bluuebunny/crossref_metadata_2025 huggingface dataset:
Microwave Simulations of a Tropical Rainfall System with a Three-Dimensional Cloud Model
Authors: Adler Robert F. , Yeh Hwa-Young M. , Prasad N. , Tao Wei-Kuo , Simpson Joanne
Published: August 2010

A three-dimensional cloud model-microwave radiative transfer model combination is used to study the relations among the precipitation and other microphysical characteristics of a tropical oceanic squall line and the upwelling radiance at pertinent microwave frequencies. Complex brightness temperature-rain rate relations are evident at the full horizontal resolution (1.5 km) of the models, with spatial avenging producing smoother, shifted relations, in most cases. Nonprecipitating cloud water is shown to be important in understanding the resulting distribution of brightness temperature. At the mature stage, convective portions of the cloud system are shown to produce different brightness temperature relations than the stratiform portion, primarily related to the distribution of cloud water. The evolution of the convective system from a small convective complex through its mature stage and the beginning of its dissipation also is shown to result in a variation of brightness temperature-rain relations, related to the distribution of cloud water and the evolution of ice in the precipitating system. The results of the study paint to the need to take into account the evolution of nonprecipitating cloud water and precipitation-sized ice in the retrieval of rain team from microwave space observations. This effect is evident for both the life cycle of individual convective elements and the life cycle of the convective system as a whole.
Information Content of AVHRR Channels 4 and 5 with Respect to the Effective Radius of Cirrus Cloud Particles
Authors: Parol F. , Buriez J. C. , Brogniez G. , Fouquart Y.
Published: August 2010

This paper investigates the important difference in the relationship between brightness temperatures between the 11-μm and the 12-μn AVHRR data and the microphysical properties of the semitransparent cirrus clouds. In the nonscattering approximation, the emittance for channels 4 and 5 are related through the absorption coefficient ratio that is the key parameter giving access to the size of cloud particles. The observed mean value of this parameter corresponds to effective radius of 18 μm for polydisperse spheres and 12 μm for polydisperse infinitely long ice cylinders. Taking the multiple scattering into account, the brightness temperature difference enhances much more for cylinders than for spheres owing to the fact that the forward peak of scattering is less large for cylinders. To obtain the size of cloud particles, the method developed in the nonscattering case is still applicable if one makes use of the effective emittance that implicitly includes the effects of mattering. Thus, an effective absorption coefficient ratio is defined and we derive a direct relationship between this ratio and the optical properties of the cloud particles. The mean value of the effective absorption coefficient ratio corresponds to ice spheres of effective radius of 26 μm or a bit less in the case of water spheres (supercooled droplets), but no agreement can be obtained for fully randomly oriented cylinders.
A New Parameterization Scheme for Shortwave Radiative Properties of Water Clouds
Authors: Zhang Guotong , Xu Lisheng , Chen Hongbin
Published: August 2010

A new parameterization scheme for the shortwave radiative properties of water clouds is presented, using versatile cloud drop size distributions ( DSDs). As for single-scattering properties, a new parameterization for cloud optical thickness is proposed by using the separation of the dependence of r on the total number of theDSDs, the cloud thickness, and the liquid water content, combined with equivalent radius. The cloud bulk radiative properties are obtained from the delta-Eddington approximation for our cloud models. It is shown that the flux reflectivity, transmissivity, and absorptivity are uniquely fitted by a dimensionless parameter X or the cloud optical thickness 7. The parameterization is compared with other schemes. The features and potential use of the scheme are discussed.
Assessment of Seeding Effects in Snowpack Augmentation Programs: Ice Nucleation and Scavenging of Seeding Aerosols
Authors: Warburton J. A. , Young L. G. , Stone R. H.
Published: August 2010

Trace chemical analysis techniques have been used in a series of cloud-seeding experiments in the central Sierra Nevada with the ultimate purpose of distinguishing whether the submicron-sized aerosol particles used for seeding are removed by nucleation or by scavenging in snowfall. The research programs used submicron-sized seeding aerosols with different nucleating characteristics. When winter storms were seeded with silver iodide in the Lake Tahoe and Lake Almanor watersheds, positive correlations were observed between silver concentrations and precipitation amounts in both catchment areas.This is considered to be evidence that the AgI aerosols are not being removed in the snowfall entirely by scavenging processes. When two separate aerosols of silver iodide and indium sesquioxide were released simultaneously from the same ground locations during winter snowstorms in the Lake Almanor watershed, it was found that considerably more of the ice-nucleating aerosol particles (AgI) were removed by the snowfall than the non-ice-nucleating ones (In203). Under the experimental conditions employed, scavenging alone of the two aerosols would lead to a chemical ratio of Ag:In in the snowfall of 0.83:l. Ratios as high as 17.2:l were observed, the mean ratio being 4: I. These results are considered to be evidence of the removal of substantial numbers of the AgI aerosol particles through direct nucleation of ice crystals.
Numerical Simulation of a Rain Shower Affected by Waste Energy Released from a Cooling Tower Complex in a Calm Environment
Authors: Guan S. , Reuter G. W.
Published: August 2010

