraft operations, close coordination with the Federal
Aviation Administration (FAA) and the Military Operating Areas (MOA) is
absolutely necessary to maximize FIRE aircraft operations. (Remote
sensing aircraft will also operate at constant altitudes of about
65,000\' (20 km) and 10,000\' (3 km.). Preliminary discussions have been
initiated with the FAA and they have unofficially recommended areas to
the east and southeast of Wichita as incurring minimal interference with
commercial, military and private airways (circled in fig. 4). Further
discussions will be held to consolidate our plans and improve
cooperation with the Kansas air traffic controllers.

The area due east of Wichita is the first choice for FIRE Cirrus IFO-II
operations as it is more completely contained within the National
Weather Service (NWS) mesoscale profiler network. This location on the
expected downwind side of the inner profiler array will also enhance our
ability to resolve the precedent development. McConnell Air Force Base
has responded positively to our inquiries with respect to locating the
FIRE aircraft there and would probably serve for either option.
Appropriate fuel and power services, hangar space (possibly), laboratory
space, and access to meteorological forecasting facilities are
available. McConnell\'s location would permit all aircraft (except for
the ER-2 which will base in San Antonio) to climb to operating altitude
during transit to the operating area with little or no wasted air time.
Roads in the area allow relatively easy commuting from the surface sites
to the airbase. The proposed operating area is a military restricted
airspace (see last paragraph for update). Coordination with the
appropriate military authorities will be initiated. Alternate locations
to the south and further east of Wichita will also be explored.

A preliminary layout of the surface remote-sensing sites (section 4.3.2)
is depicted in figure 4. Three primary sites are identified in this
preliminary configuration: the Hub site consisting of an extensive suite
of remote sensing instruments and two other remote sensing sites one of
which (R3) is located at a NOAA wind profiler station. The sites roughly
form an equilateral triangle with sides of about 50 km in length. Wind
profilers will also be deployed at the Hub and R2 sites. The locations
of four CLASS-type rawinsonde stations are also shown as are the
locations of NWS rawinsonde stations (section 4.3.4). Another
remote-sensing surface site (VIL) is shown in close (20 km) proximity to
the Hub site. The scanning cloud lidar system will operate at this
satellite location.

Although preliminary (subject to finding suitable airspace), the
configuration shown in figure 4 illustrates the basic sampling strategy
adopted for the experiment. First, all the remote sensing sites are
located within the aircraft operations area. Second, the array is
located within the expected downwind side of the NWS profiler array.
Third, the site separations approximate that of typical mesoscale cloud
features and enable resolution of finer scale dynamic features (Hub, R2
and R3) in comparison to the NWS profiler grid. Fourth, the scanning
lidar system (VIL) is located upwind and as close as operationally
feasible (section 4.3.2) to the Hub site while the extensively
instrumented R2 site is located downwind of the Hub site.

![](media/image4.wmf){width="6.5993055555555555in"
height="7.802083333333333in"}

> Possible locations for the missions operations center are:
>
> a\) McConnell AFB: This location would allow the mission scientist to
> directly interact with aircraft principal investigators and support
> crews in mission planning and to debrief airborne scientists
> immediately after a mission. Both of these advantages are highly
> desirable. Suitable office and meeting facilities are available.
>
> b\) Hub site: A majority of surface-based active and passive remote
> sensing instruments and scientists will be located at this site. This
> location would facilitate the direct participation of these scientists
> in mission planning and execution and provide direct access to unique
> information on current local cloud and meteorological conditions. The
> lack of involvement by the ground site scientists in mission planning
> and execution was a significant weakness in the Phase I Cirrus IFO.
> Potential locations for the Hub site will be investigated. Suitable
> office and meeting facilities must be available.

Another option which attempts to maximize the advantages of both the
above locations while minimizing the disadvantages is being considered.
In this scenario, the Hub site would serve as mission operations center
during actual operations. A second location would be used for pre- and
post-mission briefings and mission planning (typically in the early
morning and early evening).

This \'briefing center\' would be located relatively close to McConnell
AFB but between McConnell AFB and the Hub site. Most of the principal
investigators would be housed at this location since there is little
likelihood of securing the needed quantity of suitable housing at the
remote-sensing sites. This would facilitate daily interaction among all
the investigators, including those with airborne and surface-based
instrumentation. However, this would require some duplication in
facilities (satellite imagery and synoptic information, section 4.4).

In addition, since observations from the remote-sensing sites are
essential for the pre-mission briefing and final planning decisions,
this will require that selected investigators have sufficient personnel
to operate their instruments in the early morning (prior to aircraft
operations) while they are involved in mission planning. Good
communications, including data transmittal and display, between the Hub
and VIL sites and the \'briefing center\' would also be essential.

Logistical considerations will be important in selecting the specific
locations of the Hub and R2 remote sensing sites, including the
availability of suitable field sites with access to the required power
supplies, and the availability of suitable accommodations and eateries
within a reasonable driving distance.

\[The above discussions of the specific sites are tentative at this
time. Very recent discussions with the military and the FAA have
confirmed that the FIRE Operations Area shown in figure 4 coincides with
the Eureka MOA, which is heavily used throughout the week for F16
training up to 31K ft. Coincident F16 and FIRE operations would pose
potential conflicts, from both aircraft and upward-looking lidar
considerations. Discussions are now underway to pursue either approval
to operate within the Eureka MOA on a priority basis or to relocate to
another, more accessible area. The FAA Central Region manager has
suggested, based on preliminary information of FIRE\'s aircraft
operational requirements that the areas to the [southeast]{.underline}
of Neodesha or to the [east]{.underline} of Lamont offer the least
interference with other air traffic (fig. 10). Further discussions with
the FAA and military will continue and another site survey will be
conducted to inspect the alternate areas in order to identify a suitable
region for FIRE operations.\]

4.3 Observations

4.3.1 Satellite Observations

Satellite observations have two primary strengths. First, they provide
coverage over a wide range of spatial and temporal scales and may be
used to connect and extend cloud observations made at small space and
time scales (surface and aircraft observations) to analyses at the
larger scales resolved by regional and global models. Second, satellite
observations provide a means to directly observe the cloud radiative and
physical properties that are most critical to climate (solar albedo and
infrared emittance; and cloud height, temperature and amount,
respectively).

Satellite data acquired during Cirrus IFO-IL are integral to the
strategies adopted for achieving the objectives of FIRE Phase II
(related FIRE science objectives given in parenthesis, see section 3)
and will be used to:

> • construct verification data sets for testing models of cirrus cloud
> development (1, 2),
>
> • develop and test models of radiative transfer in cirrus (3),
>
> • quantify and develop satellite remote sensing capabilities (4, 5),
> and
>
> • support operational scheduling of aircraft and surface observations.
>
> Satellite observations provide the only means of characterizing cirrus
> cloud fields at the scales required for verifying large scale models
> of cloud development such as regional nested mesoscale models and
> global scale, models. As such, satellite-derived cloud field
> descriptions are essential for verification and predictability studies
> using these models. Satellite observations of the distribution of
> upper tropospheric water vapor may also prove valuable in
> characterizing the large-scale environment for these studies (section
> 3.2). Detailed radiative models of cirrus clouds will be also tested
> using the IFO-II data and rely critically on satellite measurements of
> upwelling radiance.
>
> The FIRE Cirrus IFO-II data will also be used to define the
> capabilities and limitations of algorithms that are used to derive
> cirrus cloud properties from satellite radiance data. The cloud
> properties are both physical (cloud height and coverage, cloud
> particle *size and* phase) and radiative (albedo, optical depth,
> emittance). Definition of satellite remote sensing capabilities relies
> on acquisition of coincident data from a) surface-based and airborne
> remote sensing instruments such as lidars, spectrometers, narrow-beam
> infrared-window radiometers, and pyranometers, b) airborne *in situ*
> measurements such as cloud particle size, and c) satellites at higher
> temporal, spatial and spectral resolution than routinely available for
> global remote sensing of clouds (i.e., Landsat, AVHRR HRPT). Satellite
> cloud remote sensing studies will focus on the current ISCCP
> methodology (Rossow et a\]., 1985) but also include tests of
> alternative cloud algorithms (Wylie and Menzel, 1989; Minnis et al.,
> 1990; Ackerman et al., 1990; Arking and Childs, 1985). Investigations
> of future satellite cloud remote sensing capabilities will also be
> conducted. For example, the MODIS scanning radiometer (Salomonson et
> al., 1989) scheduled to fly in the late 1990\'s on EOS can be
> simulated using high-spectral (HIS) resolution measurements from the
> ER-2.
>
> Finally, satellite data are crucially important to the operational
> decisions made during the field experiment (section 4.4). Time series
> of half-hourly GOES imagery will be used to characterize the movement
> and development of cirrus cloud systems (and upper tropospheric
> moisture structure) before and during their arrival over the
> operations area. Experience during IFO-I showed this to be the single
> most important piece of information required for planning and
> execution of successful missions. These data will be available for
> mission planning in near-real time via an interactive link between the
> mission operations center and the Man-Interactive Data Acquisition
> System (McIDAS) at University of Wisconsin.

Satellite data will be collected for the region from 25N to 5ON latitude
and from 75W to 125W longitude over the time period from one week before
to one week after the field experiment (section 4.2). The area of
coverage encompasses the domain required for the studies of cirrus cloud
development using regional (mesoscale) and GCM models, as well as
including some areas with water backgrounds to simplify radiative
analysis. Satellite data requirements are given in table 1.

Table 1 - Satellite Data Requirements for the FIRE Phase II Cirrus
Intensive Field Observations

  ------------------ ------------- ------------ ------------- -------------
  Satellite          Spatial       Spatial      Time          Data at the
                     Sensor                     Resolution    

                                   Coverage                   Field Hub
                                                              Site

  GOES VISSR         1 km VIS, 7   25N - 5ON    1/2 hour      Yes
                     km                                       

                     IR            75W- 125W                  

  GOES H20           7 km          "            1/2 hour      Yes

  GOES VAS           8 - 16 km     "            3 hour        Yes

  NOAA AVHRR GAC     4 km          "            4 per day     Yes

  NOAA AVHRR HRPT    1 km          "            4 per day     Yes

  NOAA HIRS/MSU      20 km         "            4 per day     No

  ISCCP CX           32 km         "            3 hour        No

  Landsat TM 30      30 meters     180 km       selected      No
  meters                           targets      times         
  ------------------ ------------- ------------ ------------- -------------

The GOES data will be collected from both GOES-7 and GOES-1 at the
highest spatial and temporal resolution available and include frequent
imagery from the infrared water vapor channel (Multi-Spectral Imagery,
MSI) in addition to routine imagery (VISSR) and sounding data (VAS). The
high-resolution AVHRR image data (GAC and HRPT) and lower resolution
HIRS and MSU sounder data will be collected from both daily overpasses
(ascending and descending) of each of the two operational NOAA polar
orbiters and include all channels. The ISCCP CX data include both the
cloud and radiance analyses derived from GOES and NOAA polar orbiter
data. The Landsat Thematic Mapper data (30 meter spatial resolution,
seven spectral bands) will be collected over the IFO surface sites and
also over the Gulf of Mexico. Data will be acquired for 30 candidate
targets (180-km areas) of which 10 will be selected for analysis.

The above data will be extensively used by FIRE investigators. Other
satellite data which will be of more limited use include:

> \- ERBS/SAGE II solar occultation observations of cloud height and
> extinction,
>
> \- ERS-l multi-angle scanning radiometer (AVHRR wavelengths),
>
> \- DMSP SSM/I microwave data for cloud liquid water content, and
>
> \- Meteor ScaRaB broadband longwave and shortwave scanner data.

4.3.2 Surface Radiation and Remote Sensing Observations

> Surface-based radiation and remote sensing observations using active
> and passive

techniques serve three main purposes (related FIRE science objectives
given in parenthesis; see section 3). They will be used to:

> • determine cirrus cloud radiative, physical, and dynamic properties
> (1, 2, 3, 4, 6),
>
> • measure the surface radiation budget in the visible, near infrared,
> and infrared spectral regions and determine the influence of cirrus
> clouds on those budgets (5), and
>
> • make high spectral resolution measurements of downwelling infrared
> radiance for comparison with radiative transfer model calculations (5,
> also see section 6.1).

Surface-based observing systems provide very high temporal (and, in many
cases, much greater horizontal and vertical) resolution in comparison to
satellite observations and significantly greater temporal (and vertical)
coverage than aircraft observations. To maximize the scientific output
of FIRE, a great variety of relevant remote sensing instruments will be
deployed at each of several sites during IFO-II. Three such remote
sensing sites are planned (fig. 4). The strategy is to field as complete
a complement of remote sensors as possible at one site, and then at a
second, and then at a third. Because some of the systems are totally
unique, it will not be possible to have an identical complement at each
site. Thus, the first site (known as the Hub) will have at least one of
every desired type of instrument available (the scanning VIL will be
located within 20 km of the Hub). The second (R2) and third (R3) sites
will necessarily have progressively less complete instrumentation. The
strategy of having the greatest possible concentration of different
instruments at the Hub, and to a lessor extent at the other sites, will
*facilitate analyses that combine data from multiple Systems.* In this
way, the total information content of the data will increase in a highly
synergistic manner in comparison to what would be achievable within the
limited capabilities of each individual sensing system.

FIRE seeks to provide as complete a description as possible of cirrus
clouds and associated radiative and meteorological conditions within a
relatively small mesoscale region. The surface-based radiation and
remote sensing data will support analysis and modeling of cirrus cloud
development and variety of radiative transfer and satellite-based remote
sensing studies. The increased coverage of a mesoscale network (as in
the Phase I Cirrus IFO) provides greater compatibility with the spatial
scales resolved by aircraft operations, satellite observations, and
cloud development models. A site spacing of -50 km corresponds to the
scale of typical mesoscale cirrus cloud features (20 - 100 km) and will
also maximize coincidence of aircraft observations for the most sites
(i.e., aircraft sampling legs are typically 30 to 100 km in length,
section 4.3.3). The most important criteria in establishing site
locations is to ensure that the *airspace* overhead is as *unrestricted*
as possible, since these sites must serve as the locus of research
aircraft operations.

