bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical simulations of the July 10 Stratospheric‐Tropospheric Experiment: Radiation, Aerosols, and Ozone/Deep Convection Experiment convective system: Kinematics and transport
Authors: Skamarock William C. , Powers Jordan G. , Barth Mary , Dye James E. , Matejka Thomas , Bartels Diana , Baumann Karsten , Stith Jeffrey , Parrish David D. , Hubler Gerhard
Published: February 2004

The observed July 10, 1996, Stratospheric‐Tropospheric Experiment: Radiation, Aerosols, and Ozone (STERAO) convective system is broadly reproduced in a nonhydrostatic cloud model simulation using an idealized horizontally homogeneous sounding and no terrain. System evolution from a multicellular line to a supercell, along with line orientation, anvil structure, horizontal wind fields, depth of convection, and derived radar reflectivity, compares well with observations. Simulated passive tracer transport of CO and ozone generally agrees with aircraft measurements and shows a small amount of entrainment of environmental air in the updrafts, and a small amount of dilution occurring with transport downwind in the anvil; the entrainment and dilution are less pronounced in the supercell stage. The horizontally integrated vertical flux divergence for CO in the simulation shows a net gain at almost all levels above 8 km mean sea level (msl). The rate of increase of CO mass above 8 km varies significantly in time, with a peak at early times, followed by a decline and minimum as the system transitions to a supercell and a steady increase as the supercell matures. Trajectory analyses show that updrafts in the simulation are ingesting air from a layer spanning from 2 km to 3.5 km msl (from 0.5 to 2km above the surface). The residence times for parcels in the updraft varies from just under 10 min to more than 20 min, with most parcels taking approximately 10 min to ascend to the anvil.
Influences of vertical transport and scavenging on aerosol particle surface area and radon decay product concentrations at the Jungfraujoch (3454 m above sea level)
Authors: Lugauer M. , Baltensperger U. , Furger M. , Gäggeler H. W. , Jost D. T. , Nyeki S. , Schwikowski M.
Published: February 2004

Concentrations of the aerosol particle surface area (SA) and aerosol‐attached radon decay products 214Pb and 212Pb have been measured by means of an aerosol and a radon epiphaniometer at the Jungfraujoch research station (JFJ; 3454 m above sea level, Switzerland). These parameters exhibit a pronounced seasonal cycle with minimum values in winter and maximum values in summer. In summer, pronounced diurnal variations with a maximum at 1800 LST are often present. Highest concentrations and most pronounced diurnal variations occur during anticyclonic weather conditions in summer. Thermally driven vertical transport over alpine topography is responsible for this observation. During this synoptic condition, concentrations vary greatly with the 500 hPa wind direction, exhibiting low concentrations for NW‐N winds and high concentrations for weak or S‐SW winds. Lead‐214 and SA are highly correlated during anticyclonic conditions, indicating transport equivalence of the gaseous 214Pb precursor, 222Rn, and of aerosol particles. When cyclonic lifting is the dominant vertical transport, wet scavenging of aerosol particles can explain the weak correlation of 214Pb and SA. This conclusion is corroborated by the 214Pb/SA ratio, being twice as high during cyclonic than during anticyclonic conditions. Lead‐212 is a tracer for the influence of surface contact on a local scale due to its short lifetime of 15.35 hours. The analysis of this parameter suggests that high‐alpine surfaces play an important role in thermally driven transport to the JFJ.
Aircraft assessment of trace compound fluxes in the atmosphere with relaxed eddy Accumulation: Sensitivity to the conditions of selection
Authors: Delon C. , Druilhet A. , Delmas R. , Greenberg J.
Published: February 2004

The Relaxed Eddy Accumulation (REA) technique, implemented aboard aircraft, may be used to measure a wide variety of trace gas fluxes at a regional scale. Its principle is rather simple: air is sampled at a constant rate and the flux is calculated by multiplying a constant β (0.58 in field experiment and 0.62 in simulations) by the standard deviation of the vertical velocity and by the difference between the average concentrations of the scalar (trace gas) for updrafts and downdrafts. The storage of the chemical compound in reservoirs allows for trace gas analysis in laboratory, when in situ measurement with fast response and high sensitivity sensors are not available. The REA method was implemented on the Avion de Recherche Atmosphérique et de Télédétection aircraft during the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) campaign. The main requirement for accurate flux determination is the measurement of the vertical component of wind velocity in real time. A simulation technique was developed to evaluate the performance of an aircraft REA. The influence of the time lag between the vertical velocity (W) measurement and REA control was tested, as well as the offset of W, the threshold, and the filtering imposed on W. Correction factors, used in a deployment of aircraft REA, were deduced from this study. An additional simulation was performed to evaluate the influence of spatial or temporal drifts on the scalar. The simulation showed that the REA method is not more disturbed than the Eddy Correlation method by low frequencies of physical origin, such as topography. The REA method was used during EXPRESSO for the measurement of isoprene fluxes over the wet savanna and the evergreen rain forest.
Springtime aerosol layers in the free troposphere over Australia: Mildura Aerosol Tropospheric Experiment (MATE 98)
Authors: Rosen James , Young Stuart , Laby Jean , Kjome Norman , Gras John
Published: February 2004

