bluuebunny/crossref_metadata_2025 huggingface dataset:
AERMOD: A Dispersion Model for Industrial Source Applications. Part II: Model Performance against 17 Field Study Databases
Authors: Perry Steven G. , Cimorelli Alan J. , Paine Robert J. , Brode Roger W. , Weil Jeffrey C. , Venkatram Akula , Wilson Robert B. , Lee Russell F. , Peters Warren D.
Published: June 2005

The performance of the American Meteorological Society (AMS) and U.S. Environmental Protection Agency (EPA) Regulatory Model (AERMOD) Improvement Committee’s applied air dispersion model against 17 field study databases is described. AERMOD is a steady-state plume model with significant improvements over commonly applied regulatory models. The databases are characterized, and the performance measures are described. Emphasis is placed on statistics that demonstrate the model’s abilities to reproduce the upper end of the concentration distribution. This is most important for applied regulatory modeling. The field measurements are characterized by flat and complex terrain, urban and rural conditions, and elevated and surface releases with and without building wake effects. As is indicated by comparisons of modeled and observed concentration distributions, with few exceptions AERMOD’s performance is superior to that of the other applied models tested. This is the second of two articles, with the first describing the model formulations.
Characteristics of Strong Updrafts in Precipitation Systems over the Central Tropical Pacific Ocean and in the Amazon
Authors: Anderson Nicholas F. , Grainger Cedric A. , Stith Jeffrey L.
Published: June 2005

Airborne in situ measurements of updrafts in tropical convective storms were analyzed to determine the similarities and differences between updrafts in a tropical continental and a tropical oceanic region. Two hundred fifteen updraft cores from the Tropical Rainfall Measuring Mission (TRMM) component of the Large Scale Biosphere–Atmosphere (LBA) experiment (tropical continental wet season) and 377 updraft cores from the Kwajalein Experiment (KWAJEX) (tropical oceanic) were analyzed in a similar manner to that of previous studies of tropical updrafts. Average speed, maximum speed, width, and mass flux of the updraft cores from the TRMM-LBA and KWAJEX were generally similar to each other and also were similar to results from previous studies of tropical updrafts.
The Influence of Meteorological Conditions and Atmospheric Circulation Types on PM10 and Visibility in Tel Aviv
Authors: Dayan Uri , Levy Ilan
Published: June 2005

A 3-yr dataset (February 2000–November 2002) of 30-min averages for particulates, visibility, and meteorological data were analyzed to examine the dependence of particulate matter of less than 10 μm in diameter (PM10) mass concentrations and visibility in Tel Aviv, Israel, on seasonal meteorological conditions, synoptic weather patterns, and airmass history. The persistent nature of the summer synoptic weather pattern in the east Mediterranean (EM) region yielded the highest mean visibility (35 km) and lowest variability in PM10 concentrations, as compared with the higher variance for the other seasons, and spring in particular. The frequent passage of the “Sharav” cyclones during spring causes natural dust outbreaks with extreme values that result in a much higher PM10 annual mean (57 μg m−3) as compared with other larger cities. The history of the air mass dictating the physical and chemical properties of the particulate matter reaching Israel was assessed by back-trajectory analysis from predetermined transport sectors. The west and central European sectors, representing mostly the summer flow regime of European clean air masses that are advected over the Mediterranean, differ significantly in their reduced PM10 loading and high visual range from the Arabian Peninsula and North African sectors, originating from the surrounding deserts. Because the most frequent direction of air masses reaching the EM is from European origins, Tel Aviv experiences most of the time satisfying visibility conditions. Applying the definition “very good visibility” for visibility exceeding 19 km shows that about 90% of the time these conditions were met for this 3-yr period.
A Detailed Evaluation of GPCP 1° Daily Rainfall Estimates over the Mississippi River Basin
Authors: Gebremichael Mekonnen , Krajewski Witold F. , Morrissey Mark L. , Huffman George J. , Adler Robert F.
Published: June 2005

This study provides an intensive evaluation of the Global Precipitation Climatology Project (GPCP) 1° daily (1DD) rainfall products over the Mississippi River basin, which covers 435 1° latitude × 1° longitude grids for the period of January 1997–December 2000 using radar-based precipitation estimates. The authors’ evaluation criteria include unconditional continuous, conditional (quasi) continuous, and categorical statistics, and their analyses cover annual and seasonal time periods. The authors present spatial maps that reflect the results for the 1° grids and a summary of the results for three selected regions. They also develop a statistical framework that partitions the GPCP–radar difference statistics into GPCP error and radar error statistics. They further partition the GPCP error statistics into sampling error and retrieval error statistics and estimate the sampling error statistics using a data-based resampling experiment. Highlights of the results include the following: 1) the GPCP 1DD product captures the spatial and temporal variability of rainfall to a high degree, with more than 80% of the variance explained, 2) the GPCP 1DD product proficiently detects rainy days at a large range of rainfall thresholds, and 3) in comparison with radar-based estimates the GPCP 1DD product overestimates rainfall.
A Method to Identify the Optimal Areal Unit for NLDN Cloud-to-Ground Lightning Flash Data Analysis
Authors: Schultz Michael D. , Underwood S. Jeffrey , Radhakrishnan Premkrishnan
Published: June 2005