An axisymmetric cloud model is used to investigate the evolution of convective cells and associated rain showers that develop due to the sensible and latent heat released into a calm atmosphere from an industrial cooling complex. The simulated convection is in fair agreement with observations for a particular cloud developingover a large cooling system. Sensitivity experiments were run for two different soundings to quantify how the convection and rainfall depend on the magnitude of the waste energy loss, the area of the source region, and other parameters. The major findings were the following:1 ) Without a sustained waste heat input the model convection ceased. The model convection became progressively more intense and developed more rain when the rate of total waste energy heat released from thepower station was increased. Doubling the total waste energy amount from its control case value caused a 15-fold increase in 3-h rainfall values. However, reducing the control case value of total waste energy release by 20% led to much weaker convection without rain.2) The triggering of cumulus clouds was reduced when the waste energy was released in terms of latent heat rather than sensible heat. As the relative contribution of sensible heat increased, so did the intensity of the circulation and the rainfall.3) The cloud formation depended on the area of the heat source: a wider source tended to delay and to weaken the convection. Specifically, when the area was doubled the simulated cloud appeared 2.5 min laterand the total accumulated rain after 3 h was reduced by 88%. Moreover, the maximum total kinetic energy was reduced by 26%.4) The model results were only slightly sensitive to the radial distribution of the sensible and latent heatfluxes within the source region.
Automated Quality Control Procedure for the "Water Equivalent of Snow on the Ground" Measurement
Authors: Schmidlin Thomas W. , Wilks Daniel S. , McKay Megan , Cember Richard P.
Published: August 2010

Snow water equivalent (SWE) has been measured daily by the United States National Weather Service since 1952, whenever snow depth is 2 in. (5 cm) or greater. These data are used to develop design snow loads for buildings, for hydrological forecasting, and as an indicator of climate change. To date they have not been subjected comprehensively to quality control. An automated quality control procedure for these data is developed here, which checks daily SWE values for common data entry errors, values beyond reasonable limits, and consistency with daily precipitation and estimated melt. Potential effects of drifting in high winds and of the intrinsic microscale variability of SWE are also considered. An SWE measurement is declared suspicious if a sufficient discrepancy is found with respect to the expected SWE. Data values flagged as potential errors are checked manually. Results of applying the procedure to available SWE data from the northeastern UnitedStates are also summarized.
Development and Testing of a Surface Flux and Planetary Boundary Layer Model for Application in Mesoscale Models
Authors: Pleim Jonathan E. , Xiu Aijun
Published: August 2010

Although the development of soil, vegetation, and atmosphere interaction models has been driven primarily by the need for accurate simulations of long-term energy and moisture budgets in global climate models, the importance of these processes at smaller scales for short-term numerical weather prediction and air quality studies is becoming more appreciated. Planetary boundary layer (PBL) development is highly dependent on the partitioning of the available net radiation into sensible and latent heat fluxes. Therefore, adequate treatmentof surface properties such as soil moisture and vegetation characteristics is essential for accurate simulation of PBL development, convective and low-level cloud processes, and the temperature and humidity of boundary layer air. In this paper, the development ofa simple coupled surface and PBL model, which is planned for incorporation into the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM4/5), is described. The soil-vegetation model is based on a simple force-restore algorithm with explicit soil moisture and evapotranspiration. The PBL model is a hybrid of nonlocal closure for convective conditions and eddy diffusion for all other conditions. A one-dimensional version of the model has been applied to several case studies from field experiments in both dry desert-like conditions (Wangara) and moist vegetated conditions(First International Satellite Land Surface Climatology Project Field Experiment) to demonstrate the model's ability to realistically simulate surface fluxes as well as PBL development. This new surface-PBL model is currently being incorporated into the MM4-MM5 system.
Cloud Model-Based Simulation of Spaceborne Radar Observations
Authors: Yeh H-Y. M. , Prasad N. , Meneghini R. , Tao W-K. , Jones J. A. , Adler R. F.
Published: August 2010