A field campaign focused primarily on free tropospheric aerosol measurements over Mildura, Australia, at 34°S (Mildura Aerosol Tropospheric Experiment (MATE 98)) was conducted in the austral spring of 1998 to test for the current presence of a seasonal aerosol layering activity observed in the 1970s and to obtain additional characteristics that would lead to a better understanding of the phenomenon. Ground‐based lidar as well as balloon‐borne optical particle counters and backscattersondes with ozone sensors were employed. The results indicate that large horizontal scale layers are present and show that their structure is highly correlated with excess ozone. Particle concentrations in the layers were sufficient to measurably affect aerosol optical depth. The probable source is distant biomass burning regions, but a detailed understanding of associated smoke transport and evolution as observed over Mildura is incomplete.
A study of gas‐aerosol equilibrium and aerosol pH in the remote marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)
Authors: Fridlind A. M. , Jacobson M. Z.
Published: February 2004

A thermodynamic equilibrium model was applied to study the interactions of gas‐phase NH3, HNO3, and HCl with size‐resolved aerosols and estimate aerosol pH in the remote marine boundary layer during the First Aerosol Characterization Experiment (ACE 1). Analysis of model results and field measurements indicates that accumulation‐mode aerosols were probably in equilibrium with NH3, HNO3, and HCl simultaneously. The largest coarse‐mode aerosols did not appear to be in equilibrium with HNO3, but may have been in equilibrium with NH3 and HCl. The estimated pH of accumulation‐mode aerosols was 0–2, a function primarily of the amount of sulfate relative to sea salt present in that mode. By contrast, the estimated equilibrium pH of coarse‐mode aerosols was 2–5, a function primarily of relative humidity and gas‐phase HCl. Prior to exposure to HCl, the estimated pH of fresh sea spray aerosols was 7–9, a function primarily of relative humidity. Sensitivity tests showed that the drying of aerosols during the sampling process may have volatilized up to 30% of NH+4.
Solar energy deposition rates in the mesosphere derived from airglow measurements: Implications for the ozone model deficit problem
Authors: Mlynczak Martin G. , Garcia Rolando R. , Roble Raymond G. , Hagan Maura
Published: February 2004

We derive rates of energy deposition in the mesosphere due to the absorption of solar ultraviolet radiation by ozone. The rates are derived directly from measurements of the 1.27‐μm oxygen dayglow emission, independent of knowledge of the ozone abundance, the ozone absorption cross sections, and the ultraviolet solar irradiance in the ozone Hartley band. Fifty‐six months of airglow data taken between 1982 and 1986 by the near‐infrared spectrometer on the Solar‐Mesosphere Explorer satellite are analyzed. The energy deposition rates exhibit altitude‐dependent annual and semi‐annual variations. We also find a positive correlation between temperatures and energy deposition rates near 90 km at low latitudes. This correlation is largely due to the semiannual oscillation in temperature and ozone and is consistent with model calculations. There is also a suggestion of possible tidal enhancement of this correlation based on recent theoretical and observational analyses. The airglow‐derived rates of energy deposition are then compared with those computed by multidimensional numerical models. The observed and modeled deposition rates typically agree to within 20%. This agreement in energy deposition rates implies the same agreement exists between measured and modeled ozone volume mixing ratios in the mesosphere. Only in the upper mesosphere at midlatitudes during winter do we derive energy deposition rates (and hence ozone mixing ratios) consistently and significantly larger than the model calculations. This result is contrary to previous studies that have shown a large model deficit in the ozone abundance throughout the mesosphere. The climatology of solar energy deposition and heating presented in this paper is available to the community at the Middle Atmosphere Energy Budget Project web site at http://heat.budget.gats.inc.com.
Arctic polar stratospheric clouds observed with the Improved Limb Atmospheric Spectrometer during winter 1996/1997
Authors: Hayashida Sachiko , Saitoh Naoko , Kagawa Akiko , Yokota Tatsuya , Suzuki Makoto , Nakajima Hideaki , Sasano Yasuhiro
Published: February 2004

The newest retrieval (version 4.20) of the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) captured more than 60 polar stratospheric cloud (PSC) profiles during the winter and early spring of 1997 in the Northern Hemisphere. That winter is well known for its long‐lasting polar vortex and significant ozone loss over the Arctic. The ILAS PSC measurements were the only spaceborne measurements made on a regular basis (about 14 times daily) during that period. PSC events were selected by comparing an individual profile with a threshold value at each altitude that was defined as an average of the extinction coefficient of background aerosols plus five standard deviations. Many of the selected PSC events correspond to temperatures lower than the nitric acid trihydrate (NAT) temperature, which was calculated using nitric acid and water vapor data observed with ILAS. The correlation between the aerosol extinction coefficient and temperature shows that the extinction data increase as the temperature decreases to a point several degrees lower than the NAT temperature, suggesting the formation of particles of a supercooled ternary solution. Some of the nitric acid profiles corresponding to intense PSC events showed a decreased mixing ratio, suggesting the uptake of nitric acid in the gas‐phase into particles. The highest probability of sighting PSCs was obtained in mid‐January at an altitude of approximately 23 km, and subsequent occurrences of PSCs were found intermittently at lower altitudes until mid‐March. The 1997 Arctic winter was characterized by the prolonged appearance of PSCs until mid‐March, associated with a long‐lasting polar vortex. The PSC data presented in this paper compensate for the gap in the long‐term PSC record from space and help to reveal the chemical mechanisms that caused the Arctic ozone loss observed that season.
Radiative forcings and global warming potentials of 39 greenhouse gases
Authors: Jain Atul K. , Briegleb Bruce P. , Minschwaner K. , Wuebbles Donald J.
Published: February 2004