Currently, no uniform method exists for determining the optimal areal unit to analyze National Lightning Detection Network (NLDN) data. To address this problem, this paper utilizes the capabilities of modern geographic information systems (GIS) software to develop a consistent method for identifying areal analysis units while considering the location accuracy of the NLDN. Five grid cells were created at spatial resolutions of 0.1°, 0.2°, 0.3°, 0.4°, and 0.5°. To create cloud-to-ground (CG) lightning strikes, random points were generated at nine densities ranging from 1 to 9 strikes per square kilometer. A buffer of 500 m was placed around each random point to account for the margin of error in NLDN location accuracy. Random points that, when buffered to 500 m, still remained completely within the study region were evaluated as a percentage of all of the strikes to determine accuracy. The greatest accuracy of 95.88% was observed in the 0.5° grid cell at a density of 9 strikes per square kilometer. The lowest accuracy of 80.59% occurred in the 0.1° grid cell at a density of 4 strikes per square kilometer. There was little variation between the accuracies in similar grid cells regardless of their density, suggesting that CG flash density will have little effect on accuracy when selecting spatial resolution. Gains in accuracy diminished as spatial resolution increased. Gains in accuracy between the 0.1° and the 0.2° grid cells are approximately 9%. Gains in accuracy between the 0.4° and the 0.5° grid cells are less than 1%. To achieve 95% accuracy, a spatial resolution of no less than 0.4° is required.
Polarimetric Tornado Detection
Authors: Ryzhkov Alexander V. , Schuur Terry J. , Burgess Donald W. , Zrnic Dusan S.
Published: June 2005

Polarimetric radars are shown to be capable of tornado detection through the recognition of tornadic debris signatures that are characterized by the anomalously low cross-correlation coefficient ρhv and differential reflectivity ZDR. This capability is demonstrated for three significant tornadic storms that struck the Oklahoma City, Oklahoma, metropolitan area. The first tornadic debris signature, based on the measurements with the National Severe Storms Laboratory’s Cimarron polarimetric radar, was reported for a storm on 3 May 1999. Similar signatures were identified for two significant tornadic events during the Joint Polarization Experiment (JPOLE) in May 2003. The data from these storms were collected with a polarimetric prototype of the Next-Generation Weather Radar (NEXRAD). In addition to a small-scale debris signature, larger-scale polarimetric signatures that might be relevant to tornadogenesis were persistently observed in tornadic supercells. The latter signatures are likely associated with lofted light debris (leaves, grass, dust, etc.) in the inflow region and intense size sorting of hydrometeors in the presence of strong wind shear and circulation.
Zonal Momentum Balance, Potential Vorticity Dynamics, and Mass Fluxes on Near-Surface Isentropes
Authors: Schneider Tapio
Published: June 2005

While it has been recognized for some time that isentropic coordinates provide a convenient framework for theories of the global circulation of the atmosphere, the role of boundary effects in the zonal momentum balance and in potential vorticity dynamics on isentropes that intersect the surface has remained unclear. Here, a balance equation is derived that describes the temporal and zonal mean balance of zonal momentum and of potential vorticity on isentropes, including the near-surface isentropes that sometimes intersect the surface. Integrated vertically, the mean zonal momentum or potential vorticity balance leads to a balance condition that relates the mean meridional mass flux along isentropes to eddy fluxes of potential vorticity and surface potential temperature. The isentropic-coordinate balance condition formally resembles balance conditions well known in quasigeostrophic theory, but on near-surface isentropes it generally differs from the quasigeostrophic balance conditions. Not taking the intersection of isentropes with the surface into account, quasigeostrophic theory does not adequately represent the potential vorticity dynamics and mass fluxes on near-surface isentropes—a shortcoming that calls into question the relevance of quasigeostrophic theories for the macroturbulence and global circulation of the atmosphere.
Linear and Nonlinear Signatures in the Planetary Wave Dynamics of an AGCM: Phase Space Tendencies
Authors: Branstator Grant , Berner Judith
Published: June 2005

To identify and quantify indications of linear and nonlinear planetary wave behavior, characteristics of a very long integration of an atmospheric general circulation model in a four-dimensional phase space are examined. The phase space is defined by the leading four empirical orthogonal functions of 500-hPa geopotential heights, and the primary investigated characteristic is the state dependence of mean phase space tendencies. Defining the linear component of planetary wave tendencies as that part which can be captured by a least squares fit linear operator driven by additive Gaussian white noise, the study finds that there are distinct linear and nonlinear signatures. These signatures are especially easy to see in plots of mean tendencies projected onto phase space planes. For some planes the mean tendencies are highly linear, while for others there are strong departures from linearity.

