Harry J. Cooper

Foundations for statistical-physical precipitation retrieval from passive microwave satellite measurements. I: Brightness-temperature properties of a time-dependent cloud-radiation model

Abstract A cloud-radiation model is used to investigate the relationship between emerging microwave brightness temperatures (TBs) and vertically distributed mixtures of liquid and frozen hydrometeors as a means to establish the framework for a hybrid statistical-physical rainfall retrieval algorithm. The focus in this study is on the microwave characteristics of an intense hailstorm in which cold-rain microphysics dominate the precipitation process. The TB calculations exhibit a high degree of intercorrelation across a wide frequency range (15–128 GHz) due to the pervasive influence of large ice particles on attenuation of upwelling radiation emerging from the rain layers. When the radiative emission source is near blackbody, fluctuations of the mixing ratios of ice particles are wholly responsible for the TB variations, as opposed to fluctuations in the cloud-or raindrop mixing ratios. Supercooled cloud drops, suspended in the graupel layers, can exert influence on the TBs but only at the higher freque...

Simulation of Microwave Brightness Temperatures of an Evolving Hailstorm at SSM/I Frequencies

A simulation of the appearance of an intense hailstorm in the passive microwave spectrum is conducted in order to characterize the vertical sources of radiation that contribute to the top-of-atmosphere microwave brightness temperatures (TB) which can be measured by satellite-borne radiometers. The study focuses on four frequencies corresponding to those used on the USAF Special Sensor Microwave Imager (SSM/I), a recently launched payload flown on the U.S. Air Force DMSP satellites. Computation of the microwave brightness temperatures is based on a vertically, angularly, and spectrally detailed radiative transfer scheme that has been applied to the highly resolved thermodynamical and microphysical output from the three-dimensional Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The RAMS model was used to carry out a 4-h simulation of an intense hailstorm that occurred on 11 July 1986 in the vicinity of Eldridge, Alabama. Initial conditions for the cloud model run were developed...

Estimation of surface heat and moisture fluxes over a prairie grassland: 3. Design of a hybrid physical/remote sensing biosphere model

This is the third in a series of papers describing a measurement program and modeling approach to the estimation of surface heat and moisture fluxes over a tallgrass prairie. We describe the design and formulation of an experimental biosphere model (Ex-BATS) which follows the development of Dickinsons biosphere-atmosphere transfer scheme (BATS). Ex-BATS has been designed to incorporate in situ measurements and satellite parameterizations of certain canopy variables which are slowly varying in the course of a growing season. All components of a multistage biosphere process model used to simulate the exchange of heat and moisture between the canopy and the atmosphere are presented here. The remote sensing aspects of the model are described in a companion paper. The procedures used to optimize the model and the validation of the model against First ISLSCP Field Experiment (FIFE) observations taken during 1987 are described. Validation was carried out for three of the four intensive field campaigns (IFCs): 1, 2 and 3. The validation intercomparison shows that the Ex-BATS model reproduces the diurnal behavior of the surface fluxes very closely. The rms differences between in situ measurements of sensible and latent heat fluxes and their model counterparts are of the order of 35 W m−2. Biases over the three IFCs, which ranged in duration from 10 to 17 days, are typically less than 20 W m −2. The overall bias for the 44 days within the first three IFCs is less than 10 W m −2. The biases and rms differences are of the same order as the accuracy and precision uncertainties of the measured fluxes, therefore the model provides a useful experimental tool to explain radiative, hydrological, and physiological controls on surface fluxes over vegetated canopies and the explicit sources of water flux to the atmosphere from transpiration, foliage evaporation, and soil evaporation.

Linking boundary-layer circulations and surface processes during FIFE89. Part 1: Observational analysis

Abstract Surface, aircraft, and satellite observations are analyzed for the 21-day 1989 intensive field campaign of the First ISLSCP Field Experiment (FIFE) to determine the effect of precipitation, vegetation, and soil moisture distributions on the thermal properties of the surface including the heat and moisture fluxes, and the corresponding response in the boundary-layer circulation. Mean and variance properties of the surface variables are first documented at various time and space scales. These calculations are designed to set the stage for Part II, a modeling study that will focus on how time–space dependent rainfall distribution influences the intensity of the feedback between a vegetated surface and the atmospheric boundary layer. Further analysis shows strongly demarked vegetation and soil moisture gradients extending across the FIFE experimental site that were developed and maintained by the antecedent and ongoing spatial distribution of rainfall over the region. These gradients are shown to hav...