Simulations of observations from potential spaceborne radars are made based on storm structure generated from the three-dimensional (3D) Goddard cumulus ensemble model simulation of an intense overland convective system. Five frequencies of 3, 10, 14, 35, and 95 GHz are discussed, but the Tropical Rainfall Measuring Mission precipitation radar sensor frequency ( 14 GHz) is the focus of this study. Radar reflectivities and their attenuation in various atmospheric conditions are studied in this simulation. With the attenuation from cloud and precipitation in the estimation of reflectivity factor (dBZ), the reflectivities in the lower atmosphere in the convective coresare significantly reduced. With spatial resolution of 4 km X 4 km, attenuation at 14 GHz may cause as large as a 20-dBZ difference between the simulated measurements of the peak (Zmp) and near-surface reflectivity (Zmp) in the most intense convective region. The Zmp occurs at various altitudes depending on the hydrometeor concentrations and their vertical distribution. Despite the significant attenuation in the intense cores, the presence of the rain maximum is easily detected by using information of Zmp. In the stratiform region, the attenuation is quite limited (usually less than 5 dBZ), and the reduction of reflectivity is mostly related to the actual vertical structure of cloud distribution. Since Zmp suffers severe attenuation and tends to underestimate surface rainfall intensity in convective regions, Zmp can be more representative for rainfall retrieval in the lower atmosphere in these regions. In the stratiform region where attenuation is negligible, however, Zmp tends to overestimate surface rainfall and Zmp is more appropriate for rainfall retrieval. A hybrid technique using a weight between the two rain intensities is testedand found potentially useful for future applications. The estimated surface rain-rate map based on this hybrid approach captures many of the details of the cloud model rain field but still slightly underestimates the rain-rate maximum.
Improvements of an Ice-Phase Microphysics Parameterization for Use in Numerical Simulations of Tropical Convection
Authors: Krueger Steven K. , Fu Qiang , Liou K. N. , Chin Hung-Neng S.
Published: August 2010

It is important to properly simulate the extent and ice water content of tropical anvil clouds in numerical models that explicitly include cloud formation because of the significant effects that these clouds have on the radiation budget. For this reason, a commonly used bulk ice-phase microphysics parameterization was modified to more realistically simulate some of the microphysical processes that occur in tropical anvil clouds. Cloud ice growth by the Bergeron process and the associated formation of snow were revised. The characteristics of graupel were also modified in accord with a previous study. Numerical simulations of a tropical squall line demonstrate that the amount of cloud ice and the extent of anvil clouds are increased to more realistic values by the first two changes.
Turbulent Mixing in Stably Stratified Shear Flows
Authors: Schumann U. , Gerz T.
Published: August 2010

Vertical mixing of momentum and heat is investigated in turbulent stratified shear flows. It is assumed that the flow has uniform shear and stratification with homogeneous turbulence and that an equilibrium is reached between kinetic and potential energy without gravity wave oscillations. A simple model is derived to estimate vertical diffusivities for Richardson numbers in between 0 and about 1. The model is based on the budgets of kinetic and potential energy and assumes a linear relationship between dissipation, shear, and vertical velocity variance for closure. Scalar fluctuations are related to shear or buoyancy frequency depending on the Richardson number. The turbulent Prandtl number and the growth rate of kinetic energy are specified as functions of this number. Model coefficients are determined mainly from laboratory measurements. Data from large-eddy simulations are used to determine the "stationary" Richardson number with balanced shear production, dissipation, and buoyancy terms. The results of the model are compared with data from laboratory experiments in air or saltwater, with measurements in the atmospheric boundary layer and in the stable troposphere, and with results from the numerical simulations. The model interpolates the observations within the scatter of the data. Theanalysis shows intrinsic relationships between several mixing parameters.
Description of Wind Field Dynamic Patterns in a Valley and Their Relation to Mesoscale and Synoptic-Scale Meteorological Situations
Authors: Guardans R. , Palomino I.
Published: August 2010

A large set of hourly meteorological data from seven towers deployed in Montesina Valley (Cordoba, Spain) is studied in relation to the prevailing synoptic situation. The complete collection of daily synoptic maps for 1985-90 has been classified in seven basic weather patterns. First-order transition probabilities and weatherpattern persistences have been calculated for each of the patterns. The behavior of the local valley wind field is described as a function of the synoptic patterns. The work reported here, based on observations of the characteristic time and space patterns of flow in the valley under different synoptic conditions, has made it possible to obtain a set of empirical rules and regression functions to produce forecasts of the local wind field as a function of the synoptic situation and the hour of the day to be used in the emergency plans. The result of this work is currently being implemented in an atmospheric dispersion module of an expert system that will be used as a tool to predict the evolution of accidental and routine hazardous emissions to the atmosphere in complex terrain such as valleys and coastal areas.
Aggregation of Selected Three-Day Periods to Estimate Annual and Seasonal Wet Deposition Totals for Sulfate, Nitrate, and Acidity. Part I: A Synoptic and Chemical Climatology for Eastern North America
Authors: Brook Jeffrey R. , Samson Perry J. , Sillman Sanford
Published: August 2010