The radiative forcings and global warming potentials for 39 greenhouse gases are evaluated using narrowband and broadband radiative transfer models. Unlike many previous studies, latitudinal and seasonal variations are considered explicitly, using distributions of major greenhouse gases from a combination of chemical‐transport model results and Upper Atmosphere Research Satellite (UARS) measurements and cloud statistics from the International Satellite Cloud Climatology Project. The gases examined include CO2, CH4, N2O, plus a number of chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, hydrochlorocarbons, bromocarbons, iodocarbons, and perfluorocarbons (PFCs). The model calculations are performed on a 5° latitude grid from 82.5°S to 82.5°N. The radiative forcings determined by the model are then used to derive global warming potential for each of the compounds, which are compared with prior analyses. In addition, the latitudinal and seasonal dependence of radiative forcing since preindustrial time is calculated. The vertical profiles of the gases are found to be important in determining the radiative forcings; the use of height‐independent vertical distributions of greenhouse gases, as used in many previous studies, produce errors of several percent in estimated radiative forcings for gases studied here; the errors for the short‐lived compounds are relatively higher. Errors in evaluated radiative forcings caused by neglecting both the seasonal and the latitudinal distributions of greenhouse gases and atmospheres are generally smaller than those due to height‐independent vertical distributions. Our total radiative forcing due to increase in major greenhouse gas concentrations for the period 1765–1992 is 2.32 Wm−2, only 2% higher than other recent estimates; however, the differences for individual gases are as large as 23%.
Relevance of mountain waves for the formation of polar stratospheric clouds over Scandinavia: A 20 year climatology
Authors: Dörnbrack Andreas , Leutbecher Martin
Published: February 2004

A climatology of meteorological conditions necessary for the existence of polar stratospheric clouds (PSCs) over Scandinavia is presented. The frequency of low enough temperatures for synoptic‐scale PSCs is compared with the frequency of additional events when mesoscale PSCs might be induced by the adiabatic cooling in mountain waves. The climatology is based on European Centre for Medium‐Range Weather Forecasts analyses of the 20 winter seasons 1979–1980 to 1998–1999 and a simple parameterization of mesoscale stratospheric mountain wave activity. The synoptic‐scale formation potential of stratospheric clouds is determined as the fraction of time when the stratospheric minimum temperature is less than the thresholds TNAT (PSC of type I) and Tfrost (PSC of type II). The potential of additional mesoscale formation is computed by reducing the synoptic‐scale temperature on dates when stratospheric mountain wave activity is predicted. This simple approach allows an estimate of the climatological relevance of mountain waves for the existence conditions of PSCs above Scandinavia. In the climatological mean, maximum stratospheric mountain wave activity is found in January, and the maximum seasonal winter average for various levels amounts to 11% of all analysis times (i.e., approximately 3 days per month). Based on the temperature analyses the existence conditions for PSCs of type I are dominated by synoptic‐scale processes (maximum 36% of all analysis times are conducive to their existence) whereas the additional mesoscale fraction is less than 6%. Suitable conditions of ice PSCs occur less frequently. It is found that episodes of temperatures low enough for the existence of PSC of type II are controlled by mesoscale temperature anomalies induced by stratospheric mountain waves; on the synoptic scale the stratosphere above Scandinavia is almost always warmer than the threshold Tfrost.
QBO, SAO, and tropical waves in the Berlin TSM GCM: Sensitivity to radiation, vertical resolution, and convection
Authors: Nissen Katrin M. , Braesicke Peter , Langematz Ulrike
Published: February 2004

The sensitivity of tropical waves in a general circulation model (GCM) to radiation, vertical resolution, and deep convection is investigated. The implications of this for the simulation of the quasi‐biennial oscillation (QBO) and the stratopause seminannual oscillation (SAO) are examined. Varying the vertical resolution produced the greatest impact. Increasing the resolution leads to a more realistic Kelvin wave dissipation, and the westerly phase of a QBO starts to develop but is ended early by the advection of unrealistically strong easterly winds from the southern summer hemisphere. The dissipation of Kelvin waves at lower altitudes leads to a weakening of the westerly phase of the SAO. Only small, not statisically significant differences have been found between the integrations using the Kuo and the Betts‐Miller convection schemes. There is slightly more westerly momentum in the integration with the Kuo parameterization. The mass flux convection scheme of Tiedtke is unsuitable for use in the Berlin Troposphere‐Stratosphere‐Mesosphere GCM. It simulates unrealistic latent heat distributions and favors instabilities which generate artifical Kelvin waves. The radiation scheme has little impact on the dissipation of the tropical waves, but replacing the Newtonian cooling approach allows more physically consistent integrations.
Airborne observations of summertime surface features and their effect on surface albedo during FIRE/SHEBA
Authors: Tschudi Mark A. , Curry J. A. , Maslanik J. A.
Published: February 2004