The results of the analysis are found to depend strongly on the lag time used to estimate tendencies with the linear component monotonically increasing with lag time. This is shown to result from the ergodicity of the system. Using the theory of Markov models it is possible to remove the lag-dependent component of the tendencies from the results. When this is done the projected mean dynamics in some planes is found to be almost exclusively nonlinear, while in others it is nearly linear.

In the four-dimensional space the linear component of the dynamics is largely a reflection of a westward propagating Northern Hemisphere pattern concentrated over the Pacific and North America. The nonlinear signature can be approximated by two linear functions, each operating in a different region of phase space. One region is centered around a Pacific blocking pattern while the other is centered on a state with enhanced zonal symmetry. It is concluded that reduced models of the planetary waves should strive to include these state-dependent dynamics.
Droplets to Drops by Turbulent Coagulation
Authors: Riemer N. , Wexler A. S.
Published: June 2005

This study addresses two central problems in cloud microphysics. The first is the source of large droplets, which initiates the rapid production of warm rain. The second is the broadening of the cloud droplet spectrum at both tails of the spectrum. The study explores how in-cloud turbulence can help to close the gaps in our understanding. With box model simulations, the development of cloud droplet spectra is calculated using a coagulation kernel that recently has been derived from direct numerical simulations. This kernel includes both the effect of turbulence on the relative velocities of the droplets and on the local increases in droplet concentration, the so-called accumulation effect. Under the assumption that this kernel can be extrapolated to atmospheric Reynolds numbers, the results show that for typical atmospheric conditions, the turbulent coagulation kernel is several orders of magnitude larger than the sedimentation kernel for droplets smaller then 100 μm. While for calm air after 30-min simulation time, only 7% of the total mass is found in droplets with sizes over 100 μm, this increases to 79% for a dissipation rate of 100 cm2 s−3 and 96% for 300 cm2 s−3 if a combined sedimentation and turbulent kernel is employed that assumes that the sedimentation and turbulent kernel can be added. Hence, moderate turbulence can enhance significantly the formation of large droplets. Furthermore, a time-scale analysis shows that broadening at the upper end of the spectrum is caused by turbulent coagulation whereas thermodynamic effects are responsible for broadening at the lower end.
Possible Effects of Collisional Breakup on Mixed-Phase Deep Convection Simulated by a Spectral (Bin) Cloud Model
Authors: Seifert Axel , Khain Alexander , Blahak Ulrich , Beheng Klaus D.
Published: June 2005

The effects of the collisional breakup of raindrops are investigated using the Hebrew University Cloud Model (HUCM). The parameterizations, which are combined in the new breakup scheme, are those of Low and List, Beard and Ochs, as well as Brown. A sensitivity study reveals strong effects of collisional breakup on the precipitation formation in mixed-phase deep convective clouds for strong as well as for weak precipitation events. Collisional breakup reduces the number of large raindrops, increases the number of small raindrops, and, as a consequence, decreases surface rain rates and considerably reduces the speed of rain formation. In addition, it was found that including breakup can lead to a more intense triggering of secondary convective cells. But a statistical comparison with observed raindrop size distributions shows that the parameterizations might systematically overestimate collisional breakup.
Numerical Simulation of Boundary Layer Structure and Cross-Equatorial Flow in the Eastern Pacific*
Authors: Small R. Justin , Xie Shang-Ping , Wang Yuqing , Esbensen Steven K. , Vickers Dean
Published: June 2005

Recent observations from spaceborne microwave sensors have revealed detailed structure of the surface flow over the equatorial eastern Pacific in the boreal fall season. A marked acceleration of surface wind across the northern sea surface temperature (SST) front of the cold tongue is a prominent feature of the regional climate. Previous studies have attributed the acceleration to the effect of enhanced momentum mixing over the warmer waters. A high-resolution numerical model is used to examine the cross-frontal flow adjustment. In a comprehensive comparison, the model agrees well with many observed features of cross-equatorial flow and boundary layer structure from satellite, Tropical Atmosphere Ocean (TAO) moorings, and the recent Eastern Pacific Investigation of Climate Processes (EPIC) campaign. In particular, the model simulates the acceleration across the SST front, and the change from a stable to unstable boundary layer. Analysis of the model momentum budget indicates that the hydrostatic pressure gradient, set up in response to the SST gradient, drives the surface northward acceleration. Because of thermal advection by the mean southerly flow, the pressure gradient is located downstream of the SST gradient and consequently, divergence occurs over the SST front, as observed by satellite. Pressure gradients also act to change the vertical shear of the wind as the front is crossed. However, the model underpredicts the changes in vertical wind shear across the front, relative to the EPIC observations. It is suggested that the vertical transfer of momentum by mixing, a mechanism described by Wallace et al. may also act to enhance the change in shear in the observations, but the model does not simulate this effect. Reasons for this are discussed.
The Role of Relative Humidity in Radiative–Convective Equilibrium
Authors: Sugiyama Masahiro , Stone Peter H. , Emanuel Kerry A.
Published: June 2005