Running 3-day periods from 1979 to 1985 were categorised into one of 20 meteorological categories. These categories were developed through the cluster analysis of 3-day progressions of 85-kPa wind flow over eastern North America. The purpose for developing the categories was to identify recurring atmospheric transport patterns that were associated with differing amounts of wet sulfate (SO2−4) and nitrate (NO−3) deposition at a variety of locations in eastern North America. Identification of these patterns was necessary to facilitate the selection of time periods for simulation by the Regional Acid Deposition Model and in the development of a method for estimating long-term acidic deposition over eastern North America from a limited number of model runs. The effectiveness of this method (referred to as the aggregation method) was expected to be dependent on the ability of the categories to separate structure in wet deposition patterns. This paper describes the determination of the 20 meteorological categories and demonstrates that there were differences in their meteorological and chemical behavior and in their frequency of occurrence. Observations of precipitation and wet SO2−4 and NO−3 deposition from 22 sites in eastern North America and multiple regression models were used to demonstrate that there were statistically significant differences in deposition among categories and that knowledge of meteorological category explained some of the variation in wet deposition. The best statistical correlation, which was based upon precipitation amount, time of year, and meteorological category, explained 35%–83% (28%– 76%) of the observed variation in wet SO2−4 (NO−3) deposition depending on location. On average, across all sites and for both SO2−4 and NO−3, knowledge of category accounted for about 4% of the variation. The minimum amount explained by category was 1% and the maximum was 13%.
Aggregation of Selected Three-Day Periods to Estimate Annual and Seasonal Wet Deposition Totals for Sulfate, Nitrate, and Acidity. Part II: Selection of Events, Deposition Totals, and Source–Receptor Relationships
Authors: Brook Jeffrey R. , Samson Perry J. , Sillman Sanford
Published: August 2010

A method for deriving estimates of long-term acidic deposition over eastern North America based on a limited number of Regional Acid Deposition Model runs has been developed. The main components of this method are the identification of a representative sample of events for model simulation and the aggregation of the deposition totals associated with the events. Meteorological categories, defined according to 3-day progressions of 850-mb wind flow over eastern North America, were used to guide the selection of events. This paper describes how events were selected from the categories and how they were combined (aggregated) to estimate long-term deposition. The effectiveness of the category-based approach was compared against alternate aggregation approaches and it was found to provide the best sample-based estimates of long-term wet sulfate deposition across eastern North America.

Thirty events from the 1982–85 time period were selected using a set of predetermined criteria and aggregated to estimate seasonal and annual SO2−4, NO−3, and H+ deposition at 20 Utility Acid Precipitation Study Program sites. The accuracy of the estimates varied geographically and depending upon whether they were for the annual or seasonal time periods. Over the main area of interest (a smaller 13-site region), the mean rms errors for annual deposition were 10%, 15%, and 12% for sulfate, nitrate, and acidity, respectively. Source–receptor relationships associated with the 30 events were examined for three sites located in Michigan, North Carolina, and upstate New York. It was found that the amount of time that transport was to these areas from the U.S. Midwest (an area of high SO2 emissions) was represented to within 20%.
Estimating the Urban Bias of Surface Shelter Temperatures Using Upper-Air and Satellite Data. Part I: Development of Models Predicting Surface Shelter Temperatures
Authors: Epperson David L. , Davis Jerry M. , Bloomfield Peter , Karl Thomas R. , McNab Alan L. , Gallo Kevin P.
Published: August 2010

Multiple regression techniques were used to predict surface shelter temperatures based on the time period 1986–89 using upper-air data from the European Centre for Medium-Range Weather Forecasts to represent the background climate and site-specific data to represent the local landscape. Global monthly mean temperature models were developed using data from over 5000 stations available in the Global Historical Climate Network (GHCN). Monthly maximum, mean, and minimum temperature models for the United States were also developed using data from over 1000 stations available in the U.S. Cooperative (COOP) Network and comparative monthly mean temperature models were developed using over 1150 U.S. stations in the GHCN. Initial correlation analyses revealed that data from 700 mb were sufficient to represent the upper-air or background climate. Three-, six-, and full-variable models were developed for comparative purposes. Inferences about the variables selected for the various models were easier for the GHCN models, which displayed month-to-month consistency in which variables were selected, than for the COOP models, which were assigned a different list of variables for nearly every month. These and other results suggest that global calibration is preferred because data from the global spectrum of physical processes that control surface temperatures are incorporated in a global model. All of the models that were developed in this study validated relatively well, especially the global models. Recalibration of the models with validation data resulted in only slightly poorer regression statistics, indicating that the calibration list of variables was valid. Predictions using data from the validation dataset in the calibrated equation were better for the GHCN models, and the globally calibrated GHCN models generally provided better U.S. predictions than the U.S.-calibrated COOP models. Overall, the GHCN and COOP models explained approximately 64%–95% of the total variance of surface shelter temperatures, depending on the month and the number of model variables. The R2's for the GHCN models ranged between 0,86 and 0.95, whereas the R2's for the COOP models ranged between 0.64 and 0.92. In addition, root-mean-square errors (rmse's) were over 3°C for GHCN models and over 2°C for COOP models for winter months, and near 2°C for GHCN models and near 1.5°C for COOP models for summer months. The results of this study—a large amount of explained variance and a relatively small rmse—indicate the usefulness of these models for predicting surface temperatures. Urban landscape data are incorporated into these models in Part II of this study to estimate the urban bias of surface ternperatures.
Estimating the Urban Bias of Surface Shelter Temperatures Using Upper-Air and Satellite Data. Part II: Estimation of the Urban Bias
Authors: Epperson David L. , Davis Jerry M. , Bloomfield Peter , Karl Thomas R. , McNab Alan L. , Gallo Kevin P.
Published: August 2010