Aircraft observations of the arctic surface were obtained during the recent Surface Heat Budget of the Arctic Ocean (SHEBA) and FIRE Arctic Clouds Experiment. A series of images were created from a downward looking video camera that was mounted on the underside of a research aircraft as it flew in the vicinity of the SHEBA camp, which drifted with the pack ice in the Beaufort and eastern Chukchi Seas. These data are processed to determine the distribution of melt ponds and open water during five flights in July 1998. Melt pond and open water coverage vary between 25–34% and 5–9%, respectively. Coincident observations of albedo derived from upward and downward looking broadband shortwave radiometers indicate that the albedo decreased through July until the last week, when the albedo began to rise. Pond fraction was observed to vary inversely with albedo through July. By relating the areal coverage of the surface features to the observed surface albedo, mean albedos of 0.26, 0.07, and 0.60 are found for melt ponds, open water, and pack ice, respectively.
A three‐dimensional model calculation of the ozone depletion potential of 1‐bromopropane (1‐C3H7Br)
Authors: Bridgeman C. H. , Pyle J. A. , Shallcross D. E.
Published: February 2004

A three‐dimensional chemical transport model has been used to investigate factors affecting the potential impact of a short‐lived bromine compound on lower stratospheric ozone. The model is used to calculate the ozone depletion potential (ODP) of 1‐bromopropane employing a previously used empirical approach, which depends on the lifetime of the compound and the amount reaching the stratosphere. We show that this approach may be unsuitable for very short‐lived compounds. Indeed, for a short‐lived compound the definition of the lifetime itself is ambiguous. The lifetime varies with season, region of emission, and depends on the method of calculation. A series of tracer experiments reveals that the amount of bromine reaching the stratosphere, and hence the calculated ODP, can also be highly dependent on the distribution of the surface emissions. Where emissions are located solely in the equatorial region, the calculated ODP is over 3 times greater than when the emissions are centered over Europe. Vigorous convection in the tropics can lift the compound rapidly into the lower stratosphere where the bromine can be released and contribute to ozone destruction. For surface releases at higher latitudes the lifetime in the troposphere is significant compared with the time to reach the stratosphere and a smaller ODP is calculated. This highlights a problem in calculating ODPs for short‐lived species. Uncertainties in the degradation mechanisms for short‐lived compounds, and the subsequent fate of the degradation intermediates, add further uncertainty to calculations of their impact on the stratosphere. Additional methods need to be developed to assess their potential impact on the stratosphere.
Airborne studies of cloud structures over the Arctic Ocean and comparisons with retrievals from ship‐based remote sensing measurements
Authors: Hobbs Peter V. , Rangno Arthur L. , Shupe Matthew , Uttal Taneil
Published: February 2004

Information on the heights and microphysical structures of two cloud systems derived from a 35 GHz radar, microwave and infrared radiometers, and a lidar aboard a ship in the Arctic Ocean are compared with simultaneous airborne in situ measurements. The cloud systems considered are a single layer of thin altocumulus with virga (June 3, 1998), and a more complex cloud system consisting of several altocumulus‐altostratus layers that precipitated into a boundary layer, stratus‐stratocumulus system (May 29, 1998). For the first cloud system the cloud top deduced from the 35 GHz radar was close to that measured from the aircraft. The radar detected virga below cloud base even when the virga was composed of very low concentrations of ice crystals; this prevented measurement of the cloud base height with the radar. Because of the sensitivity of the 35 GHz radar to ice crystals, cloud liquid water contents derived from it are confounded by just a few ice crystals. In the case of the second more complex cloud system, embedded cloud liquid water layers into which ice particles fell were not resolved by the radar. Consequently, although five altocumulus layers were intercepted by the aircraft, the radar display depicts a single deep precipitating system. These cases illustrate that liquid water content in mixed‐phase clouds cannot be retrieved reliably using the radar‐microwave radiometer technique.
Surface water and energy budgets in the NCEP regional spectral model
Authors: Roads J. O. , Chen S.‐C.
Published: February 2004