Synergism is a key theme of IFO-II. FIRE seeks to explore the full
remote sensing potential contained in temporally and spatially
consistent data sets collected by active and passive sensors operating
in various spectral regions. Simple intercomparisons of data from
similar systems, although useful for intercalibration (and system
confidence) purposes, is not the objective. The multiple remote sensing
approach using polarization lidar, Doppler radar, and passive radiometry
has been demonstrated to be a powerful research tool that permits the
increased understanding of cirrus (Sassen et al., 1989) and winter storm
(Sassen et al., 1990a) cloud systems. Additional available technologies
make this approach even more attractive. Despite the many successes of
the FIRE Cirrus IFO-I, it is clear that the value of the individual
lidar data sets would have been significantly greater if the data were
coincident due to the complementary nature of various observing systems
(e.g., coordinated polarization lidar, high spectral resolution lidar,
and infrared lidar observations).

Acquisition of coincident observations by a scanning short-wavelength
(mm) Doppler radar is a very high priority for Cirrus IFO-II. The
hardware failure of the radar on the first day of field operations was a
major setback in Cirrus IFO-I. Radar provides a relatively direct
measure of cloud water content (Sassen, 1987; Sassen et al., 1989;
Kropfli et al., 1990) which is a key parameter for most of the Phase II
science objectives. Doppler capability permits estimation of the
vertical ice mass flux which is ,t crucial determinate of the cloud
properties and cloud maintenance. Scanning capability will facilitate
use of the data for integration of *in situ* observations (aircraft) and
measurements by vertically-pointing systems (surface and airborne) into
a coherent description of the time-dependent cloud water distribution
over a mesoscale region.

Efforts to field state-of-the-art passive sensing systems, especially
accurate high spectral resolution interferometer spectrometers (Smith et
al., 1990a), will yield considerable return in comparison to Phase I,
especially when the data is combined with the observations from active
sensing systems (Grund et al., 1990). Pilot and extended-time studies
seeking to better define the potential capabilities of combined active
and passive remote sensing techniques for characterizing cirrus clouds
are ongoing in preparation for IFO-IL (sections 5.3 - 5.6).

4.3.2.1 Active Sensing Systems

Compiled in table 2 are the categories of active remote sensing systems
that offer the best capabilities for conducting a comprehensive cirrus
cloud experiment. Specific instruments are identified by the responsible
organization since, for the most part, each represents a unique
capability. The Kernel Group provides well-tested, basic information on
cloud microphysical

Table 2 - Active remote sensing systems participating in FIRE Cirrus
IFO-II. Planned location

> (see figure 4) and responsible organizations are noted. (\*) Scanning
> capabilities may not be available. (\*\*) High priority instruments
> for profiling lower tropospheric water vapor content and temperature
> in support of SPECTRE. (\*\*\*) Located within 20 km of Hub.

**Category** **Instrument** **Location/Organization**

Hub R2 R3

Kernel Polarization Lidar U. Utah LaRC

Group High Spectral Resolution Lidar U. Wise.

Scanning Doppler Radar WPL (9 mm) PSU (3 mm)\*

Highly Water Vapor Raman Lidar\*\* GSFC

Desirable Radio-acoustic sounder (RASS)\*\* WPL

Wind Profiler PSU CSU NWS

Scanning C02 Doppler Lidar WPL CSU\*

Scanning Cloud Lidar (VIL) U. Wisc.\*\*\*

content, optical thickness, and particle vertical motions. These basic
measurements are integral to the success of the FIRE Cirrus IFO-II. The
combined data allow for a unique characterization of cirrus cloud
development and structure. For example, particle phase, habit, and
orientation can be derived from polarization lidar observations. A
similar capability has recently been added to the WPL cloud-sensing
radar (Kropfli et al., 1990). These data can be combined with the
Doppler-observed particle vertical motion to estimate particle mass and
terminal velocity and, thus, updraft and downdraft wind speeds. These
data can then be combined with the radar derived ice mass content to
characterize the vertical ice mass flux and the mechanisms and scales
involved in cirrus cloud generation. Simultaneous lidar and radar
observations at multiple wavelengths will serve to refine the ice
content estimates by providing additional information on particle size
distribution in comparison to single-wavelength probing. Simultaneous
measurements will also provide information on the relationship between
ice content and optical cross-section of the cirrus clouds. Optical
depth measurements provided by high spectral resolution lidar provide a
near-absolute calibration for optical depth retrievals using
polarization (or non-polarized) lidar observations enabling a
characterization of cloud optical properties in the visible at very high
temporal and vertical resolution.

The group of highly desirable remote sensors takes advantage of
state-of-the-art and developmental type systems with additional research
capabilities. Continuous lower tropospheric water vapor profiling
(Raman) and temperature profiling (RASS) are essential for SPECTRE
(sections 6.1 and 4.3.2.2). Provided that data integration times at
cirrus cloud altitudes are not excessive and that daytime capability can
be achieved (section 3.2.4), Raman lidar water vapor measurements
preceding local cirrus cloud development, and in the vicinity of cirrus,
may help answer important questions about humidity fields and cirrus
formation. Obtaining an accurate characterization of upper tropospheric
water vapor distributions is a very high priority (section 3.2).

Infrared lidar (CO2) observations are expected to provide a measure of
the vertical structure of cirrus optical properties (emittance) in the
important 10.6 μm window region. Polarization data at this wavelength
can be used to address important questions about the infrared absorption
efficiencies of the particles. The Doppler velocity spectra from the C02
lidar has some potential for characterizing particle size distributions
(the distribution of particle fall speeds). Combining these data with
coincident radar-derived ice mass concentrations (radar response is
weighted according to the particle size distribution) and *in situ*
microphysical observations obtained by aircraft should lead to
significant improvements in the interpretation of the data from each of
these sources (e.g., Sassen, 1987; Eberhard et al., 1989; Eberhard et
al., 1990). Problems with signal attenuation that affect each system to
differing degrees can be addressed in comparative studies (Sassen et
al., 1989; Uttal et al., 1990).

A rapidly scanning lidar has considerable value for capturing
essentially Eulerian views of cirrus structure along and across the mean
wind direction within a 50 km radius. These very high resolution data
will provide a unique means to characterize cirrus cloud structure over
scales corresponding to satellite observations and the field of view
(fov) of radiative flux observations. Under conditions involving strong
vertical wind shear, the distortions present in the cloud structure
obtained from zenith operations can be assessed. Moreover, scanning
lidar data would be a very useful tool for pre-flight aircraft mission
planning, particularly if the images can be made available in near
real-time to the mission scientist and aircrews (section 4.4).
Potentially, these data could also be used to direct aircraft during
mission execution.

The planned location of the VIL at a distance of 10 to 20 km to the
southwest of the Hub site attempts to maximize collection of data in an
optimal sampling pattern for the science objectives while minimizing
potential safety problems (below) and consequent interference with
aircraft operations (section 4.3.3). Given that a southwesterly wind
direction dominates during large-scale cirrus events at this location at
this time of year , this location will permit continuous cross-wind
scanning just upwind of the Hub site and *in situ* aircraft patterns.
This crosswind VIL scanning pattern will map the time-dependent cirrus
cloud structure in a plane parallel to and of the same scale (100 km) as
that being sampled by the remote sensing aircraft which will operate
just downwind of the VIL sampling volume. The VIL site should be as
close to the Hub as possible but at a sufficient distance to permit
continuous aircraft operations over the Hub without their intersecting
the VIL scan volume. In addition, this location permits VIL scanning in
the upwind direction (away from the Hub and aircraft operations).
Situations may require further restrictions on the VIL scan volume.

Observations of vertical motions in cirrus clouds has always been very
problematic and have significantly retarded progress in testing cloud
development models (section 3.2). The array of wind prowlers at the
remote sensing sites (section 4.3.4) provides a means of deriving
area-averaged vertical motions on the scale of mesoscale cloud features
(-50 km, see fig. 4). *One of* the wind profilers (five-beam system at
R2) will have the capability to independently observe vertical motion on
a scale of about 20 km. Scanning Doppler systems provide an alternate
means to derive vertical motions at a comparable scale (-20 km) also
using kinematic techniques (VAD scans). Similar techniques can be
applied to aircraft-observed winds as in Gultepe and Heymsfield (1989).
Doppler spectra derived from the zenith-pointing C02-lidar observations
can potentially, if sufficient resolution is achieved, provide an
independent direct measure of vertical air motion and particle vertical
motions within individual convective cells for comparison to values
retrieved from Doppler radar observations at similar scale. These
observations can be compared to *in situ* aircraft observations at a
similar scale. The approach here is to apply every possible means to
definitively characterize vertical motions in cirrus clouds. Only in
this way can the present uncertainties in independent analysis of data
from individual systems be reduced to an acceptable level.

Since maximum information content is derived from the multiple remote
sensor approach when the various systems are examining the same cloud
volume, emphasis will be given to zenith-pointing operations (except for
the scanning cloud lidar\--VIL). However, the scanning Doppler systems
designed to collect Velocity-Azimuth-Display (VAD) scans will coordinate
their activities at predetermined time intervals to periodically
characterize three-dimensional cloud structure and vertical motion
fields which are of very high priority.

An important concern is that lidar operations are conducted in such a
way as to be eye­safe to ground and aircraft personnel. (The infrared
C02-lidar is eye-safe.) This is generally not a problem for vertically
pointing systems. However, the scanning lidar (VIL) can be a hazard*.*
Spotters can be used to monitor intruding commercial, private, and
military air traffic and scan patterns can be coordinated with research
aircraft activities. Operations may have to be suspended during aircraft
missions if the level of ground-to-air communications required to ensure
crew safety proves burdensome to the pilots or if situations become too
complex to insure safe operations. A plan to adequately deal with
*eye-safety* issues will be formulated and submitted to the appropriate
government agencies.

Another concern is the potential for *electronic interference* among
various active and passive sensing systems, including those borne on
aircraft or balloons, and communication equipment. In particular,
potential interference problems between wind profilers and rawinsonde
systems operating near 400 MHz and between wind profilers and aircraft
operations need to be understood and resolved, if possible. Efforts will
be initiated to investigate this issue and to recommend possible
solutions which may include altering the location of specific problem
systems. In this vein, the location and operation of the RASS system
will minimize the potential negative impacts of acoustic pulses (noise)
on project personnel.

4.3.2.2 Radiance and Radiative Flux Observations

There are four aspects to the measurements of downwelling (and
upwelling) radiances and radiative fluxes at the surface using passive
radiometric instrumentation. Although separately presented here,
exchange and cross-integration of data from each effort will be of
mutual benefit as will the availability of coincident active remote
sensing data and aircraft and satellite data. Efforts to measure the
surface radiation budget and to determine the effects of cirrus clouds
on the surface radiation budget will be conducted in collaboration with
the NASA Surface Radiation Budget (SRB) program and the Spectral
Radiation Experiment (SPECTRE, section 6. 1). A summary listing of these
instruments is given in table 3.

Data from three instruments are particularly useful for characterizing
cirrus cloud optical properties, especially in combination with lidar
and other active measurements. They are:

> \- sun photometers (Hub),
>
> \- narrow-beam infrared window (10 - 12 μm) radiometers (Hub and R2),
> and
>
> \- high spectral resolution (1 cm-1 from 4 to 16 μm) interferometer
> spectrometers (Hub and R2).

These measurements will serve to define cirrus cloud optical depth in
the visible and infrared, and cirrus cloud infrared effective radiating
temperature and emittance.

The effects of cirrus clouds on the downwelling solar radiation at the
surface will be assessed using narrowband and broadband solar flux
radiometers at the Hub and R2 sites where cloud imaging systems will
also be deployed. Besides the satellite observations (section 4.3. 1),
direct measurements of cloud and aerosol optical depths using a high
spectral resolution lidar and sun photometers at the Hub site as well as
cloud cover observations from a scanning lidar system are needed. Low
level aircraft observations of the spatial distribution of surface
albedo and solar radiative fluxes are also desired (section 4.3.3).

Table 3 -Summary of planned surface instrumentation and responsible
organizations for the FIRE Cirrus

IFO-II Campaign at the Hub, R2, and R3 surface remote sensing sites (see
figure 4).

**Location/Organization**

**Sensors** **Hub** **R2** **R3**

[Active Remote Sensing Systems]{.underline}

Polarization Lidar U, Utah LaRC

High Spectral Resolution Lidar U. Wisc.

Scanning Doppler Radar WPL (9 mm) PSU (3 mm)\*

Scanning C02 Doppler Lidar WPL CSU \*

Scanning Lidar (VIL) U. Wisc.\*\*

Laser Ceilometer LaRC CSU

Raman Water Vapor Lidar GSFC

Radio-Acoustic Temperature Sounder WPL

Dual-channel Microwave Water Vapor Sounder WPL

Microwave Wind Profiler PSU, WPL CSU NWS\*\*\*

[Passive Remote Sensors and Radiometers]{.underline}

Infrated Spectrometer GSFC, U. Wisc., CSU

> U. Denver (2)

Sun Photometer ERL

Infrared-Window Radiometer U. Utah, WPL CSU

Direct Spectral Solar Radiometer U. Denver CSU

Solar Flux Radiometers PSU CSU

Broadband Infrared Flux Radiometer many

Calibration Facility SPECTRE CSU

[Meteorological Support \#]{.underline}

Sky-Imaging Systems (VCR and 35 mm) LaRC CSU

Rawinsonde Station (CLASS) \## NCAR, LaRC NCAR

Cloud Ice Particle Replicator Sondes NCAR

Ozone Sonde ERL

Dobson Total Ozone Instrument ERL

Tethered Balloon Sounder GSFC/WPL

Instrumented Meteorological Tower TBD

Surface Observations LaRC CSU

Trace Gas Flask Samples ERL

\* - Scanning capabilities may not be available

\*\* - VIL will be located within 10-\'!O km of Hub

> \*\*\* - Up to 30 NWS wind profiler stations may participate (figure
> 10)
>
> \# - A total of 51 NWS rawinsonde sites may participate (15
> intensively, see figure 9)
>
> \## - A total of five systems will be deployed (three additional NCAR
> sites, see figure 4)

An SRB objective is to perform an intercomparison of various infrared
flux radiometers under all-sky conditions (continuation of an
international intercomparison project under the

WMO).