The following conditions are derived for the existence of a radiation limit of tropospheric origin in a nongray atmosphere, extending the work on a gray atmosphere by Nakajima et al.: 1) the atmosphere must become sufficiently optically thick, and 2) the temperature must become only a function of optical depth at each frequency, independent of surface temperature. The first condition is satisfied at high temperatures even in a window region as long as there is weak but nonzero absorption, because the optical depth of the entire atmosphere roughly scales as saturation vapor pressure. At high temperatures, the pseudoadiabatic temperature structure asymptotes to the saturation vapor pressure curve, satisfying the second condition at each frequency. A rapidly decreasing vertical gradient of water vapor mixing ratio allows temperature to asymptote faster in optical depth coordinates than in pressure coordinates.

Analyses using a radiative–convective model show that interactive relative humidity can give rise to a different kind of runaway greenhouse effect and multiple equilibria, if the strength of relative humidity feedback exceeds a critical value. The results suggest that this mechanism may be able to explain the runaway greenhouse effect found by Rennó et al. and radiative–convective multiple equilibria by Rennó. The framework employed in this study will serve as a diagnostic tool for further research on the runaway greenhouse effect and radiative–convective multiple equilibria.
Lagrangian Particle Dispersion Modeling of the Fumigation Process Using Large-Eddy Simulation
Authors: Kim Si-Wan , Moeng Chin-Hoh , Weil Jeffrey C. , Barth Mary C.
Published: June 2005

A Lagrangian particle dispersion model (LPDM) is used to study fumigation of pollutants in and above the entrainment zone into a growing convective boundary layer. Probability density functions of particle location with height and time are calculated from particle trajectories driven by the sum of the resolved-scale velocity from a large-eddy simulation (LES) model and the stochastic subgrid-scale (SGS) velocity. The crosswind-integrated concentration (CWIC) fields show good agreement with water tank experimental data. A comparison of the LPDM output with an Eulerian diffusion model output based on the same LES flow shows qualitative agreement with each other except that a greater overshoot maximum of the ground-level concentration occurs in the Eulerian model.

The dimensionless CWICs near the surface for sources located above the entrainment zone collapse to a nearly universal curve provided that the profiles are time shifted, where the shift depends on the source heights. The dimensionless CWICs for sources located within the entrainment zone show a different behavior. Thus, fumigation from sources above the entrainment zone and within the entrainment zone should be treated separately. An examination of the application of Taylor’s translation hypothesis to the fumigation process showed the importance of using the mean boundary layer wind speed as a function of time rather than the initial mean boundary layer wind speed, because the mean boundary layer wind speed decreases as the simulation proceeds.

The LPDM using LES is capable of accurately simulating fumigation of particles into the convective boundary layer. This technique provides more computationally efficient simulations than Eulerian models.
Sea Spray Impacts on Intensifying Midlatitude Cyclones
Authors: Perrie Will , Zhang Weiqing , L Andreas Edgar , Li Weibiao , Gyakum John , McTaggart-Cowan Ron
Published: June 2005

Air–sea transfer processes over the ocean strongly affect how hurricanes develop. High winds generate large amounts of sea spray, which can modify the transfer of momentum, heat, and moisture across the air–sea interface. However, the extent to which sea spray can modify extratropical or midlatitude hurricanes and intense cyclones has not been resolved. This paper reports simulations of extratropical Hurricanes Earl (1998) and Danielle (1998) and an intense winter cyclone from January 2000 using a mesoscale atmospheric model and a recent sea spray parameterization. These simulations show that sea spray can increase the sea surface heat flux, especially the latent heat flux, in a midlatitude cyclone and that sea spray’s impact on cyclone intensity depends on the storm structure and development and is strongest for cyclones with high winds.
A Simple Parameterization Coupling the Convective Daytime Boundary Layer and Fair-Weather Cumuli
Authors: Berg Larry K. , Stull Roland B.
Published: June 2005

A new parameterization for boundary layer cumulus clouds, called the cumulus potential (CuP) scheme, is introduced. This scheme uses joint probability density functions (JPDFs) of virtual potential temperature (θυ) and water-vapor mixing ratio (r), as well as the mean vertical profiles of θυ, to predict the amount and size distribution of boundary layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as noncloud updrafts. This model has several unique features: 1) cloud cover is determined from the boundary layer JPDF of θυ versus r, 2) clear and cloudy thermals are allowed to coexist at the same altitude, and 3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed.