Characteristics of the U.S. surface water and energy budgets in the National Centers for Environmental Prediction (NCEP) regional spectral model (RSM) and the driving NCEP/NCAR (NCEPR) reanalysis are compared. The RSM is similar to the global spectral model (GSM) used in the reanalyses except that it has higher resolution over a limited area. There are, however, other important differences. Besides minor parameterization differences the RSM does not have an artificial water source to maintain the surface moisture (unlike the NCEPR), and the RSM surface water dries. Although the much larger wintertime surface water difference has only a small effect on the evaporation differences and other components of the wintertime water and energy cycles, summertime effects are greater. Relative to the NCEPR the RSM develops a consistent summertime pattern of decreased surface water, precipitation, evaporation, increased sensible heating, outgoing longwave radiation, incoming solar radiation and temperature over the western Mississippi River basin and the Gulf of Mexico, south and eastern coasts. Opposite features occur over the eastern Mississippi River basin and the U.S. West in the RSM. These hydrologically and energetically consistent RSM and NCEPR spatial differences extend to interannual variations in RSM water and energy budgets, which appear to be more strongly influenced by larger interannual surface water variations than might be discerned from the NCEPR.
A new approach to modeling tree rainfall interception
Authors: Xiao Qingfu , McPherson E. Gregory , Ustin Susan L. , Grismer Mark E.
Published: February 2004

A three‐dimensional physically based stochastic model was developed to describe canopy rainfall interception processes at desired spatial and temporal resolutions. Such model development is important to understand these processes because forest canopy interception may exceed 59% of annual precipitation in old growth trees. The model describes the interception process from a single leaf, to a branch segment, and then up to the individual tree level. It takes into account rainfall, meteorology, and canopy architecture factors as explicit variables. Leaf and stem surface roughness, architecture, and geometric shape control both leaf drip and stemflow. Model predictions were evaluated using actual interception data collected for two mature open grown trees, a 9‐year‐old broadleaf deciduous pear tree (Pyrus calleryana “Bradford” or Callery pear) and an 8‐year‐old broadleaf evergreen oak tree (Quercus suber or cork oak). When simulating 18 rainfall events for the oak tree and 16 rainfall events for the pear tree, the model over estimated interception loss by 4.5% and 3.0%, respectively, while stemflow was under estimated by 0.8% and 3.3%, and throughfall was under estimated by 3.7% for the oak tree and over estimated by 0.3% for the pear tree. A model sensitivity analysis indicates that canopy surface storage capacity had the greatest influence on interception, and interception losses were sensitive to leaf and stem surface area indices. Among rainfall factors, interception losses relative to gross precipitation were most sensitive to rainfall amount. Rainfall incident angle had a significant effect on total precipitation intercepting the projected surface area. Stemflow was sensitive to stem segment and leaf zenith angle distributions. Enhanced understanding of interception loss dynamics should lead to improved urban forest ecosystem management.
Sahara mineral dust measurements from TOMS: Comparison to surface observations over the Middle East for the extreme dust storm, March 14–17, 1998
Authors: Alpert P. , Ganor E.
Published: February 2004

A comparison of the TOMS aerosol index (AI) with both measurements of dust concentrations and synoptic data over the Middle East for the extremely heavy dust storm of March 14–17, 1998, is performed. Time series over Algeria, Libya, and Israel yield the following findings: The peak values in both surface concentrations and TOMS data suggest that AI values of 2.5 and 1.2 correspond to surface mean daily concentrations of 1900 and 1000 μg/m3 respectively. Surface concentrations and TOMS maps show that AI values of 3.0 correspond to about 4000 μg/m3. TOMS AI maximum values were found to increase from Algeria to Israel moving along with the center of the dust plume from 0.9 to 2.1 and 2.5 AI values. This seems to be in contradiction with the fact that the plume moves farther away from the major mineral dust sources over the Sahara and may be caused by either an increased vertical extension of the plume, hence increasing the TOMS AI, or due to convergence of the dust plume over the eastern Mediterranean. The size distribution, morphological, and mineralogical composition of the dust analyzed in this study, as well as other aerosol parameters, are essential for improving the remote sensing methods such as the TOMS AI algorithms. Of course, surface measurements alone do not allow the refinement of the TOMS retrievals; vertical profile dust measurements as well as other physical and optical aerosol parameters are necessary.
A detailed record of high Arctic middle atmospheric temperatures
Authors: Duck Thomas J. , Whiteway James A. , Carswell Allan I.
Published: February 2004

Four hundred and twenty‐two nights of middle atmospheric temperature observations were obtained in the High Arctic at Eureka (80°N, 86°W) during six wintertime measurement campaigns from 1992–1993 to 1997–1998 by using a lidar and meteorological balloons. The measurements reveal that temperature changes of greater than 30 K occur in the stratosphere within the space of a few weeks. In a comparison with maps of wind in the Northern Hemisphere, it is clear that much of the thermal variability is associated with movements of the wintertime stratospheric vortex over the measurement site. Measurements in the vortex core show a persistently cold lower stratosphere and warm upper stratosphere. Conversely, outside of the vortex altogether the lower stratosphere is relatively warm and the upper stratosphere is cold. By separately examining measurements obtained inside the vortex core, a 25±2 K annual warming of the upper stratosphere in late December is apparent. This annual vortex core warming is distinctly different from planetary‐wave‐driven sudden stratospheric warming and the seasonal march of temperatures due to changing insolation.
Sensitivity of stationary wave amplitude to regional changes in Laurentide ice sheet topography in single‐layer models of the atmosphere
Authors: Jackson Charles
Published: February 2004