A methodology is presented for estimating the urban bias of surface shelter temperatures due to the effect of the urban heat island. Multiple regression techniques were used to predict surface shelter temperatures based on the time period 1986–89 using upper-air data from the European Centre for Medium-Range Weather Forecasts to represent the background climate, site-specific data to represent the local landscape, and satellite-derived data—the normalized difference vegetation index (NDVI) and the Defense Meteorological Satellite Program (DMSP) nighttime brightness data—to represent the urban and rural landscape. Local NDVI and DMSP values were calculated for each station using the mean NDVI and DMSP values from a 3 km × 3 km area centered over the given station. Regional NDVI and DMSP values were calculated to represent a typical rural value for each station using the mean NDVI and DMSP values from a 1° × 1° latitude–longitude area in which the given station was located. Models for the United States were then developed for monthly maximum, mean, and minimum temperatures using data from over 1000 stations in the U.S. Cooperative Network and for monthly mean temperatures with data from over 1150 stations in the Global Historical Climate Network. Local biases, or the differences between the model predictions using the observed NDVI and DMSP values, and the predictions using the background regional values were calculated and compared with the results of other research. The local or urban bias of U.S. temperatures, as derived from all U.S. stations (urban and rural) used in the models, averaged near 0.40°C for monthly minimum temperatures, near 0.25°C for monthly mean temperatures, and near 0.10°C for monthly maximum temperatures. The biases of monthly minimum temperatures for individual stations ranged from near −1.1°C for rural stations to 2.4°C for stations from the largest urban areas. There are some regions of the United States where a regional NDVI value based on a 1° × 1° latitude–longitude area will not represent a typical “rural” NDVI value for the given region, Thus, for some regions of the United States, the urban bias of this study may underestimate the actual current urban bias. The results of this study indicate minimal problems for global application once global NDVI and DMSP data become available. It is anticipated that results from global application will provide insights into the urban bias of the global temperature record.
Mobile Microwave Radiometer Observations: Spatial Characteristics of Supercooled Cloud Water and Cloud Seeding Implications
Authors: Huggins Arlen W.
Published: August 2010

Previous studies of the spatial distribution of supercooled liquid water in winter storms over mountainous terrain were performed primarily with instrumented aircraft and to a lesser extent with scans from a stationary microwave radiometer. The present work describes a new technique of mobile radiometer operation that was successfully used during numerous winter storms that occurred over the Wasatch Plateau of central Utah to determine the integrated depth of cloud liquid water relative to horizontal position on the mountain barrier. The technique had the advantage of being able to measure total liquid from the terrain upward, without the usual terrain avoidance problems that research aircraft face in cloudy conditions. The radiometer also collected data during several storms in which a research aircraft could not be operated because of severe turbulence and icing conditions.

Repeated radiometer transects of specific regions of the plateau showed significant variability in liquid water depth over 30–60-min time periods, but also revealed that the profile of orographically generated cloud liquid was consistent, regardless of the absolute quantities. Radiometer liquid depth generally increased across the windward slope of the plateau to a peak near the western edge of the plateau top and then decreased across the relatively flat top of the plateau. These observations were consistent with regions where maximum and minimum vertical velocities were expected, and with depiction of cloud liquid by accretional ice particle growth across the mountain barrier. A comparison of data from the mobile radiometer and a stationary radiometer verified the general decrease in liquid depth from the windward slope to the top of the plateau and also showed that many liquid water regions were transient mesoscale features that moved across the plateau.

Implications of the results, relative to the seeding of orographic clouds, were that seeding aerosols released from valley-based generators could at times be inhibited by stable conditions from reaching appropriate super-cooled liquid water regions and, as found by others, the region of cloud most likely to be encountered by an AgI seeding agent released from the ground was also relatively warm compared to the ice-forming capability of the particular agent used in these experiments. Also, one convective case study that exhibited relatively warm temperatures in the cloud layer indicated that, even in conditions that permit vertical transport to supercooled liquid zones, sufficient time for ice particle growth and fallout from seeded plumes on this plateau may be lacking.
Considerations for Modeling Thin Cirrus Effects via Brightness Temperature Differences
Authors: Schmidt E. O. , Arduini R. F. , Wielicki B. A. , Stone R. S. , Tsay S-C.
Published: August 2010