Using data from Boundary Layer Experiment 1996 and a model intercomparsion study using large eddy simulation (LES) based on Barbados Oceanographic and Meteorological Experiment (BOMEX), it is shown that the CuP model does a good job predicting cloud-base height and cloud-top height. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model.
Low-Order Stochastic Mode Reduction for a Realistic Barotropic Model Climate
Authors: Franzke Christian , Majda Andrew J. , Vanden-Eijnden Eric
Published: June 2005

This study applies a systematic strategy for stochastic modeling of atmospheric low-frequency variability to a realistic barotropic model climate. This barotropic model climate has reasonable approximations of the Arctic Oscillation (AO) and Pacific/North America (PNA) teleconnections as its two leading principal patterns of low-frequency variability. The systematic strategy consists first of the identification of slowly evolving climate modes and faster evolving nonclimate modes by use of an empirical orthogonal function (EOF) decomposition. The low-order stochastic climate model predicts the evolution of these climate modes a priori without any regression fitting of the resolved modes. The systematic stochastic mode reduction strategy determines all correction terms and noises with minimal regression fitting of the variances and correlation times of the unresolved modes. These correction terms and noises account for the neglected interactions between the resolved climate modes and the unresolved nonclimate modes. Low-order stochastic models with only four resolved modes capture the statistics of the original barotropic model modes quite well. A budget analysis establishes that the low-order stochastic models are dominated by linear dynamics and additive noise. The linear correction terms and the additive noise stem from the linear coupling between resolved and unresolved modes, and not from nonlinear interactions between resolved and unresolved modes as assumed in previous studies.
Control Parameters for the Influence of a Mesoscale Mountain Range on Cyclone Track Continuity and Deflection
Authors: Lin Yuh-Lang , Chen Shu-Yun , Hill Christopher M. , Huang Ching-Yuang
Published: June 2005

In this study prospective control parameters are identified for diagnosing the continuity and deflection of cyclone tracks across a mesoscale mountain range. Based on idealized simulations of a westward-moving cyclone, it was found that the cyclone track becomes a discontinuous (continuous) track and the cyclone experiences more (less) deflection with a combination of small (large) values of Vmax/Nh, U/Nh, R/Ly, U/fLx, and Vmax/fR, and large (small) value of h/Lx. The symbols are defined as follows: Vmax the maximum tangential wind, N the Brunt–Väisälä frequency, h the mountain height, U the basic wind speed, R the radius of Vmax, f the Coriolis parameter, and Lx and Ly the horizontal scales of the mountain in x and y directions, respectively.

A conceptual model is proposed to explain track deflection and continuity for a westward-moving cyclone encountering idealized topography representative of the Central Mountain Range of Taiwan. With weak orographic blocking, a cyclone crosses over the mountain range with some northward deflection. With moderate orographic blocking, northward deflection of a cyclone is greater upstream of the mountain range and a secondary, leeside vortex forms to the southwest of the mountain range, indicative of discontinuity in the cyclone track. With strong orographic blocking, a westward-moving cyclone is deflected southward and a secondary cyclone forms to the northwest of the mountain range. The northward or southward deflection of a cyclone track is explained by the orographic blocking on the outer circulation of the cyclone.
The Planetary- and Synoptic-Scale Interactions in a Southeast Pacific Blocking Episode Using PV Diagnostics
Authors: Burkhardt John P. , Lupo Anthony R.
Published: June 2005