The SRB measurements will be obtained *only at the Hub site.*
Instrumentation will include:

> \- various broadband infrared flux radiometers,
>
> \- dual-band microwave water vapor sounder, and
>
> \- cloud ceilometer.

In addition to the VCR all-sky cloud imaging system, a *dedicated
CLASS-type* *rawinsonde station* will also be deployed at the Hub site
(section 4.3.4). Soundings will be made during each overpass (ascending
and descending) of the two operational NOAA polar orbiting satellites.
TOVS temperature soundings will also be collected for comparison.

SPECTRE\'s main objective is to accurately measure the zenith infrared
radiance at high spectral resolution while simultaneously profiling the
radiatively important atmospheric characteristics. These data can then
be used to meaningfully test detailed infrared radiative transfer models
of the gaseous atmosphere (section 6.1). The key features of SPECTRE are
continuous, instantaneous profiles of atmospheric temperature, humidity,
aerosol and cloud; continuous high resolution spectra of downwelling
infrared radiation; radiance rather than flux measurements; careful and
frequent radiometric calibration in the field; and redundant
measurements. Daytime and nighttime observations will be taken in clear
sky situations and when cirrus are present. Measurements will be taken
during FIRE missions.

> Measurements will include:
>
> \- high spectral resolution observations of downwelling infrared
> radiance,
>
> \- cirrus cloud and aerosol optical depths,
>
> \- atmospheric profiles of temperature, water vapor, and ozone, and
>
> \- trace gas concentrations at the surface.
>
> Radiometric instrumentation at the Hub site will include:
>
> \- two interferometer spectrometers (0.2 cm-1 and \< 1 cm-1 from 4 to
> 16 μm, respectively),
>
> \- grating spectrometer (0.5 cm- I from 8 to 25 μm),
>
> \- direct solar spectrometer (0.002 cm- I from 2 to 20 μm),
>
> **-** sun photometer, and
>
> \- on-site blackbody radiometric calibration facility.

Additional instrumentation and measurements will include Raman lidar
(water vapor profiling to at least 6 km), radio-acoustic sounder (RASS,
virtual temperature sounding to 6 km), ozone sondes (ozone sounding to
30 km), Dobson total ozone instrument, tethersonde (temperature, water
vapor, and ozone sounding to I km), meteorological tower (temperature,
water vapor, and ozone sounding to 10 m), surface meteorological
observations (pressure, temperature, humidity, and aerosols), and flask
samples of trace gases (carbon dioxide, ozone, methane, nitrous oxide,
and freons).

The use of multiple spectrometers provides redundancy and together with
a strong effort at absolute calibration will insure that the highest
possible radiometric accuracy is achieved. The tethersonde and tower
observations will fill the near-field \'blind spot\' typical of remote
sensing prowlers while FIRE rawinsonde observations will serve to define
the atmospheric state above the 6-km level. Data from the FIRE lidars
will be used to determine atmospheric aerosol optical depths and to
characterize the properties of cirrus clouds.

4.3.3 Aircraft Observations

Airborne observations are an essential component of the FIRE Cirrus
IFO-II strategy. As remote sensing platforms, research aircraft provide
a means to collect very high resolution, multispectral radiometric
measurements at wavelengths used by present and future satellite
observing systems (satellite simulators) and at viewing geometries
(scanning systems) specifically designed to facilitate use of the data
in resolving fundamental uncertainties with respect to cirrus radiative
properties (science objective 3, section 3). The airborne remote sensing
observations are a key element in efforts to quantify the capabilities
and limitations of satellite observing systems (science objective 4).
The value of the passive sensor data is greatly enhanced by the
acquisition of simultaneous observations of cloud physical properties
using active systems such as lidar and radar. The total information
derived from coincident lidar and passive radiometric observations is
significantly greater than the sum of the independent data sets as
shown, for example, by Spinhirne and Hart (1990). Direct measurements of
broadband radiative fluxes enable the effects of cirrus on the radiative
budget of the atmosphere and surface (science objective 5) to be
quantified without the deconvolution required of satellite observations.

As *in situ* observing platforms, aircraft provide a means to directly
observe cloud physical properties, especially the microphysical and
radiative properties, and to characterize the dynamic, thermodynamic,
and radiative structure of the cloud and its environment. Acquisition
and analysis of these observations is crucial to the success of efforts
to characterize and model cirrus cloud development (science objectives I
and 2), particularly with respect to understanding some of the most
problematic but critical aspects (section 3.2). Major improvements in
aircraft instrumentation have been made in response to the findings from
IFO-I (section 2,) specifically, a capability to observe the numbers of
ice crystals down to about 10 μm in size and to accurately observe
atmospheric water vapor contents. Significant improvements have also
been made in the capability for resolving the spectral and angular
distribution of radiance, for characterizing the turbulent fluctuations
of dynamic and thermodynamic quantities, and for observing the habits of
ice crystals. Efforts to develop an airborne polar nephelometer to make
direct measurements of cirrus particle phase functions are in progress
and, hopefully, the nephelometer will be installed on the NCAR King Air
in time for the Cirrus IFO-II. When combined with satellite, airborne,
and surface-based remote sensing observations, the *in situ*
observations provide \'ground truth\' for analysis of these data and
improving remote sensing capabilities (science objectives 4 and 6) as
shown, for example, by Sassen et al., (1999), Wielicki et al., (1990),
Spinhirne and Hart (1990), Kinne et al., (1990), Ackerman et al.,
(1990), and Hammer et al., (1990). The microphysical data provide basic
information needed to characterize fundamental relationships between
cirrus cloud optical and physical properties (science objective 3).

4.3.3.1 Aircraft Platforms and Instrumentation

> Deployment of five aircraft is proposed for FIRE Cirrus IFO-II. They
> are:
>
> • the NASA ER-2,
>
> • the University of Washington C-131 Convair,
>
> • the NCAR Sabreliner,
>
> • the NCAR King Air,
>
> • and one additional aircraft.

The ER-2 and the C-131 are *remote sensing platforms* and will operate
at constant altitudes well above (ER-2 at 65,000\') and below (C-131 at
10,000\') the cirrus clouds. The Sabreliner, King Air, and the other
aircraft are primarily *in situ* platforms. The Sabreliner will be
utilized to sample high cirrus layers and the upper portion of cirrus
above the operating ceiling of the King Air. The King Air will sample
lower cirrus (and altostratus) layers and the lower portions of high
cirrus to its operating ceiling. In some instances, the Sabreliner and
King Air have conflicting mission objectives with respect to the
required sampling strategies. For example, the flight patterns needed to
properly sample radiative, dynamical, and moisture fields can preclude
adequate sampling of the microphysical composition of the subject clouds
(section 4.3.3.2). Similarly, patterns designed to observe the water
budget of cloud layers may not permit adequate sampling of cloud
radiative characteristics. This is particularly problematic given the
prevalence of cirrus occurring in multiple layers over a deep region (5
km or more) of the atmosphere (Sassen et al., 1989; Starr and Wylie,
1990) with a high degree of spatial variability in both the vertical and
horizontal distribution of ice water *(*Heymsfield et al., 1990).
Accurate knowledge of the vertical distribution of cirrus ice water
content is essential for achieving many of the FIRE Phase H science
objectives (section 3). Thus, a [third *in situ* platform is
needed]{.underline}. This aircraft will be used solely for obtaining
vertical profiles of microphysical parameters. The platform should be a
jet aircraft capable of operating at higher levels (and greater range)
than the Sabreliner since extended systems of subtropical jet stream
cirrus are anticipated where cloud top heights may exceed the operating
ceiling of the Sabreliner on some occasions. The cloud top region is of
great concern for satellite-based remote sensing studies. The
[University of North Dakota Citation]{.underline} would satisfy these
requirements.

> The ER-2 instrument complement will include:
>
> • nadir-pointing, cloud lidar system (CLS);
>
> • scanning, visible and infrared (window) imaging radiometer
> (Daedalus);
>
> • scanning, multispectral cloud radiometer (MCR);
>
> • scanning, multispectral, solar radiometer;
>
> • scanning, multispectral, millimeter imaging radiometer;
>
> • nadir-pointing, high spectral resolution, interferometer
> spectrometer (HIS); • broadband infrared flux radiometer;
>
> • broadband solar flux radiometers (total and near infrared);
>
> • nadir-pointing, narrow-beam, infrared (split window) radiometer;
>
> • spectral electro optic camera; and
>
> • 35 mm photography (hand-held by pilot).

The scanning radiometers will duplicate channels found on present and
future satellites and other channels potentially important for the
remote sensing of cirrus and other clouds, specifically in the near
infrared (MCR). These sensors have very high spatial resolution. The CLS
will provide \'cloud-truth\' as in IFO-I. The HIS instrument is nearly
identical to one of the interferometer spectrometers at the Hub site.
The high spectral resolution and accurate calibration of the HIS
provides a capability to simulate most any present or future satellite
instrument operating in the infrared. The broadband instruments measure
both the upwelling and downwelling fluxes.

> Instrumentation on the UW C- 131 Convair will include:
>
> • scanning (zenith to nadir), multispectral, cloud absorption
> radiometer;
>
> • uplooking and downlooking, Ka-band (8.6 mm) radar;
>
> • dual-wavelength, uplooking polarization lidar (Nd-YAG);
>
> • broadband solar (pyranometer) and ultraviolet flux radiometers;
>
> • narrow-beam, uplooking and downlooking, infrared (window)
> radiometer;
>
> • meteorological sensors (pressure, temperature, humidity, wind, and
> turbulence);
>
> and
>
> • cloud physics instrumentation including
>
> \- liquid water content (Johnson-Williams and King/PMS probes),
>
> \- cloud particle size spectrum \[FSSP, PMS OAP ID-C, 1D-P, 2D-C, and
> 2D-P\], and
>
> \- various aerosol and cloud condensation nucleus \[CCN\] detectors
> and analyzers.

As discussed in section 4.3.2.1, the potential scientific return from
combining polarization lidar observations with coincident millimeter
wavelength radar observations for characterizing cirrus is very high as
these systems are quite complementary. The solar flux measurements at a
relatively low altitude should prove very useful in efforts to
characterize the radiative effects of cirrus on the surface. The
meteorological and cloud physics instrumentation are not really needed
for the remote sensing objectives of this platform but are in-place and
may provide useful measurements of low and middle level cloud properties
if deemed desirable for a particular mission (This turboprop aircraft
generally cannot operate at sufficiently high altitude to serve as an
effective *in situ* cirrus observing platform). The aerosol observations
will provide a \'ground truth\' for remote sensing observations of the
vertical profile of aerosol concentrations and types which is needed by
SPECTRE (sections 6.1 and 4.3.2.2).

> Sabreliner instrumentation will include:
>
> • cloud physics instrumentation including
>
> \- liquid water content probes (Johnson-Williams and King/PMS),
>
> \- cloud particle size spectrometers \[improved FSSP, PMS OAP 2D-C and
> 2D-P\],
>
> \- continuous ice particle sampler (CIPS, new),
>
> \- ice particle collection devices (NCAR ice rod, Mohaer ice stick,
> new),
>
> \- cloud condensation nucleus chamber (new), and
>
> \- aerosol detectors (ASASP),\'bag\' samplers and composition
> analyzers (new);
>
> • meteorological sensors (pressure, temperature, and wind components
> at 20 Hz
>
> • Lyman-alpha hygrometer;
>
> • cryogenic frost point hygrometer (new);
>
> • uplooking, total direct-diffuse, multispectral, solar flux
> radiometer (TDDR)
>
> • uplooking and downlooking, multi-angle (bugeye), visible radiance
>
> radiometer;
>
> • nadir-pointing, spectrally scanning solar radiometer (SPERAD);
>
> • broadband solar flux radiometers (total and near infrared);
>
> • broadband infrared flux radiometers;
>
> • narrow-beam, nadir-pointing infrared (window) radiometer and
>
> • forward and side-viewing VCR cameras.

The broadband flux radiometers are being upgraded to the same
instruments flown on the ER-2 and will monitor upwelling and downwelling
fluxes. The TDDR and SPERAD instruments also represent major
improvements in radiometric instrumentation on this platform. Both have
high spectral resolution and high accuracy and are specifically designed
for application in sensing cloud radiative properties.

> The King Air instrument complement will include:
>
> • cloud physics instrumentation including
>
> \- liquid water content probes(Johnson-Williams and King/PMS),
>
> \- cloud particle size spectrometers \[improved FSSP, PMS OAP 2D-C and
> 2D-P\],
>
> \- continuous ice particle sampler (CIPS, new), and
>
> \- ice particle collection devices;
>
> • meteorological sensors (pressure, temperature, and wind components
> at 20 Hz);
>
> • frost point and Lyman-alpha hygrometers;
>
> • uplooking and downlooking, broadband solar, visible, and infrared
> flux radiometers;
>
> • narrow-beam, nadir-pointing, infrared (window) radiometer;
>
> • chaff dispenser (new, wind field mapping via surface-based radar);
>
> • sulfur hexafloride dispenser and detector (new, wind field mapping);
> and
>
> • forward and side-viewing VCR cameras.