Brightness temperature difference (BTD) values are calculated for selected Geostationary Operational Environmental Satellite (GOES-6) channels (3.9, 12.7 µm) and Advanced Very High Resolution Radiometer channels (3.7, 12.0 µm). Daytime and nighttime discrimination of particle size information is possible given the infrared cloud extinction optical depth and the BTD value. BTD values are presented and compared for cirrus clouds composed of equivalent ice spheres (volume, surface area) versus randomly oriented hexagonal ice crystals. The effect of the hexagonal ice crystals is to increase the magnitude of the BTD values calculated relative to equivalent ice sphere (volume, surface area) BTDs. Equivalent spheres (volume or surface area) do not do a very good job of modeling hexagonal ice crystal effects on BTDs; however, the use of composite spheres improves the simulation and offers interesting prospects. Careful consideration of the number of Legendre polynomial coefficients used to fit the scattering phase functions is crucial to realistic modeling of cirrus BTDs. Surface and view-angle effects are incorporated to provide more realistic simulation.
Geometrical Thickness, Liquid Water Content, and Radiative Properties of Stratocumulus Clouds over the Western North Pacific
Authors: Hayasaka Tadahiro , Nakajima Teruyuki , Fujiyoshi Yasushi , Ishizaka Yutaka , Takeda Takao , Tanaka Masayuki
Published: August 2010

An algorithm was developed for retrieving cloud geometrical thickness from a measured liquid water path and equivalent width of 0.94-µm water vapor absorption band. The algorithm was applied to aircraft observations obtained by a microwave radiometer and a spectrometer in the winter of 1991 over the western North Pacific Ocean. Retrieved values of the cloud geometrical thickness are apt to be smaller than those observed by eye, especially for horizontally inhomogeneous clouds. Measured cloud albedos in the visible and near-infrared spectral region were also compared with calculated values. For homogeneous clouds there exists a single droplet size distribution that satisfies both spectral regions. However, for inhomogeneous clouds no single size distribution exists that satisfies the albedo observed in both spectral regions.
Measurement of Vertical Liquid Water Path by Means of an Airborne Radiometer and the Shortwave Albedo of Marine Low-Level Clouds during WENPEX in Japan
Authors: Fujiyoshi Y. , Ishizaka Y. , Takeda T. , Hayasaka T. , Tanaka M.
Published: August 2010

Special observations were made over the southwest island area of the East China Sea from 12 to 27 January 1991 as part of the World Climate Research Program in Japan (WENPEX—Western North Pacific Cloud–Radiation Experiment). Two aircraft were used to determine the air truth of the total vertical liquid water path (LWP) using a microwave radiometer. One airplane was fitted with a 37-GHz radiometer and flew above planetary boundary layer clouds. The other flew inside the clouds with a cloud droplet spectrometer. These aircraft flew simultaneously along the same flight path when planetary boundary layer clouds were formed over the warm sea during an outbreak of cold air.

The result of the air truth of the LWPradiometer indicates that the 37-GHZ microwave radiometer gives an estimation of the LWP accurate to 100 mg cm−2. The shortwave cloud albedo was related to the LWPradiometer. The albedo increases with the LWP, independent of cloud type, when measured just above the cloud tops. The measured albedo is nearly the same as the calculated albedo when the LWPradiometer is larger than 60 mg cm−2 but much smaller than the calculated albedo when the LWPradiometer is less than 40 mg cm−2. Cloud-top irregularity is suggested to be the primary cause of this discrepancy. The degree of inhomogeneity of the horizontal distribution of liquid water appears to be correlated with the amount of precipitable water in the planetary boundary layer.
Numerical Simulation on the TIBL Structure in Shoreline Areas with a 2D Higher-Order Turbulence Closure Model
Authors: Weimei Jiang , Xiaoming Wu , Jingnan Zhou
Published: August 2010

A 2D higher-order turbulence closure model for research on the structure of the thermal internal boundary layer (TIBL) has been developed in this paper. The mean quantities (temperature and wind), as well as their turbulent moments and their distribution under the TIBL, were computed. Results of numerical simulation show that under the initial condition of onshore flow and surface temperature on land being higher, than on water. 1) the profile of the TIBL on shore can be identified by the distributions of the mean wind and temperature, and during the integration hours there is an unstable stratified region over land that extends upward and inland gradually; 2) the shape of the profiles of the TIBL is roughly in concordance with observed profiles, but there are some differences, obviously, between the results computed by the formula of h ∼ x1/2 and the results of the numerical experiment; and 3) u′2, v′2, w′2, and u′w′, θ′w′ and their general features are well reproduced by the model. It is shown that the numerical model is feasible and effective.
Characteristics of Convective Turbulence in the Surface Layer Investigated by Principal Component Analysis
Authors: Weijers E. P. , Van Delden A. , Vugts H. F. , Meesters A. G. C. A.
Published: August 2010