The synoptic- and planetary-scale forcing in two blocking anticyclones occurring over the southeast Pacific Ocean was examined using potential vorticity diagnostics. While many studies have examined the dynamic and thermodynamic forcing associated with blocking events in the Northern Hemisphere (NH), very few studies have examined blocking in the Southern Hemisphere (SH). Climatological analysis suggests SH blocking events in the Pacific region have similar characteristics to their NH counterparts. However, the occurrence of blocking is rare elsewhere in the SH, and these events are relatively short-lived. Some studies of NH blocking dynamics have also shown that the extent to which the planetary- and synoptic-scale and planetary–synoptic-scale interaction forcing that contribute to the genesis and maintenance of Pacific and Atlantic region events can be different. Thus, a study of the relevant atmospheric dynamics associated with blocking events in the SH was carried out in order to determine whether or not these events are associated with similar dynamic mechanisms to those in the NH. Using the National Center for Atmospheric Research and National Centers for Environmental Prediction (NCAR–NCEP) reanalyses dataset and applying a low-pass filter to the relevant variables, the authors examined the scale interactions associated with two blocking events that occurred during July and August 1986 and applied potential vorticity diagnostics. Results demonstrate that blocking in the southeast Pacific was associated with similar synoptic features, and the forcing mechanisms on the planetary, synoptic scales, and interactions were more similar to North Pacific blocking events rather than those occurring over the NH Atlantic region. However, these results also demonstrated that blocking events in the NH were associated with synergistically interacting synoptic- and planetary-scale waves, while in the SH, blocking events resulted from the superposition of synoptic and planetary waves. This result may explain the paucity of blocking occurrences and their tendency to be weaker and less persistent over much of the SH.
Impact of Cloud Cover on Solar Radiative Biases in Deep Convective Regimes
Authors: Di Giuseppe F. , Tompkins A. M.
Published: June 2005

Conflicting claims have been made concerning the magnitude of the bias in solar radiative transfer calculations when horizontal photon transport is neglected for deep convective scenarios. The difficulty of obtaining a realistic set of cloud scenes for situations of complex cloud geometry, while certain characteristics such as total cloud cover are systematically controlled, has hindered the attempt to reach a consensus. Here, a simple alternative approach is adopted. An ensemble of cloud scenes generated by a cloud resolving model are modified by an idealized function that progressively alters the cirrus anvil coverage without affecting the realism of the scene produced. Comparing three-dimensional radiative calculations with the independent column approximation for all cloud scenes, it is found that the bias in scene albedo can reach as much as 22% when the sun is overhead and 46% at low sun angles. The bias is an asymmetrical function of cloud cover with a maximum attained at cirrus anvil cloud cover of approximately 30%–40%. With a cloud cover of 15%, the bias is half its maximum value, while it is limited for coverage exceeding 80%. The position of the peak occurs at the cloud cover coinciding with the maximum number of independent clouds present in the scene. Increasing the cloud cover past this point produces a decrease in the number of isolated clouds because of cloud merging, with a consequential bias reduction.

With this systematic documentation of the biases as a function of total cloud cover, it is possible to identify two contributions to the total error: the geometrical consequences of the effective cloud cover increase at low sun angles and the true 3D scattering effect of photons deviating from the original path direction. An attempt to account for the former geometrical contribution to the 1D bias is made by performing a simple correction technique, whereby the field is sheared by the tangent of the solar zenith angle. It is found that this greatly reduces the 1D biases at low sun angles. Because of the small aspect ratio of the cirrus cloud deck, the remaining bias contribution is small in magnitude and almost independent of solar zenith angle.
Bimodal Behavior in the Zonal Mean Flow of a Baroclinic β-Channel Model
Authors: Kravtsov S. , Robertson A. W. , Ghil M.
Published: June 2005

The dynamical origin of midlatitude zonal-jet variability is examined in a thermally forced, quasigeostrophic, two-layer channel model on a β plane. The model’s behavior is studied as a function of the bottom-friction strength.

Two distinct zonal-flow states exist at realistic, low, and intermediate values of the bottom drag; these two states are maintained by the eddies and differ mainly in terms of the meridional position of their climatological jets. The system’s low-frequency evolution is characterized by irregular transitions between the two states.

For a given branch of model solutions, the leading stationary and propagating empirical orthogonal functions are related to eigenmodes of the model’s dynamical operator, linearized about the climatological state on this branch. Nonlinear interactions between these modes are instrumental in determining their relative energy level. In particular, the stationary modes’ self-interaction is shown to vanish. Thus, these modes do not exchange energy with the mean flow and, consequently, dominate the lowest-frequency behavior in the model. The leading stationary mode resembles the observed annular mode in the Southern Hemisphere.

The bimodality is due to nonlinear interactions between nearly equivalent barotropic, stationary, and propagating modes, while the synoptic eddies play a modest role in determining the relative persistence of the two states. The role of synoptic eddies is very substantial only at unrealistically high values of the bottom drag, where they give rise to ultralow frequency variability by modifying the jet in a way that reinforces generation of the eddy field. This type of behavior is related to the presence of a homoclinic orbit in the model’s phase space and is not apparent for more realistic, lower values of the bottom drag.
Southern Hemisphere Annular Mode Variability and the Role of Optimal Nonmodal Growth
Authors: Rashid Harun A. , Simmonds Ian
Published: June 2005

The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large amplitudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.
Reliability of ENSO Dynamical Predictions
Authors: Tang Youmin , Kleeman Richard , Moore Andrew M.
Published: June 2005

In this study, ensemble predictions were constructed using two realistic ENSO prediction models and stochastic optimals. By applying a recently developed theoretical framework, the authors have explored several important issues relating to ENSO predictability including reliability measures of ENSO dynamical predictions and the dominant precursors that control reliability. It was found that prediction utility (R), defined by relative entropy, is a useful measure for the reliability of ENSO dynamical predictions, such that the larger the value of R, the more reliable the prediction. The prediction utility R consists of two components, a dispersion component (DC) associated with the ensemble spread and a signal component (SC) determined by the predictive mean signals. Results show that the prediction utility R is dominated by SC.