Of particular note are the new small ice particle probes for measurement
of the particle number density size distribution down to 10 μm particle
sizes. The 2D-C probes are unable to respond to particles smaller than
about 50 μm at Sabreliner airspeeds and 25 μm at King Air airspeeds
where significant and uncertain \'corrections\' are required for
particles smaller than about 100 μm. This basic limitation is associated
with much of the remaining uncertainty in analysis and modeling of IFO-I
data (section 2). For IFO-II, the Sabreliner will also be equipped with
ice particle collection devices. The new cryogenic frost point
hygrometer (Sabreliner) will permit accurate observations of upper
tropospheric water vapor content which have not been available. The
meteorological data will be recorded at 20 Hz (analysis at 10 Hz) to
better resolve turbulence characteristics and turbulent fluxes in
comparison to IFO-I (1 Hz). In addition, measurements of aerosol and
cloud condensation nuclei (CCN) concentrations and composition will be
obtained from the Sabreliner which will contribute to resolving basic
uncertainties with respect to cirrus generation and microphysical
development (e.g., Sassen and Dodd, 1989; Sassen, 1989; and Heymsfield
and Sabin, 1989). A cold room and laboratory facilities for analysis of
ice crystal and aerosol samples will be needed at (or near to) the
airfield. A suitable laboratory exists at the University of
Missouri-Rolla.

> Instrumentation on the third *in situ* platform should minimally
> include:
>
> • cloud physics instrumentation including
>
> \- liquid water content probes,
>
> \- cloud particle size spectrometers \[FSSP, PMS OAP 2D-C, and 2D-P
> probes\],
>
> • meteorological sensors (pressure, temperature, humidity, and wind
> components at 1 Hz), and
>
> • sulfur hexafloride detector.

Addition of the new small ice particle probe and the new cryogenic frost
point hygrometer is highly desirable. Instruments for detecting aerosol
and CCN concentration are also desirable.

More detailed descriptions of aircraft instrument characteristics are
given in appendix A.

4.3.3-2 Aircraft Sampling Strategies

Three general principles guided development of sampling strategies for
the aircraft platforms. First, analysis of results from the Phase I
field deployment indicate that *greater* *coordination of aircraft
operations is highly desirable* (e.g., Kinne et al., 1989; Wielicki et
al., 1990) to facilitate the utility of the data obtained in achieving
the science objectives. What is required is that all aircraft operate in
close proximity to the surface sites (and each other), especially over
the Hub site, and that operations be coordinated with overpasses of the
NOAA polar orbiting satellites (or Landsat) to the extent permitted by
operational considerations and cloud conditions. Second, the patterns
flown should be based on the *simplest possible* *configuration* suited
to the mission objectives and platform and analysis capabilities in
order that mission planning be -streamlined and that execution of the
mission proceed in a well-coordinated fashion. In essence, too many
options create confusion which can negatively impact the success of a
mission. Third, *calibration and intercomparison* of radiometric
instrumentation on the aircraft platforms, on satellites, and at the
surface sites will have a high priority and will seek to reference all
measurements to a single standard. In particular, as much commonalty as
possible is desired for radiative flux sensors with respect to
instrument design and bandpass. Besides an increased effort at absolute
calibration (blackbody and integrating sphere calibrations on the
ground), it is important that intercomparisons be performed in the field
over [the actual range of]{.underline} [operating
conditions]{.underline} In this way, the effects temperature and density
(airspeed and altitude) variations on sensor performance can be firmly
established.

[NASA ER-2 and UW C-131 Flight Patterns]{.underline}

For most missions, the remote sensing aircraft will each fly *the same
basic racetrack* *pattern* (fig*.* 5). To the extent permitted by
navigational accuracy, the patterns will be flown over exactly the same
geographical coordinates with the ER-2 at an altitude of 65,000\' and
the C­131 at about 10,000\'. The starting time of each leg will be
coordinated to ensure the greatest coincidence of me data in the
vicinity of the surface remote sensing sites. Adjustments in the length
of the racetrack legs or turn patterns will be used to compensate for
differences in operating airspeeds. In general, the racetrack legs
(level straight lines during which data is obtained) will be *100 to 150
km* in length. Longer legs (up to 500 km in length) may be flown for a
few selected missions. Except for a few special missions, the
approximate midpoint of the racetrack legs will be located *over or just
downwind of the Hub site* (fig. 6) subject, of course, to the continued
presence of clouds over the target area. One of two possible
orientations (aircraft heading) will be selected for a given mission
based upon the science objectives for that mission. In the first, the
racetrack will be oriented *perpendicular to the solar plane.* Thus, the
orientation of successive racetracks will change in time as the solar
position changes. This will permit the cross-track scanning radiometers
on the ER-2 (Daedalus and MCR) to resolve the principal forward and
backward scattering peaks of the cloud bidirectional reflectance
pattern. Knowledge of the bidirectional reflectance of cirrus is
critically needed for satellite-based remote sensing applications.
Knowledge of the cloud bidirectional reflectance also places significant
constraints on possible ice particle scattering phase functions which is
a major source of uncertainty in OUT understanding of radiative transfer
in cirrus and the relationship between cirrus cloud optical and physical
properties. In the second orientation, the flight legs will be *flown
perpendicular to the wind direction at cirrus altitude.* This provides
the most effective mapping of cirrus cloud structure in time and space,
as illustrated in figure 7. Movement of the clouds in time is used to
provide a second horizontal dimension in this Eulerian cloud mapping
mode. Data taken in this manner will be highly useful for characterizing
the cloud scene in comparison to satellite observations, for use in
cirrus cloud development studies, and for characterizing the effects of
cirrus clouds on the surface and atmospheric radiative budgets. It is
expected that the *cross-wind tracks will often be approximately
perpendicular to the solar plane* during some part of a mission since
extended cirrus systems often occur in southwesterly flow at this time
of year (November 13 - December 7) and missions will usually overlap the
time of the afternoon NOAA polar orbiter overpass (-1430 LST).

![](media/image5.wmf){width="6.495833333333334in" height="6.85625in"}

Figure 5 - Basic aircraft flight patterns for FIRE Cirrus IFO-II.

![](media/image6.wmf){width="6.495833333333334in"
height="5.490277777777778in"}

> Figure 6 - Schematic of coordinated aircraft flight patterns for FIRE
> Cirrus IFO-II Eulerian Mission (Type A) - see fig. 7. In situ aircraft
> #1 is the NCAR King Air (fig. 8). In situ aircraft #2 is the NCAR
> Sabreliner. In situ aircraft #3 is TBD.

![](media/image7.wmf){width="6.497222222222222in"
height="6.0368055555555555in"}

Figure 7 - Schematic plan-view of the FIRE Cirrus IFO-II aircraft and
surface observing site sampling patterns for a Eulerian mission (Type A,
fig. 6) transformed into a coordinate system moving with the mean wind
(and clouds) at cirrus altitude. Wind direction is from the top of the
page to the bottom. Thus, observations at the bottom of the illustration
were taken first. The scanning lidar (VIL) provides near-continuous
coverage over the entire span.

[*In situ* Aircraft Flight Patterns]{.underline}

There are three basic flight patterns that will be used for the *in
situ* aircraft (fig. 5). They are:

> • the racetrack pattern,
>
> • the step-down/step-up pattern, and
>
> • the spiral descent.

The racetrack pattern is primarily used for observing the radiative
budgets of cirrus clouds and the radiative impact of the clouds on the
radiative budgets of the atmosphere and surface. This pattern is also
well-suited for characterizing the statistical properties of bulk cloud
radiative properties and horizontal cloud structure and for observing
turbulence characteristics and turbulent fluxes. it can be used to
generate vertical profiles but the number of levels is limited by the
time consumed in making the reverse heading legs at each altitude (the
reverse heading legs provide a means of normalizing for the cosine
response of the flux radiometers).

The step-down/step-up pattern (vertically stacked straight-line legs,
fig. 5) is used primarily for observing vertical profiles of cloud
microphysical properties and atmospheric state parameters such as
temperature and humidity. Useful radiative flux measurements are
obtained if the legs are sufficiently long. The spiral descent at a rate
of about 1 in s-1 is primarily used for making microphysical
measurements (Heymsfield et al., 1990) and is also well-suited for
deriving vertical motions from the observed horizontal winds (Gultepe
and Heymsfield, 1990). However, the accuracy of the radiative flux
measurements may be compromised during such a descent. Similarly,
radiative measurements taken during descent or ascent from one level leg
to the next in the step-down/ step-up pattern will not be useful. The
advantage of the spiral descent versus the step-down/step-up pattern is
that the observed profiles of cloud microphysical properties are more
representative of the conditions present in an atmospheric column at a
particular time because the profiles are obtained in about 15 minutes
rather than the time period of an hour or more required for
step-down/step-up profiling. Each of these pattern may be flown in both
a Eulerian and a Lagrangian manner (e.g., fig. 5). In the Eulerian mode,
the orientation of the racetrack and step-down/step-up legs will be
along the wind direction at citrus altitude. The upwind end of each leg
will generally coincide with the a location just upwind of the Hub (or
R2) remote sensing site. Thus, the flight tracks of the *in situ*
aircraft will cross the flight tracks of the remote sensing aircraft,
often at a right angle. The Eulerian spiral descent will be flown over
the Hub site. The diameter of the spiral is usually about 10 km. Data
obtained from Eulerian sampling patterns are best suited for combination
with satellite and surface-based remote sensing data.

In the Lagrangian mode, the flight legs are also oriented along the
wind. The patterns may be initiated in the vicinity (or upwind) of the
Hub site but the upwind terminus of each

In the Lagrangian mode, the flight legs are also oriented along the
wind. The patterns may be initiated in the vicinity (or upwind) of the
Hub site but the upwind terminus of each successive leg is moved
downwind to account for horizontal advection of the cloud by the wind
(wind drift). Thus, the legs get progressively farther from where the
pattern was initiated. Although the flight legs may remain within the
view of the cross-track scanning instruments on the remote sensing
aircraft, all the flight tracks may not cross the remote sensing flight
tracks. Similarly, coordination with the surface-based observing sites
is necessarily less. The Lagrangian spiral also drifts with the wind but
can generally be confined to a small area close to the Hub site since
each descent takes only about 15 minutes, i.e., the entire pattern can
then be repeated beginning at the initial location. An entire step-down
pattern may take more than an hour and consequently drift some distance
during that time (50 m s-1 wind speeds yield a 180 km displacement in I
hour).

The lengths of flight legs for the racetrack and step-down/step-up
patterns vary depending on the mission objectives, as described below,
and the operating characteristics of the aircraft. While the turboprop
King Air can fly relatively short legs (20 km), it is not practical to
operate jet aircraft in this manner (50 km legs are a minimum).
Moreover, only the King Air is suited to performing the spiral descent
pattern. Based on experiences from IFO-I, the King Air will often
perform two or more of the above patterns in a given mission (below).
This takes advantage of the operational flexibility of this platform and
maximizes the overall applicability of the data obtained. Although this
may at first seem to violate the maxim of simplicity, experience has
shown that a set multi-mode pattern is operationally practical and
yields significant scientific advantage.

4.3.3.3 Aircraft Missions

Approximately fourteen (14) coordinated multi-aircraft missions will be
flown during the 25 days of FIRE Cirrus IFO-II. \[A mission may involve
more than one flight of a given aircraft.\] *Other single-aircraft
missions may be conducted on a non-interference basis at the discretion
of* *the Aircraft Principal Investigator.* Non-interference means not
only that such a mission will not be conducted during a coordinated
mission but that it will not impact the operational capabilities for
conducting a coordinated mission, e.g., on the same or following day.
One of three basic multi-aircraft mission plans will be executed on most
aircraft experiment days. These coordinated missions are designed to
satisfy the sampling and data requirements for achieving most of the
scientific objectives of FIRE Phase II (section 3). Special missions
focused on specific objectives requiring unique sampling strategies or
conditions will also be performed.

[Mission Type A]{.underline}

The Type A mission is essentially a Eulerian mission (figs. 6 and 7). It
is the mission that will be executed for the three large-scale case
studies (intensive rawinsonde launches, section 3.3.4). The primary
objectives of this mission are to characterize the time-dependent cloud
and radiative fields, cirrus cloud radiative and physical properties,
and upper tropospheric water vapor and cloud water budgets within a
fixed mesoscale region. The coordination of aircraft flight patterns and
surface-based remote sensing observations are maximized in this mission.
Obtaining coincident satellite observations, especially by NOAA polar
orbiting satellites, is a high priority in planning and executing this
mission. Thus, besides supporting studies of cirrus cloud development
(science objectives I and 2), this mission will also provide data
well-suited for satellite and surface-based remote sensing studies (4,
6) and for quantifying the radiative impact of the clouds on the TOA,
atmosphere and surface radiation budgets (5). The data will be useful,
but not optimal, for characterizing relationships between cirrus cloud
optical and physical properties (3).

In a Type A Mission, the remote sensing aircraft will fly coordinated
cross-wind racetracks (100 to 150 km in length) with one leg passing
over the Hub site (midpoint). The Sabreliner and King Air will fly
combination along-the-wind flight patterns over the Hub site while the
third *in situ* aircraft will operate in coordination with the remote
sensing aircraft and R2 remote sensing site. The Sabreliner will begin
with a 50-km racetrack just above cloud top followed by a step-down
pattern (50 km legs at 2000\' intervals) to below the operating ceiling
of the King Air (or cloud base). This pattern is then repeated. The King
Air will begin with a Lagrangian spiral (initiated over or upwind of the
Hub site) from its operating ceiling (or cloud top) to cloud base
followed by a 30-km racetrack just below cloud base. A step-up pattern
(20 km legs at 2000\' intervals) to operating ceiling (or cloud top)
will then executed. This pattern is repeated one or more times. At the
conclusion of the mission, an Eulerian spiral is performed over the Hub
site. This King Air flight pattern was successfully executed in IFO-I
and is illustrated in figure 8. The third *in situ* aircraft will
perform 50-km along-the-wind step-up/step­down profiles between cloud
base and cloud top on a parallel heading. The flight track will
intersect the flight track of the remote sensing aircraft and extend
toward or over the R2 remote sensing site.