Climate variability on millennial timescales has been observed in many geologic records covering the last glacial cycle. A potential source of this variability is the Laurentide ice sheet (LIS) in its periodic discharge of large quantities of icebergs to the North Atlantic. The present analysis considers whether regional variations in LIS topography could exert a significant influence on the atmosphere's stationary wave circulation. The maximum effect that regional changes in LIS topography have on the atmosphere's stationary wave circulation is determined using single‐layer models of the atmosphere. Model experiments measure the individual contribution of 4.5°×7.5° sections of the LIS and Greenland topography to global mean stationary wave amplitude. Results show the possibility for a limited region of topography to control a disproportionate amount of the atmosphere's total response to topography. Moreover, the possibility exists for a reduction in topographic forcing to increase stationary wave amplitude. These results can be understood by considering how the mean flow controls the horizontal propagation of wave energy and superposition of wave amplitude. The location of regions with enhanced stationary wave sensitivity to topographic alteration is found to be sensitive to mean topographic height but not mean wind strength. The latter is found to primarily affect the overall amplitude of sensitivity rather than the pattern. The impact of two hypothetical changes in LIS topography is considered, and they are found to have widely different effects on the global stationary wave field. Stationary wave sensitivity to topography within the single‐layer models suggests that variations in the size or shape of the LIS can be one factor important to climate variability on millennial timescales.
Combined differential‐integral approach for the radiation field computation in a spherical shell atmosphere: Nonlimb geometry
Authors: Rozanov Alexei V. , Rozanov Vladimir V. , Burrows John P.
Published: February 2004

A new combined differential‐integral approach (CDI) has been developed, which is more accurate than commonly used pseudospherical models but not so complicated and computer time‐consuming as a fully spherical model. The radiative transfer equation is solved in its integral form. The multiple‐scattering source function is obtained by solving the integrodifferential radiative transfer equation in a pseudospherical atmosphere. Relative differences between the new model and the pseudospherical model for a variety of solar zenith angles, viewing angles, and azimuth angles for a set of wavelengths are presented. Furthermore, the GDI model is compared with a Gauss‐Seidel spherical model (GSS) and with MODTRAN. The difference between GDI and GSS is found to be less than 2% and between GDI and MODTRAN up to 12%.
Aerosol production and growth in the upper free troposphere
Authors: de Reus M. , Ström J. , Curtius J. , Pirjola L. , Vignati E. , Arnold F. , Hansson H. C. , Kulmala M. , Lelieveld J. , Raes F.
Published: February 2004

During the Second Aerosol Characterization Experiment (ACE 2), aircraft measurements of aerosol number concentration and size distribution were performed in the upper free troposphere. Temporal changes in aerosol number concentration and size distribution were studied by repeatedly probing three different altitude levels during one single flight. The observations provide indirect evidence for particle formation and growth. Simulations with a coupled gas‐phase chemistry and aerosol dynamical model, including binary homogeneous nucleation of sulfuric acid and water vapor, suggest that the ultrafine particles observed in the early morning were most likely formed when the air mass was lifted over a cold front, one day prior to the measurements. Particle growth was studied using two different aerosol dynamical models assuming particle growth by condensation of sulfuric acid and water vapor. The models were unable to simulate the observed particle growth, which suggests that another process might be important for aerosol growth, or multicomponent condensation involving species not observed and modeled play a key role in the evolution of the aerosol size distribution.
Transpolar structure of polar mesospheric clouds
Authors: Carbary J. F. , Morrison D. , Romick G. J.
Published: February 2004

Middle‐ultraviolet (210‐ to 252‐nm) images have revealed the transpolar structure of polar mesospheric clouds (PMCs) at a spatial resolution of 3 km. The ultraviolet and visible imaging and spectrographic imaging instrument on the Midcourse Space Experiment (MSX) satellite collected over 27,000 mid‐UV images of PMCs during 26 passes over the north and south polar regions during the summer seasons of 1997, 1998, and 1999. A Lomb periodogram analysis of PMC radiance projected to an 83‐km altitude reveals periodic structures with wavelengths ranging from ∼100 to ∼3000 km. In either hemisphere, more PMCs have features with wavelengths shorter than 1000 km than longer, and a crude spectrum of the PMC structures suggests a spectral peak between 500 and 1000 km. There is little evidence of structures having wavelengths short of ∼100 km, and the longer wavelengths generally have more spectral “power” than the shorter wavelengths. PMC structures do not remain stable over time periods of weeks, but may retain similar structural features for at least as long as 24 hours. The clouds may be considered markers of gravity waves, which carry energy from the lower atmosphere to the mesosphere and modulate the appearance of PMCs.
Phase transitions of sea‐salt/water mixtures at low temperatures: Implications for ozone chemistry in the polar marine boundary layer
Authors: Koop Thomas , Kapilashrami Abha , Molina Luisa T. , Molina Mario J.
Published: February 2004