Using a linear stochastic dynamical system, SC was examined further and found to be intrinsically related to the leading eigenmode amplitude of the initial conditions. This finding was validated by actual model prediction results and is also consistent with other recent work. The relationship between R and SC has particular practical significance for ENSO predictability studies, since it provides an inexpensive and robust method for exploring forecast uncertainties without the need for costly ensemble runs.
A New Double-Moment Microphysics Parameterization for Application in Cloud and Climate Models. Part I: Description
Authors: Morrison H. , Curry J. A. , Khvorostyanov V. I.
Published: June 2005

A new double-moment bulk microphysics scheme predicting the number concentrations and mixing ratios of four hydrometeor species (droplets, cloud ice, rain, snow) is described. New physically based parameterizations are developed for simulating homogeneous and heterogeneous ice nucleation, droplet activation, and the spectral index (width) of the droplet size spectra. Two versions of the scheme are described: one for application in high-resolution cloud models and the other for simulating grid-scale cloudiness in larger-scale models. The versions differ in their treatment of the supersaturation field and droplet nucleation. For the high-resolution approach, droplet nucleation is calculated from Kohler theory applied to a distribution of aerosol that activates at a given supersaturation. The resolved supersaturation field and condensation/deposition rates are predicted using a semianalytic approximation to the three-phase (vapor, ice, liquid) supersaturation equation. For the large-scale version of the scheme, it is assumed that the supersaturation field is not resolved and thus droplet activation is parameterized as a function of the vertical velocity and diabatic cooling rate. The vertical velocity includes a subgrid component that is parameterized in terms of the eddy diffusivity and mixing length. Droplet condensation is calculated using a quasi-steady, saturation adjustment approach. Evaporation/deposition onto the other water species is given by nonsteady vapor diffusion allowing excess vapor density relative to ice saturation.
A New Double-Moment Microphysics Parameterization for Application in Cloud and Climate Models. Part II: Single-Column Modeling of Arctic Clouds
Authors: Morrison H. , Curry J. A. , Shupe M. D. , Zuidema P.
Published: June 2005

The new double-moment microphysics scheme described in Part I of this paper is implemented into a single-column model to simulate clouds and radiation observed during the period 1 April–15 May 1998 of the Surface Heat Budget of the Arctic (SHEBA) and First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment–Arctic Clouds Experiment (FIRE–ACE) field projects. Mean predicted cloud boundaries and total cloud fraction compare reasonably well with observations. Cloud phase partitioning, which is crucial in determining the surface radiative fluxes, is fairly similar to ground-based retrievals. However, the fraction of time that liquid is present in the column is somewhat underpredicted, leading to small biases in the downwelling shortwave and longwave radiative fluxes at the surface. Results using the new scheme are compared to parallel simulations using other microphysics parameterizations of varying complexity. The predicted liquid water path and cloud phase is significantly improved using the new scheme relative to a single-moment parameterization predicting only the mixing ratio of the water species. Results indicate that a realistic treatment of cloud ice number concentration (prognosing rather than diagnosing) is needed to simulate arctic clouds. Sensitivity tests are also performed by varying the aerosol size, solubility, and number concentration to explore potential cloud–aerosol–radiation interactions in arctic stratus.
On Weak Zonally Symmetric ENSO Atmospheric Heating and the Strong Zonally Symmetric ENSO Air Temperature Response
Authors: Clarke Allan J. , Kim Kwang-Y.
Published: June 2005

Observations show that regions of anomalous deep convective El Niño–Southern Oscillation (ENSO) heating tend to be balanced by anomalous ENSO cooling elsewhere so that, averaged around the globe from (say) 10°S to 10°N, the net anomalous heating is nearly zero. The zonally symmetric heating is weak because it is approximately proportional to vertical velocity that, when averaged over a constant pressure surface S around the earth from 10°S to 10°N, is nearly zero. The horizontally averaged vertical velocity over S is small because the net horizontal geostrophic convergent flow across 10°S and 10°N is zero.