There is a potential conflict in that the Sabreliner and King Air are
operating in close proximity. It is anticipated that such conflicts can
be readily resolved in a safe manner. In a situation of two or more
distinct cloud layers, the operating altitude ranges will not overlap,
i.e., the King Air will work the lower cirrus layer and the Sabreliner
will work the upper cloud layer(s). In this situation, which will likely
occur during some missions, an additional racetrack pattern must be
flown at cloud top (King Air) and cloud base (Sabreliner) of the
respective cloud layers. The third *in situ* aircraft will work both
cloud layers. Furthermore, a capability for some

![](media/image8.wmf){width="6.495833333333334in"
height="7.502083333333333in"}

> Figure 8 - Schematic of FIRE Cirrus IFO-II Eulerian Mission (Type A)
> flight pattern (altitude vs time) for NCAR King Air *(in situ).* The
> entire pattern is flown over the Hub site. Time permitting, the A-B
> sequence is repeated.

adjustment of aircraft flight levels based on concurrent surface-based
observations (lidar and radar) may be desirable.

[Mission Type B]{.underline}

The Type B Mission is primarily a Lagrangian mission. The primary
objectives for this mission are to characterize the radiative budgets
and broadband radiative properties of individual cirrus clouds in
relationship to the horizontal and vertical structure of the cloud
(science objective 2), to characterize the turbulent structure and
associated vertical transports within the cloud, especially in the cloud
boundary layers where turbulent interactions have greatest consequence
*(2*), and to characterize the microphysical development of the cloud
(2). Thus, this mission is focused on some of the more problematic
aspects of understanding cirrus cloud development in support of
cloud-scale modeling studies and parameterization development (1). The
data will be directly applicable for satellite-based studies (4) and for
characterizing the radiative impacts of cirrus (5). Coordination with
the surface sites is highly desirable but will be necessarily somewhat
limited in the case of *the in situ* platforms. Nonetheless, the data
will provide useful information for improving the utilization of
surface-based remote sensing observations for quantitative studies of
cirrus clouds (6).

In a Type B Mission, the remote sensing aircraft will operate in the
same manner as in a Type A mission, i.e., cross-wind racetracks over the
Hub site. The *in situ* aircraft will conduct along-the-wind Lagrangian
flight patterns (drifting with the wind) on specific advecting cloud
targets.

The Sabreliner will fly 70 to 100-km racetracks just above cloud top,
and at 2000\' and 4000\' below cloud top. The King Air will fly a
Lagrangian spiral from its operating ceiling (or cloud top) to cloud
base followed by 70 to 100-km racetracks just below cloud base and at
2000\' and 4000\' above cloud base. These patterns should cross the
flight track of the remote sensing aircraft and overfly one of the
remote sensing sites during portions of the mission. Ideally, the
Lagrangian spiral would be performed over the Hub site. In this mission,
the Sabreliner and King Air may work on the same cloud target or in an
independent fashion on separate targets. The third *in situ* aircraft
will perform along-the-wind Lagrangian step-down/step-up microphysical
profiling below the Sabreliner (to cloud base) or above the King Air (to
cloud top) when they are working separate targets or over the Hub or R2
remote sensing sites (cloud top to cloud base) when they are working the
same target.

[Mission Type C]{.underline}

The Type C Mission is strongly focused on two important objectives. It
provides observations essential for quantifying the capabilities and
limitations of satellite-based cirrus cloud retrievals (science
objective 4) and for characterizing relationships between cirrus cloud
optical and physical properties (3). At least three such missions are
planned. Two will be conducted over the Gulf of Mexico providing a
uniform and weakly reflecting background to simplify analysis of the
radiative observations. The missions will be performed during overpass
of Landsat and/or one of the NOAA polar orbiting satellites. Two
aircraft will be deployed for these missions, the ER-2 and one *in situ*
platform. The Sabreliner is best suited in terms of instrumentation,
specifically the small ice particle probe, ice crystal collector
(crystal habit), and scanning multispectral radiometer. The third *in
situ* platform might be better suited in terms of range and altitude
capability but the potential lack of the small ice particle probe is
very limiting. The ER­2 will fly racetrack patterns perpendicular to the
solar plane. The *in situ* platform will fly a 50 ­km step-down/step-up
vertical profiling pattern. Although a flight track directly under and
along the ER-2 flight track has strong conceptual appeal, a number of
factors make this choice suspect. Absolute coincidence is difficult to
achieve because of uncertainty in navigation and differences in aircraft
flight speeds, Also, when the flight track i, across-the-wind (as will
probably be the case), the presence of even moderate vertical wind shear
greatly complicates the analysis in that it is difficult to establish
whether data taken at one level is representative of conditions when
another level is being sampled, especially if the clouds are deep and
horizontally inhomogeneous as is often the case for cirrus. Thus, an
along-the-wind, Lagrangian crossing pattern is more suitable with
respect to providing coincident measurements of a target region within
the view of the cross-track scanning ER-2 instruments.

The third satellite remote sensing mission will be a nighttime mission
involving both remote sensing aircraft and one or more of the *in situ*
platforms. It will be conducted in conjunction with the nighttime
overpass of the NOAA polar orbiter (-0230 LST). The purpose is to
provide a severe test case for satellite cirrus cloud retrieval
techniques in which the surface and atmospheric boundary layer present a
cold background (nocturnal cooling). This situation is significantly
more challenging than simply considering only infrared channels during
the day when the background contrast is much higher. Since it is at
night, the sampling patterns will be oriented with respect to the wind
at cirrus level and the mission will be conducted the same as a Type B
mission. If feasible, more than one such mission may be flown.

[Special Missions]{.underline}

> Four special missions are planned.
>
> \(1\) The first is special only in that it is a clear sky mission in
> support of SPECTRE (section 6. 1). Due to its radiometric
> instrumentation (especially the HIS), the ER­2 is required. One of the
> *in situ* platforms is also desired. The Sabreliner is best suited for
> this mission given its higher operating ceiling and, thus, the
> capability to observe upper tropospheric moisture content as well as
> radiative fluxes and radiance fields. The mission would be conducted
> in essentially the same manner as a Type A mission with the *in situ*
> aircraft profiling the upper troposphere just downwind of the Hub site
> (Eulerian along-the-wind racetracks) and the ER-2 flying crosswind
> racetracks just *downwind* of the Hub site. At least one such mission
> will be flown.
>
> \(2\) The second special mission is a low level surface albedo mapping
> mission. The King Air is best suited for this mission in which a
> grid-pattern must be flown at low altitude within the boundary layer
> (as low as feasible). This could be conducted in conjunction with the
> SPECTRE mission. An optically thin cirrus cloud cover would also be
> acceptable. One such mission would be flown. In the event of that snow
> occurs, a second mission over the snow-covered surface is also
> required. Such a mission would not necessarily preclude conduct of
> other missions using the King Air since the airtime required for this
> mission is less than two hours.
>
> \(3\) The third special mission is designed to investigate potential
> improvements in our ability to utilize data from the 6.7 μm water
> vapor channel on the GOES and the three water vapor HIRS-2 channels on
> the operational polar orbiter for quantitatively characterizing upper
> tropospheric water vapor fields. Imagery from the GOES 6.7 μm channel
> has long proven very useful to forecasters but has proven very
> difficult to quantify and incorporate into operational or research
> models. In essence, the detected radiances depend on the temperature
> of the upper 2 mm of precipitable water vapor (McGuirk et al., 1987;
> Blackwell et al., 1988). Thus, the signal does not depend on moisture
> alone. In addition, clouds also affect the detected radiances. Similar
> problems have precluded use of HIRS water vapor measurements in
> operational forecast models due to the resulting uncertainties. The
> absence of any accurate independent observations of upper tropospheric
> water vapor has precluded attempts to unravel the different effects.
> The new capability for making accurate *in situ* water vapor
> measurements in the upper troposphere (airborne cryogenic frost point
> hygrometer) and, potentially, upper tropospheric water vapor
> measurements from the surface (Raman lidar) provides, for the first
> time, a means to investigate the applicability of satellite
> observations for quantitatively characterizing the three dimensional
> structure of upper tropospheric water vapor.
>
> The Sabreliner is best suited for this mission. The flight pattern
> will consist of 300-km legs flown at three levels (Lagrangian
> step-down/step-up) either up or down the anticipated gradient of water
> vapor under generally clear conditions. The wind drift should be taken
> as the mean 400 mb level wind. The nominal altitude of the legs will
> be at 17,000\', 23,000\', and 30,000\'. The flight tracks should
> extend over the surface Raman lidar (Hub site) and the mission should
> coincide with release of rawinsondes at NWS and the special CLASS
> stations (section 4.3,4) and GOES observations (half-hourly) using the
> 6.7 μm channel (section 4.3.1). Coincidence with an overpass of the
> NOAA polar orbiting satellite is essential. Maximizing the utility of
> the Raman lidar data would require a nighttime mission, e.g., 0230
> NOAA overpass. The potential value of the mission to the objectives of
> FIRE (section 3.2) and more generally to the issues of observation and
> prediction of upper tropospheric cloudiness and climate change dictate
> that at least two such missions be flown. One could be conducted in
> conjunction with the SPECTRE mission described above.
>
> \(4\) The fourth special mission is designed to investigate convective
> scale motions in cirrus clouds using in-cloud release of chaff and an
> inert chemical tracer (SF6). These missions will be conducted using
> the King Air over the Hub site. The chaff will be tracked by surface
> radar and this experiment could easily be conducted in conjunction
> with multi-aircraft missions. It is anticipated that, due to the
> relatively long station time of the King Air, this experiment could be
> performed at the conclusion of a multi-aircraft mission.
>
> The *in situ* sampling required to track the chemical tracer is less
> compatible with the conduct of a highly coordinated multi-aircraft
> mission. The Citation or some other aircraft with appropriate
> capabilities would serve as the second aircraft. However, two aircraft
> are required (release and detection). There will be ample
> opportunities for independent aircraft missions involving the King Air
> during Cirrus IFO-II, as occurred during IFO-I, since this platform is
> not involved in Type C missions.
>
> An estimate of the number of planned aircraft missions is given in
> table 4.

4.3.4 Rawinsonde and Profiler Observations

Acquisition of rawinsonde and other surface-based remote sounding data
will be increased substantially for FIRE Cirrus IFO-II in comparison to
IFO-I. These data are absolutely crucial to the success of efforts to
model cirrus cloud development on the large scale (science objective 1,
section 3) and to characterize cirrus cloud development at smaller
scales using detailed cloud models and analysis (I and 2). The data will
be used to define atmospheric conditions on scales ranging from the
synoptic scale to the scale of individual mesoscale cloud features.
Moreover, accurate knowledge of atmospheric temperature structure is
essential for analysis and modeling of infrared radiative transfer in
cirrus clouds (3, 4 and 6), for inferring

> Table 4 - Summary of planned aircraft mission allocations for the FIRE
> Cirrus IFO-II. More than one flight (SL, KA) may be involved in a
> given mission.

  ------------- --------------- --------- -------- --------------- --------- --------
  **Mission**   **Type**                           **Aircraft/**             

                                                   **No. of                  
                                                   Missions**                

                                ER-2      C-131    SL              KA        IS#3

  A             Eulerian        5         5        5               5         5

  B             Lagrangian      5         5        5               5         5

  C             Gulf of Mexico  2                  2                         (or 2)

                Night time      1         1        1               and/or 1  

  Special       SPECTRE/SRB     1                  1               1         

                Water Vapor                        2                         

                Chaff/SF6                                          4         

                TOTAL           14        11       16              16        10
  ------------- --------------- --------- -------- --------------- --------- --------

cirrus cloud properties from satellite observations (4), for assessing
the effects of cirrus clouds on the surface and TOA radiation budgets
(5), for analysis and modeling of atmospheric radiative transfer (5),
and for analysis of surface-based remote sensing observations of cirrus
clouds (6).

Enhancements to routine NWS rawinsonde operations are required over a
large area encompassing the most of the continental United States (fig.
9), excluding only the eastern region, in support of the large-scale
modeling efforts. *Three intensive case studies are planned* *at this
scale. Each case will be 36 hours in duration.* The data will be used
directly for model initialization and verification and as input to
model-based FDDA analysis studies, and indirectly for deriving cloud
fields from satellite observations over a wide area (model
verification). For these cases, 6-hourly data are required over the
large area (Type A and Type B stations; table 5 and appendix B) while
3-hourly soundings are required over a smaller region (Type A stations
only) encompassing the south-central U.S. during the last 12 hours
(i.e., 6-hourly for the first 24 hours). Participation of additional
stations (13 more Type B stations) along the northern tier of the U.S.
(8) and in northern Mexico (5) is highly desirable (fig. 9).
Understanding the development of cirrus clouds and associated dynamic
and thermodynamic structure requires enhanced temporal coverage over a
large area given the wind speeds prevalent at cirrus altitude
(sub-tropical jet stream), especially for considering the nature and
characteristics of mesoscale dynamic disturbances at cirrus altitude
(e.g., Starr and Wylie, 1990; Stevens and Ciesielski, 1988). In
addition, the lack of moisture information at levels above the -40°C
isotherm may be compensated by additional temporal coverage in a FDDA
analysis.

NWS rawinsonde stations (Type A stations) will also operate in an
enhanced mode (a supplemental mid-day sounding at 1800 UTC) over the
south-central U.S. (fig. 9) on most other operational days (aircraft
operations) during the experiment (a total of 10 days), These data will
be used to characterize cirrus cloud development and to analyze cirrus
cloud conditions over the region.