Principal component analysis (PCA) was applied to 182 half-hour runs containing time series of turbulent wind velocity and temperature measured in the convective atmospheric surface layer. A field experiment with four sonic anemometers on the vertices and one in the centroid of a square (with sides of 80 m) was performed to obtain the necessary dataset. Physical explanations of the most important eigenvectors are presented. Two of the major principal components (PCs) identify the variance in wind speed along and across the background wind direction. Always, one major PC accounts for the presence of large-scale thermal activity: periods with higher (lower) temperatures coincide with lower (higher) wind speeds, convergence (divergence) in the wind fields, and upward (downward) movements. As an application, variance in the velocity fields was expressed in terms of horizontal divergence and vertical vorticity. These can be derived directly from the eigenvectors when PCA is combined with a planimetric method. Using the PC that identifies thermal activity, it is found that the magnitude of divergence increases and the magnitude of vorticity decreases when atmospheric conditions become more unstable. It is found that the (absolute) ratio between vorticity and divergence scales with a function of the friction velocity divided by the convective vertical scaling velocity. Both kinematic parameters are larger for updrafts than for downdrafts. It is concluded that PCA can be a useful tool to distinguish variance of thermal and nonthermal origin and in the estimation of the kinematics of dominant flow fields.
Numerical Simulation of Boundary Layer Variables Using ē–ε Closure Scheme
Authors: Ramanathan N. , Srinivasan K. , Seshasayee B. V.
Published: August 2010

In this study, a one-and-a-half-order ē–ε closure scheme is used to study the planetary boundary layer development over a full diurnal cycle using Wangara 33d-day observations as initial conditions. The simulated results are compared with a first-order closure model and higher-order closure model results. A scheme of this kind has the advantage of taking into account the history of turbulence state in terms of a prognostic equation for turbulence kinetic energy and provides a better basis for the representation of clouds. The results of the model simulations compare favorably with other investigators’ results.
Horizontal Heat Fluxes over Complex Terrain Computed Using a Simple Mixed-Layer Model and a Numerical Model
Authors: Kimura Fujio , Kuwagata Tuneo
Published: August 2010

The thermally induced local circulation over a periodic valley is simulated by a two-dimensional numerical model that does not include condensational processes. During the daytime of a clear, calm day, heat is transported from the mountainous region to the valley area by anabatic wind and its return flow. The specific humidity is, however, transported in an inverse manner. The horizontal exchange rate of sensible heat has a horizontal scale similarity, as long as the horizontal scale is less than a critical width of about 100 km.

The sensible heat accumulated in an atmospheric column over an arbitrary point can be estimated by a simple model termed the uniform mixed-layer model (UML). The model assumes that the potential temperature is both vertically and horizontally uniform in the mixed layer, even over the complex terrain. The UML model is valid only when the horizontal scale of the topography is less than the critical width and the maximum difference in the elevation of the topography is less than about 1500 m.

Latent heat is accumulated over the mountainous region while the atmosphere becomes dry over the valley area. When the horizontal scale is close to the critical width, the largest amount of humidity is accumulated during the late afternoon over the mountainous region.
Boundary Layer Characteristics over Areas of Inhomogeneous Surface Fluxes
Authors: Doran J. C. , Shaw W. J. , Hubbe J. M.
Published: August 2010

This paper describes results from a June 1992 field program to study the response of the boundary layer over a site with well-defined extreme differences in sensible and latent heat fluxes over clearly separated areas, each with characteristic length scales of 10 km or more. The experiment region consisted of semiarid grassland steppe and irrigated farmland. Sensible heat flux maxima over the steppe regularly reached values in excess of 300 W m−2 and were typically a factor of 4 or more greater than those over the farmland. Two days were selected for analysis: one with moderate winds of 7–10 m s−1 and one with lighter winds of 4–7 m s−1 over the steppe. In both cases the wind directions were nearly perpendicular to the boundary between the steppe and farm. An analysis of potential temperature soundings showed that mixed-layer characteristics over both the farm and the steppe were largely determined by heating over the steppe, with advection from the steppe to the farm playing a significant role. On the day with the lighter winds, a secondary circulation related to the thermal contrasts between the two areas was observed. A simple conceptual model is described that predicts the extent of the cooler area required to generate such circulations. The observations illustrate how predictions of boundary layer structure in terms of local surface sensible heat fluxes may be compromised by advective effects. Such difficulties complicate efforts to obtain accurate representations of surface fluxes over inhomogeneous surfaces even if parameterizations of mesoscale contributions to the heat flux are included.
The Roughness Length for Heat of Sparse Vegetation
Authors: Blyth E. M. , Dolman A. J.
Published: August 2010

A dual-source model that solves the energy balance over vegetation and soil separately can be inverted to obtain the roughness length for heat z0h of a single-source model. Model parameters for the dual-source model were taken from previous analysis of data from a sparse canopy in semiarid terrain. In these circumstances, the value of z0h, is shown to be dependent on the humidity deficit, the available energy, the vegetation fraction, and the surface resistance of the soil and the vegetation.
The 1990 Valentine's Day Arctic Outbreak. Part I: Mesoscale and Microscale Structure and Evolution of a Colorado Front Range Shallow Upslope Cloud
Authors: Rasmussen Roy M. , Bernstein Ben C. , Murakami Masataka , Stossmeister Greg , Reisner Jon , Stankov Boba
Published: August 2010