We present laboratory experiments employing differential scanning calorimetry as well as flow cell microscopy to study the microphysics of aqueous NaCl and sea‐salt solutions and droplets at temperatures below 273 K. The freezing and melting points of ice and other precipitates were determined in NaCl and sea‐salt bulk samples as well as in emulsion samples. Using flow cell microscopy, we have determined the deliquescence and efflorescence relative humidities of NaCl and sea‐salt droplets at temperatures between 249 and 273 K, extending the existing room temperature data to polar conditions. Our measurements suggest that sea‐salt aerosols will most likely be liquid most of the time under polar marine boundary conditions. In addition, we show that sea‐salt aerosols or seawater spray deposited on the polar ice pack will remain partly liquid down to 230 K, with concentrations of Cl− and Br− increasing by more than an order of magnitude upon cooling when compared to normal seawater concentrations. This is likely to enhance the rate at which heterogeneous bromine activation reactions occur in the sea‐salt deposits. Such reaction rate enhancements with decreasing temperatures are currently not implemented in chemical models, and might help explain the fast bromine activation and subsequent ozone destruction observed during ozone depletion events in the polar marine boundary layer in spring.
Comparison of scaling parameters from spatial and temporal distributions of cloud properties
Authors: Feijt Arnout , Jonker Harm
Published: February 2004

The most common method to evaluate the quality of cloud parameter retrievals from satellite data is comparison with time series of ground‐based measurements. For highly variable cloud parameters, such as liquid water path (LWP), however, a direct comparison of spatial and temporal distributions has limited value. It is questionable which period in the time series is representative of an area in the satellite image. A necessary but not sufficient boundary condition for statistical analysis is that the variability of both subsets is similar. In this paper we study the variability in terms of scaling properties of both spatial distributions and timeseries of LWP and focus on the relation between them. The time series are obtained from ground‐based microwave radiometer measurements at 1 Hz. The spatial distributions are derived from Advanced Very High Resolution Radiometer 0.6 μm radiances. It is shown that the scaling properties of both distributions are similar for a stratocumulus field, which exhibits scale invariance, and for a cumulus field for which scales of preference were identified. A double logarithmic representation was appropriate to obtain the spectral exponent. A log‐linear representation was appropriate to identify a scale of preference. In the cumulus case it was possible to calculate a feasible conversion factor to map variability in time to variability in space. This enables an estimation of the spatial variability at very high resolutions.
Study of mesosphere lower thermosphere (MLT) mean winds over Yamagawa (31.2°N, 130.6°E) during 1996–1998
Authors: Kishore P. , Namboothiri S. P. , Igarashi K.
Published: February 2004

The mean motions in the mesosphere and lower thermosphere from 70 to 98 km were observed with the recently installed MF radar at Yamagawa (31.2°N, 130.6°E). We have utilized the continuous wind data collected during the 3‐year period 1996–1998. Seasonal variations of mean winds over the site are constructed and the climatologies are then compared with observations from similar latitudes and the latest empirical model results. The mean zonal circulation in the mesosphere is characterized by a clear annual variation with summer westward (easterly) flow and winter eastward (westerly) flow. The summer westward jet closes at a height in between 80 and 85 km. Southward motion exhibits its dominance at 90–98 km during all times of the year, and at lower heights (70–90 km), southward/northward motions predominate during summer/winter season. The annual mean zonal wind has an eastward motion in the 70–98 km. It maximizes at around 90 km with a strength of 15–17 m/s. The annual mean meridional circulation shows both poleward and equatorward motions. Harmonic analysis of mean zonal winds clearly demonstrates the dominance of annual oscillation over the other periods. Comparison of mean winds over Yamagawa, Japan, and London, Ontario, Canada, demonstrates that the zonal circulation over these two sites has some similar features. Similarly, the results compared for Yamagawa and Urbana show some consistency in mean wind circulation. The paper also presents the results of the comparison of MF radar‐derived winds and the HWM93 model outputs. There is reasonable agreement between the observation and the model values.
An investigation of boundary‐forcing sensitivities in a regional climate model
Authors: Lynch Amanda H. , Cullather Richard I.
Published: February 2004

The sensitivity of a regional climate model to lateral, upper, and lower dynamical forcing is evaluated for a pan‐Arctic domain. The experiments focus on a month‐long case study of May 1990, but the results are shown to be extendable to January and July 1990. Although the control simulation is found to adequately represent major tropospheric circulation features, a large negative geopotential height bias of up to 700m is found at the upper levels of the model. Experiments find these biases are remedied through the replacement of a reflective rigid lid upper boundary condition with a local wave‐absorbing layer. Geopotential height biases are reduced to a maximum underestimation of only 50m at the upper boundary. Substantial improvement is also found in the simulated temperature and zonal wind profiles, transient heat and momentum fluxes, and stationary eddies as seen through the Eliassen‐Palm flux divergence. With this wave‐absorbing upper boundary condition in place, additional improvement is found using a gravity wave drag parameterization appropriate for this domain's 100km horizontal resolution. Additional sensitivity to lateral forcing is examined, and the relative role of the upper boundary condition to skillful simulations within the Arctic basin domain is discussed.
Anisotropic reflection of UV radiation at the top of the atmosphere: Characteristics and models obtained from Meteor 3/TOMS
Authors: Pubu Ciren , Li Zhanqing
Published: February 2004