Data from the NWS operational wind profiler network (400 MHz), which is
presently under construction in the central U.S., will also be acquired
(fig. 10). The availability of these observations and their potential
impact on efforts to analyze and model cirrus cloud development was i
prime factor in determining the location and timing of the experiment.
These data will enable greatly increased resolution of the dynamic
structure of the upper troposphere, especially at the mesoscale, both
through increased spatial resolution (the inner ring) and increased
temporal resolution (-10 min.). Starr and Wylie (1990) have shown the
potential benefit of increased temporal resolution in the analysis of
cirrus cloud development (see also Westphal and Toon, 1990; Nicholls et
al., 1990). The inner ring of stations will be fully operational well
before the field campaign and it is anticipated that all stations will
be completed by that time. The inner ring and the stations to the west
of the FIRE operations area are most critical and these have been
targeted for first completion by the NWS. Incorporation of these data
into operational (and FIRE) analyses should be straightforward. However,
this has not been demonstrated. As a result, FIRE has initiated a pilot
project using a three-site network to gain experience in preparation for
Cirrus IFO-II (section 5.5).

FIRE will also field up to three additional wind profilers for IFO-II
(fig. 4, section 4.3.2.1). A 50 MHz system (NOAA/WPL) will be deployed
at the Hub site and will also serve as a RASS. A 5-beam 400 MHz system
(CSU) will be deployed at the R2 remote sensing site. An additional 50
MHz system (PSU) will also be deployed. Together with the operational
NWS profiler at the R3 site, these data will serve to characterize upper
tropospheric dynamic structure at a scale corresponding to smaller scale
cirrus cloud features such as longitudinal jet stream hands.

**FIRE PHASE II**

**CIRRUS INTENSIVE FIELD OBSERVATIONS**

**NOAA-NWS RAWINSONDE NETWORK**

![](media/image9.wmf){width="6.595138888888889in"
height="4.817361111111111in"}

Figure 9 -National Weather Service rawinsonde stations coded to indicate
their level of participation in the FIRE Cirrus IFO-II. See table 5.

> Table 5. Modes of operation of NWS rawinsonde stations in support of
> Cirrus IFO-II. All times are UTC. (See fig. 9).

  ----------------- ------------- ---------------------------------------
  **Modes of        **Station**   **Operations**
  Operation**                     

                                  

  Routine:          All Stations  • no change in usual NWS operations

                                  • 9 days

  Enhanced:         Type A        • a supplemental sonde at 1800 UTC
                    Stations      

                                  • 15 NWS stations

                                  • 10 days

  Intensive:        Type A        • 4 routine sondes (1200, 0000, 1200,
                    stations      0000)

                                  • 5 supplemental sondes

                                  (1900,0600,1500,1800,2100)

                                  • 15 NWS stations

                                  • 3 events (36-hour duration over 2
                                  days)

                    Type B        • 4 routine sondes (1200, 0000, 1200,
                    stations      0000)

                                  • 3 supplemental sondes (1800, 0600,
                                  1800)

                                  • 28 NWS stations

                                  • 3 events (36-hour duration over 2
                                  days)
  ----------------- ------------- ---------------------------------------

**FIRE PHASE II**

**CIRRUS INTENSIVE FIELD OBSERVATIONS**

**NOAA-NWS WIND PROFILER SITES**

![](media/image10.wmf){width="6.595833333333333in"
height="4.735416666666667in"}

Figure 10 - Planned National Weather Service wind profiler sites.

FIRE will also field five CLASS-type rawinsonde stations (table 6). Four
of these systems will be deployed in an arrangement designed to
facilitate combination with NWS and FIRE wind profiler data to enable a
complete analysis of the dynamic and thermodynamic development over
triangular areas defined by four NWS wind profiler stations (fig. 4).
These data support the application of small-scale cloud models used to
develop cirrus parameterizations for large-scale models. The CLASS-type
systems make measurements of atmospheric water vapor content to a higher
level than standard NWS sondes. Although the upper limit is not well
established, the observations are useful to at least the -50°C level
(Starr and Wylie, 1990), which is more than I km higher than for NWS
sondes (to -40°C). This is often sufficient to include the lower portion
of cirrus clouds. The rawinsonde observations will also directly support
analysis and modeling of radiative processes and remote sensing
observations (satellite, airborne and ground-based). These studies are
highly focused in the area of the remote sounding sites where the CLASS
sites will be located. These four stations follow the same launch
schedules as the Type A NWS rawinsonde stations. However, the station at
the Hub site will also provide on demand support for remote sensing
activities at the request of the mission or lead scientist
(non­interference basis).

The fifth CLASS-type rawinsonde station will also be located at the Hub
site to support the SRB activities. This station will provide routine
Soundings on all days during overpasses (ascending and descending
orbits) of the two operational NOAA polar orbiting satellites (4
soundings per day).

Other systems include a Raman lidar water vapor sounder, a
radio-acoustic virtual temperature sounder, and a dual-channel microwave
water vapor sounder at the Hub site (section 4.3.2.1). A Dobson
instrument (total ozone) will be operated at the Hub site where a
limited number of ozone sondes will be launched (section 4.3.2.2). In
addition, ice particle replicator sondes will be launched from that
location. A surface observing station, a meteorological tower, and a
tethered balloon system will also be deployed there to monitor
meteorological conditions in the boundary layer (section 4.3.2.2).

4.4 Operational Forecasting

A capability to adequately forecast Cloud and weather conditions for
operational planning and execution is absolutely essential. Two
operational decisions involve appreciable lead times and are
particularly forecast sensitive. The NASA ER-2, that will base in San
Antonio, requires a two-hour lead time to prepare the pilot. This and
the transit time from San Antonio to the FIRE operations area result in
a need to schedule take-off more than four hours in advance. Once
prepared, take-off can be delayed but delays cannot be excessive. Thus,
final decisions to

> Table 6 - Modes of operation of CLASS-type rawinsonde stations in
> support of FIRE Phase II Cirrus Intensive Field Observations. All
> times are UTC. There will be two (2) stations located at the Hub site.
> One will perform the routine launches while the other will conduct the
> enhanced, intensive, and special launches. There will be three (3)
> additional CLASS-type stations conducting enhanced and intensive
> soundings (see fig. 4).

+-------------------+--------------------------------------------------+
| **Modes of        | **Operations**                                   |
| Operation**       |                                                  |
+-------------------+--------------------------------------------------+
|                   |                                                  |
+-------------------+--------------------------------------------------+
| Routine:          | • 4 sondes per day (at times of NOAA polar       |
|                   | orbiter overpass)                                |
|                   |                                                  |
|                   | • every day                                      |
+-------------------+--------------------------------------------------+
|                   | • 1 station at Hub site                          |
+-------------------+--------------------------------------------------+
| Enhanced:         | • 3 sondes per day (1200, 1800 and 0000 UTC)     |
+-------------------+--------------------------------------------------+
|                   | • 10 days                                        |
+-------------------+--------------------------------------------------+
|                   | • 4 Stations                                     |
+-------------------+--------------------------------------------------+
| Intensive:        | • 10 sondes per event                            |
|                   | (12                                              |
|                   | 00,1800,0000,0600,0900,1200,1500,1800,2100,0000) |
|                   |                                                  |
|                   | • 3 events (6-hour duration)                     |
+-------------------+--------------------------------------------------+
|                   | • 4 stations                                     |
+-------------------+--------------------------------------------------+
| Special:          | • 20 to 30 additional on-demand launches         |
+-------------------+--------------------------------------------------+
|                   | • 1 station (Hub site)                           |
+-------------------+--------------------------------------------------+
|                   | • non-interference with 'enhanced\' or           |
|                   | 'intensive\' launches                            |
+-------------------+--------------------------------------------------+

commit the ER-2 to flight require a firm *4 to 10-hour forecast.* There
are also requirements with respect to surface weather conditions
expected for landing (a forecast for excessive winds will cause a
mission to be scrubbed). The expectation of more favorable surface wind
conditions was a prime reason for basing the ER-2 in San Antonio rather
than Topeka, This latter forecast decision will be the responsibility of
flight support personnel in San Antonio and not FIRE.

The second forecast-sensitive decision is activating the large-scale NWS
rawinsonde network for intensive operations (section 4.3.4) in support
of each of three planned large-scale experiment events. A commitment to
launch the supplemental 1800 UTC midday sondes (43 stations) would be
required by about 1200 UTC that morning. A commitment for the late-night
0600 UTC launch would be needed by early afternoon (-1800 UTC). A
commitment for full intensive mode launches (3-hourly from 1200 UTC to
0000 UTC at 15 stations and a supplemental 1800 LTC launch at 28
stations) would be required before 0000 LTC. Obviously, operations could
be cancelled in mid-event if conditions do not proceed as forecast.
However, this is highly undesirable as limited resources would be
depleted. Thus, a good *36-hour forecast* of expected conditions,
particularly cirrus cloudiness and surface weather, is crucial.
Experience from IFO-I shows that this is a very challenging, but
achievable, requirement. A conservative forecasting approach and
selection of a large-scale target system, such as extended cirrus
associated with the subtropical jet stream, are prudent given that there
are 25 experiment days in which to execute. these three experiments.

In addition, forecasting support is also necessary for daily operations.
A good *12 to 24* *hour forecast* is needed each evening to plan
operations for the next day. This is also important for scheduling
maintenance and calibration activities and ensuring high performance of
project personnel over the entire experiment given the limited personnel
(some down days will be required). A longer range outlook is also
essential, since conduct of a mission may impact the ability to conduct
another mission of the following day (some missions over the Gulf of
Mexico and at least one night mission are planned). Except for the ER-2
(see above), the research aircraft are generally quite flexible.
Operations may be delayed until conditions improve with little real
consequence. Thus, a firm 0 to 12-hour forecast at 1200 UTC will
generally be adequate. This will also be satisfactory for committing to
enhanced NWS rawinsonde launches (a supplemental 1800 UTC sonde at 15
stations on -10 days).

FIRE will be responsible for providing forecasting support. Access to
the following information is required for this purpose:

> • NWS rawinsonde observations,
>
> • NMC analysis products (weather maps),
>
> • NMC forecast-model output data base (including all upper
> tropospheric levels), and
>
> • GOES imagery (full resolution visible, infrared and water vapor
> channels).

The satellite imagery must be available in video time-sequence format
and all information must be current. A dedicated priority link to the
Man-interactive Data Acquisition System at the University of Wisconsin -
Madison will provide near-real-time access to this information in a
highly useable format. In the event of system crash or hardware failure
(including the communication link) an alternate source is required.
Access to a NWS Forecast Office (with AFOS) would satisfy this need and
was found generally useful during IFO-I. Thus, locating the Mission
Control Center near a WSFO or WSO (Topeka or Wichita, respectively) is
very desirable.

The field observations also provide an important source of information
for short-term forecasting as is often required just prior to a mission.
Near-reassume access to the following data is highly desirable:

> • lidar observations (especially the scanning VIL lidar),
>
> • rawinsonde soundings (CLASS),
>
> • Raman lidar observations of mid- and upper tropospheric water vapor,
>
> • wind profiler observations.

A capability to electronically access and display this information would
be ideal. A hardcopy capability is also desirable. Some means to
transmit information, especially satellite and lidar images to aircrews
prior to take-off and also *during* flight Would enhance operations.

> 5.0 EXTENDED TIME OBSERVATIONS
>
> 5.1 Satellite Validation Studies
>
> An objective of FIRE Phase II is to explicitly quantify the
> capabilities and limitations of methods used to derive the physical
> and optical properties of cirrus clouds from satellite observations,
> especially the ISCCP algorithm (science objective 4, section 3). The
> strategy is to rigorously test the assumptions (models) upon which
> algorithms air based (section 3.4). While the intensive field
> observations (FIRE Cirrus IFO-II, section 3) will provide data to
> conduct a most detailed examination of the cloud-radiative models that
> are invoked, the number of cases and types of situations observed
> during IFO-II will be necessarily limited. Thus, the
> representativeness of those observations and findings needs to be
> established, especially given the intended application to global
> studies. Data sets representative of a wider range of geographical,
> seasonal and synoptic conditions (i.e., greater time and space
> coverage) are needed. In general, a larger and more statistically
> significant data base is required for investigating the performance of
> satellite cirrus cloud retrieval algorithms under diverse situations.
> Two approaches will be employed.
>
> First, coincident \'ground truth\' and satellite observations will be
> collected and analyzed over an extended time period for a number of
> sites in the continental United States. Surface sites with up-looking
> lidar and narrow-beam infrared window (11 μm) radiometers, and
> ancillary observations (rawinsonde soundings) will be *used to observe
> the cloud height, effective radiative* temperature (or height),
> infrared emittance, and visible optical depth of cirrus clouds at the
> overpass times of the NOAA polar orbiting satellites. In combination
> with cirrus cloud properties derived from coincident observations by
> the operational NOAA polar orbiter and geosynchronous (GOES)
> satellites, these observations will be used to:
>
> • establish the limits of cirrus delectability from satellites,
>
> • quantify the limitations and applicability of cirrus cloud heights
> and visible optical depths inferred front satellite observations, and
>
> • determine the relationship between cirrus cloud visible optical
> depth and infrared emittance.
>
> Five potential lidar sites are: Salt Lake City, UT; Madison, WI;
> Hampton, VA; Ft. Collins, CO; and Boulder, CO. Most of these sites are
> already participating in ECLIPS (section 6.2) which is a program to
> monitor the temporal variations of cloud base height for all cloud
> types. At two of these sites (Salt Lake City and Madison),
> extended-time remote sensing investigations of cirrus clouds are
> presently ongoing (sections 5.3 and 5.4, respectively) as part of
> FIRE.