The mesoscale and microscale structure and evolution of a shallow, upslope cloud is described using observations obtained during the Winter Icing and Storms Project (WISP) and model stimulations. The upslope cloud formed within a shallow arctic air mass that moved into the region east of the Rocky Mountains between 12 and 16 February and contained significant amounts of supercooled liquid water for nearly 30 h. Two distinct layers were evident in the cloud. The lower layer was near neutral stability (boundary layer air) and contained easterly upslope flow. The upper layer (frontal transition zone) was thermodynamically stable and contained southerly flow. Overlying the upslope cloud was a dry, southwesterly flow of 20–25 m s −1, resulting in strong wind shear near cloud top. Within 10 km of the Rocky Mountain barrier, easterly low-level flow was lifted up and over the mountains. The above-described kinematic and thermodynamic structure produced three distinct mechanisms leading to the production of supercooled liquid water: 1) upslope flow over the gently rising terrain leading into the Colorado Front Range, up the slopes of the Rocky Mountains and over local ridges, 2)upglide flow within a frontal transition zone, and 3) turbulent mixing in the boundary layer. Supercooled liquid water was also produced by 1) upward motion at the leading edge of three cold surges and 2) vertical motion produced by low-level convergence in the surface wind field. Large cloud droplets were present near the top of this cloud (approximately 50-µm diameter), which grew by a direct coalescence process into freezing drizzle in regions of the storm where the liquid water content was greater than 0.25 g m −3 and vertical velocity was at 10 cm s −1

Ice crystal concentrations greater than 1 L−1 were observed in the lower cloud layer containing boundary layer air when the top of the boundary layer air when the top of the boundary layer was colder than −12°C. The upper half of the cloud was ice-free despite temperatures as low as −15°C, resulting in long-lived supercooled liquid water in this region of the cloud.
Clustering of Satellite Sounding Radiances to Investigate Intense Low-Level Humidity Gradients
Authors: Fuelberg Henry E. , Rao P. Anil , Hillger Donald W.
Published: August 2010

Satellite-derived profiles of temperature and dewpoint (retrievals) are obtained using radiance data from the Visible-infrared Spin Scan Radiometer Atmospheric Sounder. Individual fields of view that are input to the retrieval algorithm must be horizontally averaged to provide suitable signal-to-noise ratios. This paper investigates three methods for performing this averaging: 1)a blocking approach that is employed operationally, 2) a manual procedure that seeks to maximize atmospheric gradients, and 3) an objective procedure called clustering that takes advantage of similarities in satellite measurements to avoid smearing the gradient information. The three techniques are examined on 10–11 July 1989 when intense gradients of humidity were present over the Florida peninsula.

Results show that the clustering scheme produced retrievals that were very similar to those obtained manually. Both schemes indicated strong humidity gradients in the lower troposphere. The blocking procedure produced less intense gradients. The retrieval information is used to examine conditions leading to fair weather on 10 July but intense thunderstorm development on 11 July.
Quantitative Assessment of the Sampling Properties of a Spaceborne Lidar (ATLID)
Authors: Ackermann Jörg
Published: August 2010

The European Space Agency plans to install the backscatter lidar system ATLID (atmospheric lidar) on a polar-orbiting platform at the beginning of the next century. This kind of active remote sensing will provide highly accurate information about cloud-top height, which, in addition to collocated passive sounder's measurements of brightness temperature, might improve retrieved vertical temperature profiles and serve as a supplementation of present cloud climatologies. Due to technical constraints, ATLID will not provide spatially continuous information about cloud-top height. The representativeness of the lidar measurements for the whole cloud field constitutes the sampling problem and is investigated in two steps: first, a scan mode for ATLID is developed, which on the assumption that the cloud field is a two-dimensional random variable gives an equal pixel spacing along and across the flight track of the orbiter. Second, the simulated lidar measurements given by the elaborated scan mode are contributed to a spatially continuous cloud field represented by Advanced Very High Resolution Radiometer images. From the dispersed lidar measurements with a footprint diameter of about 1 km the cloud field is restored by a spatial interpolation scheme and compared with the original cloud field by a linear regression analysis. It turns out that the sampling error and hence the benefits of ATLID strongly depend on the meteorological situation: if the required vertical accuracy of the lidar measurement is about 250 m corresponding approximately to half of the vertical resolution of present retrieval schemes, the probability for a meaningful ATLID information is between 40% and 70%. Since an imager cannot provide a useful brightness temperature in case of multilayered or broken clouds within one imager pixel, the synergism of ATLID with a passive instrument also depends on the homogeneity of cloud-top height within the range of 1 km. To cheek this small-scale variability of cloud tops data from the European Lidar Airborne Campaign 1990 are evaluated. Results show that for optically thick clouds the variability exceeds in 3% to 38% of all considered cases a threshold of 250 m. Additionally, power-spectrum analyses confirm the result of the sampling analyses.