Reflection of solar radiation usually exhibits a strong dependence on viewing geometry relative to the Sun's position. Such a dependence needs taking into account in remote sensing studies employing satellite measurements made with scanning radiometers. While many investigations have been conducted concerning the angular variation of radiation in the total solar, visible, and IR wavelengths, no study was devoted to the UV radiation whose dependence on viewing geometry may differ from radiation in other spectral regions. On the basis of two and half years of Meteor‐3/TOMS data (from January 1992 to May 1994) the angular characteristics of UV radiation reflected at the top of the atmosphere (TOA) were studied extensively. A set of angular dependence models (ADMs) was developed for UV radiation at 340, 360, and 380 nm over 10 scene types. The results show that the angular dependence of UV‐reflected radiation is strong and rather different from that for total shortwave (SW) radiation as derived for the Earth Radiation Budget Experiment (ERBE). The largest discrepancy occurs under clear conditions over oceans for overhead Sun and large viewing zenith angles. The discrepancy in TOA mean albedos derived using the two sets of ADM can reach up to 48%. The discrepancy diminishes to about 10% for clear land scenes. Comparisons were also made for regional monthly mean TOA albedos over the whole globe derived from the ERBE SW ADM, TOMS UV ADM, and the assumption of isotropic reflection from an independent data set acquired by Nimbus 7/TOMS in January and July 1991. The differences exhibit an apparent zonal trend, large at high latitudes, moderate at midlatitudes, and small in the tropics.
The stable carbon isotope fractionation for reactions of selected hydrocarbons with OH‐radicals and its relevance for atmospheric chemistry
Authors: Rudolph J. , Czuba E. , Huang L.
Published: February 2004

Measurements of the kinetic isotope effect (KIE) for the reactions of light n‐alkanes as well as for several unsaturated hydrocarbons, including alkenes, dienes, benzene, and ethyne with OH‐radicals are presented. All measured KIEs are positive; that is, molecules containing only C‐12 react faster than the C‐13 labeled molecules. However, the KIEs for n‐alkanes are quite small; between one and four permil. They can be explained mainly by the mass dependence of the collision frequency between the n‐alkanes and OH‐radicals. KIEs for the reaction of alkenes with OH‐radicals are considerably higher. They can be explained by a fractionation of 24.5±1.1‰ for the addition of an OH‐radical to a double bond. Inverse dependence on number of carbon atoms and mass dependence of the collision frequencies explain our observations. For benzene the KIE is slightly higher; for ethyne it is somewhat lower than expected from this simple model. For the reaction of many light nonmethane hydrocarbons (NMHC), especially of unsaturated hydrocarbons, with OH‐radicals the KIEs are sufficiently large to have significant impact on the isotopic composition of atmospheric NMHC. A small series of stable carbon isotope ratio measurements of atmospheric NMHC were made in the greater Toronto area. Traffic related NMHC emissions were also studied for their stable carbon isotope ratios. From these data it is possible to quantitatively determine the extent of photochemical processing due to OH‐radical reactions that the individual NMHC has experienced. Thus such measurements allow quantitative evaluation of the extent of chemical processing the different NMHC have gone through. This also includes the possibility to differentiate between the impact of local sources and regional or large scale transport. It is shown that in combination with concentration measurements isotope ratio measurements are extremely valuable to study the complex interaction between chemical removal mechanisms, mixing, and dilution processes.
Ozone episode analysis by four‐dimensional variational chemistry data assimilation
Authors: Elbern Hendrik , Schmidt Hauke
Published: February 2004

A chemical four‐dimensional variational data assimilation system has been developed and applied for the study of an episode with enhanced summerly ozone levels. In this study the optimization parameters are the initial values of the chemical constituents. The case study focuses on an ozone episode over central Europe during August 1997. The associated observational database consists mainly of surface observations of ozone and nitrogen oxides, but also a limited number of ozone radiosonde records. The four‐dimensional data assimilation algorithm is composed of the chemistry transport model, its adjoint model with the adjoint versions of the Regional Acid Deposition Model (RADM2) chemical mechanism, horizontal and vertical advection schemes, implicit vertical diffusion, and a limited memory quasi‐newton minimization routine. The underlying model of the spatio‐temporal data assimilation scheme is the comprehensive mesoscale‐α EUropean Air pollution Dispersion model (EURAD), which is based on the RADM2 gas phase mechanism. On the basis of a 6 hours data assimilation interval, analyses of the chemical state of the atmosphere were obtained, where the skill is verified in two different ways: (1) improvements of forecasts subsequent to the assimilation procedure, and (2) validation of the analyses with observational data which is withheld from the variational data assimilation algorithm. It is demonstrated that significant improvements are achieved for short‐term forecasts including the afternoon ozone peak abundances. The analysis skill is further corroborated by examinations with retained measurement data.