Second, higher spatial, temporal, and spectral resolution satellite
observations are needed to test the capabilities of the coarser
resolution data used for global climate studies such as ISCCP.
Additional Landsat scenes will he acquired, These 30-meter resolution
data will be used to investigate the effects of sensor spatial
resolution and horizontal cloud inhomogeneities on retrieved cirrus
cloud properties (Parker and Wielicki, 1989). Multispectral NOAA/HIRS or
GOES\[VAS data (sounding channels) can be used to estimate Cirrus cloud
heights even for optically thin cirrus (Wylie and Menzel, 1989).
Alternatively, NOAA/AVHRR observations at 3.7 μm can be used in
combination with the visible and infrared window channel data to select
appropriate \'cloud models\' for the retrievals (Arking et al., 1989).
Estimates by these and other methods, including those using new Cloud
models derived from IFO-I and IFO-II observations (e.g., new cirrus
bidirectional reflectance or spectral emittance models), will be
compared and evaluated using the lidar \'ground truth\' observations
(above). This will provide a basis for meaningful comparisons to ISCCP
analyses over a wide range of geographical locations, seasons, and
meteorological situations, including tropical environments.

Potential satellite data sources are the same as given in section 4.3.1
(table 1). However, since the satellite data requirements for these
studies are generally episodic in space and time and specific to
individual investigations, ETO satellite data will be obtained only on
an as-needed basis from the operational NOAA and NASA archives.

5.2 Comparisons of GCM and Satellite-Derived Cirrus Cloud Statistics

One of the essential uses of satellite data is to test the capability of
global circulation models to accurately produce the climatological
distribution of clouds in the Earth\'s atmosphere. ISCCP (Rossow et al.,
1985) provides a global data set of cloud amount, height, and optical
depth designed to be useful for ISCCP tests. Other climatological data
sets, that are more limited in time and space (Wylie and Menzel, 1989),
ire focused more specifically on cirrus cloud systems and provide
another tool for GCM comparisons. More advanced analysis algorithms
(multispectral) can be used in generating these smaller data sets.
Several of these more limited satellite-based cirrus cloud climatology
data sets will be produced by FIRE research. Comparisons of these and
the ISCCP analyzed cirrus cloud statistics to cirrus cloud statistics
produced by GCMs will be conducted in FIRE Phase II.

An alternative use of satellite-derived cloud data is for investigating
dependencies of cirrus cloudiness and cloud properties on the
large-scale fields resolved by GCMs. Studies are currently underway to
investigate potential relationships between satellite-observed cirrus
and the large scale dynamic and thermodynamic fields analyzed for
initialization of operational forecast models.

5.3 The University of Utah ETO Project

Cirrus cloud research based at the University of Utah Facility for
Atmospheric Remote Sensing (FARS) is designed to support satellite
validation objectives of FIRE Phase II (section 5.1) by providing
routine extended time observations (ETO) of cirrus clouds on a 10
\'primary\' days per month basis at and around the times of NOAA polar
orbiting satellite overpasses and GOES satellite observations. In this
way, a much greater number of cirrus cloud/satellite observation
coincidences are obtained than possible during intensive field
observation campaigns (IFO-I or IFO-II). Routine lidar observations are
used to provide a direct measure of cirrus cloud base height and often
cloud top height, as well as derived estimates of visible cloud optical
thickness, These observations of cirrus cloud properties are compared to
satellite-derived cloud properties and provide a data base encompassing
a broad range of meteorological and surface conditions.

The cirrus cloud observations are also used to enhance basic knowledge
of the physical and microphysical properties of cirrus clouds and the
characteristic cloud structures and generating mechanisms (Sassen,
1989). Observational periods are typically two to four hours in duration
but may be extended to six hours or more on some occasions. In addition
to regularly scheduled observations (\~10 per month), observations
bracketing satellite overpass times are often collected on other days
when cirrus ,ire present. Special attention is given to the
identification of supercooled liquid water regions (Sassen et al., 1989)
within or associated with cirrus, and to the occurrence of oriented
planar ice crystals in cirrus which creates a highly anisotropic
scattering media. These data enable statistical characterization of
cirrus cloud properties, such as areal coverage, thickness, and
structure/morphology as functions of season, synoptic conditions, and
Cirrus Cloud type. The findings from this basic research component of
FIRE Phase II will be applied in generating comprehensive cirrus cloud
climatologies and will aid in cirrus cloud modeling studies ranging
front the microscale to the regional scale (mesoscale).

In addition to initiating collection of the satellite validation data
base observations, ETO activities at FARS during FIRE Phase I focused
mainly on the visible light scattering properties of cirrus using dual
polarization lidar (ruby), net flux radiometers, and all-sky
photography. For Phase II, a broader range of passive remote sensing
measurements will be obtained (table 7). A narrow-beam, infrared window
radiometer will be co-aligned with the lidar system. These observations
together with ancillary rawinsonde observations will be used to derive
the infrared emittance of the cirrus clouds (the LIRAD method, Platt,
1979; Platt and Dilley, 1981). Observations using a greatly expanded
complement of solar radiometers and the infrared observations will serve
to quantify the radiative effects of cirrus clouds on the surface energy
budget. Coincident observations using a Ka-band (0.86 mm) radar will
also be obtained on

> Table 7 - University of Utah Facility for Atmospheric Remote Sensing
> instrumentation for extended time cirrus cloud observations in FIRE
> Phase II.

+------------------+---------------------------------------------------+
|                  | **Facility for Atmospheric Remote Sensing         |
|                  | (FARS)**                                          |
|                  |                                                   |
|                  | **University of Utah**                            |
+------------------+---------------------------------------------------+
|                  |                                                   |
+------------------+---------------------------------------------------+
| Active Remote    | Dual Polarization Lidar (0.694 μm)                |
| Sensors:         |                                                   |
+------------------+---------------------------------------------------+
|                  | Ka-band radar (0.86 m)                            |
+------------------+---------------------------------------------------+
| Passive Remote   | PRT-5 narrow-beam infrared radiometer (9.5 - 11.5 |
| Sensors:         | μm)                                               |
|                  |                                                   |
|                  | Precision Infrared Radiometer                     |
|                  |                                                   |
|                  | Broadband Solar Flux Radiometer                   |
|                  |                                                   |
|                  | Broadband Near-Infrared Solar Flux Radiometer     |
|                  | Solar-tracking Pyrheliometer                      |
+------------------+---------------------------------------------------+
|                  | Direct and Diffuse Narrowband Visible Flux        |
|                  | Radiometer                                        |
|                  |                                                   |
|                  | All-sky VCR Imaging System                        |
|                  |                                                   |
|                  | All-sky photography (35 mm[)]{.underline}         |
+------------------+---------------------------------------------------+

selected occasions. These observations will be used to advance our basic
knowledge of cirrus clouds and to explore the potential benefits of
multiple wavelength observations *for investigating* cirrus (e.g.,
Sassen et al., 1999) as discussed in section 4.2.

5.4 University of Wisconsin Cirrus Remote Sensing Pilot Experiment

The University of Wisconsin at Madison began an extended time study of
the optical and physical properties of cirrus clouds in November 1989.
The experiment lasted approximately one month (Grund et al., 1990). An
additional experiment is planned for the spring of 1990. The objectives
of this pilot experiment are:

> • to characterize cirrus Cloud structure, including spatial
> inhomogeneities and vertical layering;
>
> • to characterize the relationship between cirrus cloud visible
> optical depth and infrared emittance; and
>
> • to provide validation data sets for satellite-retrieved cirrus cloud
> properties, including areal coverage, cloud height, and cloud optical
> properties.

Instrumentation for the pilot experiments are listed in table S. The VIL
scanning lidar produces near-continuous 100-km wide cross-sections of
cirrus cloud backscatter at a 100 m resolution. Time sequences of these
data are used to reconstruct a three-dimensional images of cirrus cloud
fields. The data are used to determine cloud height and structure over
the region. The vertically pointing HSRL provides calibrated
measurements of the vertical profile of cirrus backscatter cross
section, optical depth, and backscatter phase function at a wavelength
of 0.532 μm within a 100-m field of view. Two vertically pointing high
spectral resolution (better than 1 cm-1) infrared interferometer
spectrometers are used to observe downwelling spectra over the spectral
region from 3.5 to 20 pm. One is the same instrument that will be flown
on the ER-2 in Cirrus IFO-II (section 4.3.3) and was also flown in IFO-I
(down-looking), The second is a compact model that is being developed
for deployment at the surface sites (Hub and R2, section 4.3.2) in
IFO-II. These observations provide estimates of the variability of the
infrared spectral emittance of cirrus clouds (Ackerman et al., 1990).
The field of view is about 100 meters at cirrus altitude. Satellite
observations are collected from all channels on the operational NOAA
polar orbiting and geosynchronous satellites, including the
multispectral infrared observations used for temperature and moisture
sounding (HIRS and VAS). The satellite observations are contained within
the sampling domain of the scanning lidar system. A CLASS rawinsonde
system is used to observe the vertical structure of pressure,
temperature, humidity, and winds for use in processing the data (3-D
lidar displays), and analyzing the infrared observations (HIS and
satellite). Routine meteorological observations are also collected.

> Table 8 - Instrumentation and satellite observations for the
> University of Wisconsin at Madison Cirrus Remote Sensing Pilot
> Experiments

  --------------------------------------- -------------------------------
  **Cirrus Remote Sensing Pilot           
  Experiment**                            

  **University of Wisconsin at Madison**  

  Volume Imaging Lidar (VIL)              scanning lidar

  High Spectral Resolution Lidar (HSRL)   zenith pointing

  High Spectral Resolution Interferometer 3.5 to 20 μm
  Sounder (HIS)                           

  NCAR CLASS rawinsonde system            

  Digital All-sky Imaging System          VCR and 35 mm display

  NOAA- 10 and -1 I Satellite             AVHRR/HRPT (and GAC) HIRS/2
  Observations                            

  GOES-7 Satellite Observations           VISSR VAS
  --------------------------------------- -------------------------------

Besides addressing the experiment objectives (above), analysis and
integration of this diverse suite of simultaneous remote sensing
observation will provide needed experience in use of these data and
facilitate similar analysis of Cirrus IFO-II observations. The potential
gain in total information content of the combined observations
(synergism) will be explored.

5.5 Pennsylvania State University Cirrus Remote Sensing Pilot Experiment

One of the key questions to be addressed in the context of FIRE Phase U
is me relationship between synoptic and mesoscale circulations and
cirrus formation and maintenance. Recognizing the importance of defining
the regional circulation, the FST chose to locate the planned experiment
in the immediate vicinity of the proposed NWS wind profiler network
(figure 10). This network will provide hourly\--average horizontal wind
speeds and directions. In addition to the NWS network, two or three
additional profilers will be deployed for the experiment (sections
4.3.2.1 and 4,3.4).

A crucial variable to be obtained from the wind profilers is the
vertical velocity (section 3.2.3). While it is possible to measure
updraft vertical velocities on the convective scale, direct measurement
of the vertical velocity on the mesoscale is extremely difficult due to
the typically small magnitudes on the order of a few cm per second.
Alternatively, vertical velocities can be deduced from a network of
profilers by computing the divergence of the horizontal wind, and then
computing the vertical velocity from continuity. This latter approach
has been demonstrated successfully for a few case studies in Colorado
and Pennsylvania using arrays of three profilers, but has not been
thoroughly examined, particularly for cirrus conditions.

From the perspective of cloud studies, measurements of the vertical
velocity alone are inadequate. Cloud occurrence and cloud radiative
properties such as optical depth must also be measured. These cloud
parameters can then be correlated with mesoscale vertical velocity and
average atmospheric thermodynamic structure, leading to quantitative
understanding of the cirrus formation and maintenance problem.

Based on these concepts, the Pennsylvania State University has begun a
2-year pilot program designed to address the following objectives:

> \- develop appropriate diagnostic techniques and software to compute
> mesoscale vertical velocity profiles from wind profiler arrays.
>
> \- investigate the relationship between this average mesoscale
> vertical velocity, the average atmospheric thermodynamic state, and
> cirrus bulk properties.
>
> \- evaluate techniques for effective use of wind profiler data during
> Cirrus IFO-II, including analysis of the profiler data itself and how
> it can be used to maximum advantage with aircraft and other
> ground-based remote sensing instrumentation.

Preliminary data was acquired during the 1989-90 winter using Penn
State\'s three 50 Hz wind profilers. The prowlers were deployed in
central Pennsylvania at the vertices of an approximately equilateral
triangle 140 km on a side. Hourly-average wind profiles were recorded
continuously for a 3-month period from December to February. Analysis of
these data suggests that the vertical velocity can be deduced using the
divergence approach, but that missing or incorrect data pose a
considerable problem. Fairly sophisticated data smoothing and filtering
techniques are being examined to solve these problems.

A second and more extensive phase of the pilot program will be carried
out during the 1990-91 winter. The instrumentation to be deployed for
this phase is listed in table 9. The three wind profilers will be used
again in a triangular array to obtain an average vertical velocity.
Temperature and humidity structure will be obtained locally it low
levels using the 400 MHz profiler and RASS system. Upper level structure
will be found from a combination of both routine NWS radiosondes and
special sondes released locally in central Pennsylvania. The 94 GHz (3
mm) Doppler radar will be used primarily to determine cirrus cloud
presence and height. In addition, some analysis of cloud structure may
be carried out. Cirrus infrared emissivity will be determined by
measuring the downward radiance between 9.5 and 11.5 microns with a
PRT-5. Broadband solar and infrared downwelling fluxes will be measured
with standard Epply instruments. With the exception of the three 50 MHz
profilers, all the equipment will be collocated near State College,
Pennsylvania.

The Penn State mesoscale model will be run in a real-time forecasting
mode for Pennsylvania for some part of the experimental period. Periods
of special interest with regard to cirrus presence will be selected.
Temperature, humidity, and vertical velocity fields will be extracted
from the model runs for comparison with the data.