Although the zonally symmetric ENSO heating is weak, the observed ENSO tropospheric air temperature anomaly has a large zonally symmetric component. Past work has shown that with weak momentum and thermal damping, Kelvin and Rossby waves can travel around the earth without significant loss of amplitude so that a zonally symmetric response is favored. This physical interpretation depends on knowing temperature and momentum anomaly damping times over the depth of the troposphere. Such times are not well known. Here a Gill tropical atmospheric model is generalized to include realistic surface friction and so theoretically estimate a frictional spindown time. Using this spindown time (approximately 3 weeks), together with an estimate of the Newtonian cooling time (1 month) the authors show, in agreement with observations, that the extremely weak zonally symmetric heating anomaly generates a symmetric air temperature anomaly comparable to the asymmetric one.
Filtering of Parameterized Nonorographic Gravity Waves in the Met Office Unified Model
Authors: Warner Christopher D. , Scaife Adam A. , Butchart Neal
Published: June 2005

This paper investigates the vertical filtering of parameterized gravity wave pseudomomentum flux in the troposphere–stratosphere version of the Met Office Unified Model. Gravity wave forcing is parameterized using the Warner and McIntyre spectral gravity wave parameterization. The same amount of isotropic pseudomomentum flux per unit mass is launched from the planetary boundary layer at each grid point. The parameterization models the azimuthally dependent Doppler shifting and breaking of the gravity wave spectrum as it is filtered by the background atmosphere. The result is an anisotropic distribution of pseudomomentum flux among azimuthal sectors that varies greatly with altitude and location. This gives an idealized global climatology of nonorographic gravity waves. The filtering effect of the atmosphere in this climatology is diagnosed using the “zonal anisotropy.”

Results show areas where observational measurements could be targeted to find the most prominent features in the gravity wave field. Such areas include, for example, the summer stratosphere where zonal anisotropy is very large and where there is a significant localization in latitude and longitude of patches of high zonal anisotropy. Comparisons are also made with recent observational estimates of gravity wave fluxes and test whether wind filtering of a homogeneous, azimuthally isotropic source is enough to reproduce observed features of the gravity wave field.
Cell Broadening Revisited: Results from High-Resolution Large-Eddy Simulations of Cold Air Outbreaks
Authors: Schröter M. , Raasch S. , Jansen H.
Published: June 2005

Large-eddy simulations (LES) have been carried out in order to investigate the structure and development of organized mesoscale cellular convection (MCC) during cold air outbreaks (CAOB) and to reevaluate results by Müller and Chlond. Some limitations of this and other earlier LES studies of CAOBs have been removed by using a parallelized model with both a large horizontal domain and a fine grid resolution. These model simulations reveal a hitherto undiscovered insight into the development of MCC.

It is shown that MCC with aspect ratios larger than 10 only develop in the presence of diabatic heat sources, that is, latent heat release within the clouds and cloud-top radiative cooling, which confirms results from previous studies. Simulated cells are of closed type. The wind field is seen to be correlated with the liquid water field, and thus, dynamic variables are organized on the mesoscale as well. Updrafts predominantly occur in regions with high liquid water content. Two-dimensional spectral analysis confirms clear peaks at the wavelength of the MCC. The dynamic variables are characterized by an organized conglomeration of randomly distributed up- and downdrafts, where each convective cell can hardly be detected by eye. Whereas scalar variables like temperature and the liquid water path field are characterized by large scales with aspect ratios of 9 to 11, in the vertical velocity field multiple scales are present simultaneously, with small scales having aspect ratios between 1 and 3 contributing most to the total energy, but also large scales correlated with scales seen in the thermodynamic variables.
A New Multiscale Model for the Madden–Julian Oscillation
Authors: Biello Joseph A. , Majda Andrew J.
Published: June 2005

A multiscale model of the MJO is developed here that accounts, in a simplified fashion, for both the upscale transfer from synoptic to planetary scales of momentum and temperature from wave trains of thermally driven equatorial synoptic-scale circulations in a moving convective envelope as well as direct mean heating on planetary scales. This model involves idealized thermally driven congestus synoptic-scale fluctuations in the eastern part of the moving wave envelope and convective superclusters in the western part of the envelope. The model self-consistently reproduces qualitatively many of the detailed structural features of the planetary circulation in the observations of the MJO, including the vertical structure in both the westerly onset region and the strong westerly wind burst region, as well as the horizontal quadrupole planetary vortex structure. The westerly midlevel inflow in the strong westerly region and the quadrupole vortex are largely produced in the model by the upscale transport of momentum to the planetary scales, while the midlevel easterly jet in the westerly onset region is substantially strengthened by this process. The role of wave trains of tilted organized synoptic-scale circulations is crucial for this fidelity with observations. The appeal of the multiscale models developed below is their firm mathematical underpinnings, simplicity, and analytic tractability while remaining self-consistent with many of the features of the observational record.