Preliminary data analysis should be completed by late summer, 1991. The
focus of this analysis will be on determining how well deduced vertical
velocities agree with model vertical velocities and correlate with cloud
Occurrence. Assessments of cirrus predictability based on model
simulations and/or measured wind fields will also be made. This
information will be made available to participants in the Cirrus IFO-II
field program. It is anticipated that this information will be useful in
designing operational strategies for the field campaign.

Table 9 - Instrumentation for the Pennsylvania State University Cirrus
Remote Sensing Pilot

Experiment

**Cirrus Remote Sensing Pilot Experiment**

Pennsylvania State University

Active Remote Sensors: 50 MHz wind profilers (3)

> 400 MHz wind profiler
>
> Radio-Acoustic Sounding System (RASS)
>
> 94 GHz (3 mm) Doppler cloud radar

Passive Remote Sensors: 9.5-11.5 μm radiometer (PRT-5)

> All-sky camera and VCR
>
> Broadband solar and infrared flux radiometers

5.6 NOAA/WPL Cloud Lidar and Radar Exploratory Test (CLARET)

NOAA\'s Wave Propagation Laboratory conducted the field phase of the
Cloud Lidar and Radar Exploratory Test for one month beginning September
6, 1989 (Eberhard et al., 1990). The overall objectives were to:

> \(1\) Evaluate new instruments and techniques for measuring cloud
> properties, especially cirrus. This included relatively \'young\'
> individual instruments, like the C02 Doppler lidar. It also included
> multiple devices operated in synergistic combination, such as
> simultaneous radar and lidar profiling of cloud structure.
>
> \(2\) Perform cloud and radiation research, including participation in
> ECLIPS.
>
> \(3\) Prepare for contributions to FIRE and other cloud microphysics
> and radiation research programs.
>
> CLARET instruments and measurements were:
>
> • Pulsed C02 lidar (10.6 μm wavelength) and ruby lidar (0.69 μm
> wavelength), both with depolarization sensitivity.
>
> • Doppler radar (3.2 mm wavelength).
>
> • Three-channel microwave radiometer to measure liquid water and
> precipitable water.
>
> • Pyranometer, pyrgeometer, and pyrheliometer.
>
> • Narrow field IR radiometer (bandpass 10.7 [+]{.underline} 0.7 μm)*.*
>
> • GOES and AVHRR radiance data.
>
> • All-sky camera and zenith-viewing video camera.
>
> • Meteorological data from surface station, Denver NWS radiosonde and
> ceilometer, and WPL sensors in the vicinity (wind profilers, RASS for
> temperature profiles, and 6-channel microwave radiometer for
> continuous but coarse temperature profiles).

Good data were acquired for 22 episodes (9 cirrus, 5 water cloud, and 6
clear air), where each episode was 1-3 hour duration. Data for many
additional hours are available from unattended instruments (including
radar) and sometimes one lidar.

CLARET involves both analytical and experimental investigations. Some
specific research tasks are:

> a\) Cloud microphysics from lidar depolarization. The high
> absorptivity and smaller size parameter of ice particles interrogated
> by the C02 lidar are expected to yield different depolarization
> characteristics than by the conventional lidars operating in or near
> the visible (Intrieri et al., 1990). These differences have potential
> for providing additional information on particle sizes and shapes.
>
> b\) Complementary view of cloud structure by lidar and radar (Uttal et
> al., 1990). A lidar detects even very thin cirrus and aerosol layers,
> whereas a radar can penetrate lower cloud layers that block a lidar.
>
> c\) Cirrus particle size information by simultaneous lidar and radar
> measurements.
>
> Multiple-wavelength probing will provide some description of the size
> distribution. Optimum wavelengths and methods to interpret the signals
> are being investigated for obtaining information on equivalent radius,
> refined radar estimates of ice content, and the relationship between
> ice content and optical depth.

A follow-on field experiment is planned (pending adequate funding) for
spring 1991 that will include the upgraded 8 mm wavelength,
dual-polarization, Doppler radar (Kropfli et a\]., 1990) and *in situ*
measurements by aircraft for validation of concepts and algorithms.
Participation of CLARET components in FIRE Cirrus IFO-IL will be
important for validation and application of new remote sensing
techniques and achievement of the FIRE Phase II cirrus .science
objectives.

6.0 COLLABORATIVE EXPERIMENTS

6.1 Spectral Radiance Experiment (SPECTRE)

The ongoing program known as ICRCCM (Intercomparison of Radiation Codes
in Climate Models, now a part of the World Climate Research Program) has
conducted intercomparisons of infrared radiative transfer models ranging
from line-by-line models to me highly parameterized models used in GCMs.
These comparisons have revealed substantial disagreements in computed
fluxes (20% and more) not only among GCM radiation models but also among
the so-called \'narrow-band\' models that were presumed accurate. The
line-by-line models were able to achieve 1-2% relative accuracy, but
only after agreeing to make certain common assumptions regarding line
shape, line cutoff, and continuum absorption. Thus, these models should
not be regarded as an absolute standard. SPECTRE, sponsored by NASA and
DOE is an outgrowth of these findings.

SPECTRE\'s main objective is to accurately measure the zenith infrared
radiance at high spectral resolution while simultaneously profiling the
radiatively important atmospheric characteristics . In this way, the
radiative models may be tested in a meaningful way. High priority is
placed upon improving and calibrating line-by-line models, since these
are the models against which cruder radiative models (Such as those in
GCMs) are tested. SPECTRE aims to close the loopholes by which radiation
models have eluded incisive comparison with measurements in the past by:

> • using remote profiling technologies that continuously sense the
> whole atmospheric column thereby minimizing the \'uncertainty of
> atmospheric characteristics\' associated with the sampling
> characteristics of rawinsondes and aircraft.
>
> • considering the downwelling radiation) and thereby eliminating the
> \'uncertainty in surface temperature and emittance\'. This also makes
> atmospheric emission stand out in stark relief and provides the
> severest test of the models.
>
> • using laboratory class spectrometers (several for redundancy) and an
> onsite blackbody calibration facility to eliminate the \'uncertainty
> in measurement accuracy\'.

A second related objective is to demonstrate that radiation measurements
in the field can approach their potential accuracy of 1%. This is
important because 1% changes in radiation energy are climatically
significant. This increased accuracy will further deny models any tuning
latitude.

Thus, the key features of SPECTRE are simultaneous, instantaneous
profiles of temperature, humidity, aerosol, and cloud; spectral detail
in the form of continuous high resolution infrared spectra; radiance
rather than flux measurements; careful and frequent radiometric
calibration in the field; and redundant measurements.

The field measurement activities of SPECTRE will be conducted in
conjunction with the FIRE Phase II Cirrus Intensive Field Observation
Campaign (section 4.3.2). In addition to the uplooking spectrometers
(spectral resolution better than 1 cm-1 over the spectral region from
about 4 μm to 20 μm), a Raman lidar for profiling water vapor contents,
a radio-acoustic sounder (RASS) for profiling temperature, and Dobson
sonde for profiling ozone will be deployed. Rawinsondes and satellites
will be used to characterize the atmospheric state at upper levels while
a tethersonde system and instrumented tower will be used to monitor
conditions in the boundary layer where the remote sensing systems have
difficulties. Flask samples will be taken and analyzed for trace gas
concentrations. As part of FIRE IFO-II, lidar and *in situ* (aircraft)
observations will be obtained to characterize aerosol profiles and
cirrus clouds (when present) and airborne observations of downwelling
radiance will also be obtained in some cases.

6.2 Experimental Cloud Lidar Pilot Study (ECLIPS)

The Experimental Cloud Lidar Pilot Study (ECLIPS) is an international
research program in support of ISCCP. ECLIPS uses Intensive periods of
lidar and ancillary measurements to categorize all clouds (low, middle,
and high) present during overpasses of NOAA polar orbiter satellites.
The main objective is to provide \'ground truth\' data on cloud
properties (heights and coverage) for validation of cloud properties
inferred from satellite radiance observations (AVHRR and HIRS),
especially those derived by ISCCP. An auxiliary objective is to provide
detailed statistics on the temporal variations of cloud base altitude
which exerts a significant radiative influence on the earth\'s surface
temperature. ECLIPS also seeks to encourage international collaboration
and uniform data processing methods among lidar investigators in order
to facilitate a possible future program of long-term global cloud
monitoring using lidars. The first experimental stage of ECLIPS was
conducted over a 1-month period in the fall of 1989. International
participation appeared strong and meetings are underway to organize the
data processing effort.

ECLIPS and FIRE share some objectives and will mutually benefit one
another. Increased utilization of lidar observations to monitor
cloudiness worldwide would provide extended time observations of cirrus
(and other) clouds for a variety of geographical areas and
climatological regimes. Such information would be quite useful to FIRE
researchers both with respect to validation of algorithms for retrieving
cirrus cloud properties from satellite observations (section 5.1) as
well as for investigating possible differences in cirrus cloud structure
(i.e., assessing the global representativeness of FIRE observations).
Greater commonalty in lidar data processing and interpretation is a
mutual concern, especially with respect to the integration of lidar and
radiometric observations. FIRE researchers are, active participants in
ECLIPS. FIRE lidar groups, who are regularly collecting extended time
observations (ETO) of cirrus (section 5.0), augmented their observation
schedules to obtain more comprehensive data sets (all clouds) during
ECLIPS. Other groups conducted intensive ETO research efforts during the
ECLIPS observational period.

6.3 TOGA/COARE Cirrus Research

A tropical cirrus experiment has been proposed as part of the intensive
observational period (IOP) of TOGA/COARE. This experiment proposes to
measure the radiative, microphysical, and macroscopic properties of
cirrus clouds using a combination of aircraft and ground-based (island)
observations. The specific concerns of the experiment are identified to
be:

> • the impact of cirrus clouds on the radiative budget of the tropical
> atmosphere,
>
> • the relationship between tropical cirrus and mesoscale deep
> convection over the Western Pacific warm pool region,
>
> • the influence of clouds on the, radiative fluxes into the oceanic
> warm pool,
>
> • the remote sensing of cirrus clouds, including retrieval of their
> optical properties, and
>
> • the parameterization of cirrus clouds in atmospheric general
> circulation models.

The objectives of the experiment are:

> • to describe and understand the radiative properties of tropical
> cirrus clouds in relation to their influence on both the radiative
> budget of the tropical atmosphere and the radiative budget at the
> air-sea interface over the Western Pacific region,
>
> • to characterize the microphysical properties of tropical cirrus
> clouds in relation to the environments in which they form and to
> establish their impact on the radiative budgets of the clouds
> themselves, and
>
> • to establish an understanding of the physical processes that link
> tropical cirrus to the deep convective systems that form over a more
> limited area of the warm pool region.

The plan is to make detailed radiometric, lidar, and radar measurements
of cirrus clouds using the NASA ER-2 aircraft along with *insitu*
measurements (perhaps using a Learjet) as well as ground-based
measurements, including lidar and passive radiometric instruments, from
an island site. The experiment is being proposed as a joint effort with
a tropical rainfall measurement (TRMM) field experiment that is also
planned for COARE. An umbrella proposal detailing this experiment and
its relationship to the TRMM field experiment is planned for the early
part of 1990. The use of ground-based sensors is crucial to the program
given the limited accessibility of high tropical cirrus (cloud top
altitudes to 17 km) to *in situ* research aircraft. It is hoped that the
understanding of remote sensing observations and their interpretation
gleaned from the Cirrus IFO-II experiment will be directly applicable
for the analysis of similar observations from the tropical cirrus
experiment. The proposed experiment closely associates with the
objectives of FIRE Phase II (section 3).

6.4 STORM Cirrus Research

> [TBD - W. Cotton/CSU]{.underline}

6.5 Atmospheric Radiation Measurements Program (ARM)

The Atmospheric Radiation Measurements (ARM) Program sponsored by the
Department of Energy is a recently initiated effort with the primary
goal of improving the parameterization of clouds and cloud-radiative
interactions in general circulation models (GCMs). In concept, ARM may
be thought of as an instrumented grid point. A large number of
sophisticated ground-based, remote-sensing instruments will be
co-located at some site. These instruments will be used to monitor on a
continuous basis atmospheric wind, temperature, and humidity profiles;
cloud physical and optical properties; and surface spectral and
broadband radiometric quantities. Current plans call for the
establishment of at least four such sites and a program duration of some
10 years.

Although FIRE and ARM have different primary goals, there is obviously
substantial overlap among their objectives and considerable potential
collaboration. Some examples of the

overlap are:

> \- many of the instruments proposed for the Cirrus IFO-II Hub site
> (table 3) are logical candidates for deployment at an ARM site.
>
> \- operation of the Cirrus IFO-II Hub site will provide a test of the
> ARM site concept both in terms of instrument deployment and
> synergistic data reduction and analysis.
>
> \- *in situ* aircraft measurements will provide basic understanding
> and calibration of remote sensing measurements that will be vital for
> the success of ARM.
>
> \- Cirrus IFO-II will be a test of concept that will need to be very
> carefully examined in the context of ARM.
>
> \- cloud process modelling based on the Cirrus IFO-II data set will
> undoubtedly shape ARM measurement strategies and interpretation of
> data.
>
> \- further FIRE programs may very likely be coordinated with ARM
> ground sites to take advantage of existing instrumentation and ARM
> funding, as well as provide further calibration of ARM measurements.

FIRE and ARM are related programs that have the potential for
considerable interaction and a great deal of mutual benefit. FIRE is a
fairly mature field operation, about to conduct its third intensive
field campaign. ARM is just beginning to get underway, but with
considerable momentum. Due to the close relationship between the science
of the two projects, they will undoubtedly share participants. At the
current time, these shared participants will be able to provide
sufficient information flow between the two programs to keep each
abreast of the other\'s plans and to provide for some general
collaboration. However, as ARM becomes more clearly defined, a more
formal collaborative effort should be considered.

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