Estimation of surface heat and moisture fluxes over a prairie grassland: 4. Impact of satellite remote sensing of slow canopy variables on performance of a hybrid biosphere model

Herein, we present the results of a series of numerical experiments using the Ex-BATS biosphere model, which is an adaptation of Dickinsons biosphere-atmosphere transfer scheme (BATS). These simulations are used to assess how the model performs when remotely sensed data are used to estimate three key canopy variables. These canopy variables, which effectively represent the slowly changing boundary conditions of a vegetated surface, consist of the total surface albedo, leaf area index, and the nondiurnally varying component of stomatal resistance, referred to as stressed stomatal resistance. The surface albedo is retrieved from NOAA-AVHRR (advanced very high resolution radiometer) channel 1 spectral reflectance information in conjunction with a directional reflectance model which accounts for the strong diurnal variations in surface reflectance. A 4-channel vegetation index also retrieved from AVHRR measurements is used to estimate the leaf area index. A similar index derived from high-resolution SPOT visible and near-infrared information has been used to describe the spatial variations in such indices which impact the retrieval of the leaf area index. Satellite retrieval of stomatal resistance is based on split-window skin temperatures from AVHRR channels 4 and 5 from the afternoon overpass (∼1630 LT). Variables derived from the hourly skin temperature observations of GOES-VISSR have also been examined with respect to retrieval of stomatal resistance. It was found that although stomatal resistance has little correlation with the diurnal amplitude of skin temperature, it is closely related to the daily maximum of skin temperature. Numerical experiments have been conducted to examine model sensitivity to each of these canopy variables. Results indicate that Ex-BATS is not sensitive to small variations of surface albedo or leaf area index within the range of estimation uncertainty. The rms measurement-model flux differences in every numerical trial were within 6 W m−2 of the rms differences obtained for the simulations performed using measured albedo and leaf area index. Measured stomatal resistance values were obtained using an inversion form of the model. The resulting stomatal resistances were used to perform a control experiment simulating an ideal satellite retrieval scenario involving one observation per day. The control experiment resulted in improvements of approximately 20 W m−2 in the rms flux differences over the model using a purely hypothetical formulation for stomatal resistance. Simulations using the remotely retrieved stomatal resistances produced significantly reduced rms differences for latent and sensible heat fluxes over the model using the hypothetical formulation. Based on a 55-day composite involving all days from the four FIFE intensive field campaigns, the sensible and latent heat flux improvements are approximately 25 and 20%, respectively (11 and 8 W m−2). The satellite retrievals are only 20 and 30% less accurate (7 and 10 W m−2) than the idealized results of the control experiment.

Modeling the impact of averaging on aggregation of surface fluxes over BOREAS

Surface meteorological observations, soil and vegetation parameters, and satellite-retrieved surface radiation fluxes are used as input to a biosphere-atmosphere flux exchange model to estimate the fine-scale surface fluxes over a 75,000 km 2 region within the large-scale BOREAS study area under cloud-free conditions. The size of the study area chosen for the analysis is synonymous with the characteristic grid size of a modern global climate model. The variables used to force the model are area-averaged and applied in such a fashion to assess the errors in area-wide fluxes that would arise in flux aggregation schemes based on the area-averaged inputs. In the absence of large spatial variance of downwelling radiative fluxes, characteristic of the clear sky case under study, progressive area averaging of the input parameters indicates that the most important quantities influencing the aggregated fluxes are root layer depth, soil moisture, and soil type, all of which are interrelated in the formulation of the transpiration capacity. Meteorological variance over the site and variability in a number of other parameters relating to vegetation characteristics and initial soil temperatures are not critical to the aggregation process. Notably, major errors in aggregated surface fluxes arise due to area-averaging soil moisture alone, even when all other parameters are not area-averaged and allowed to maintain their spatial variability. The errors materialize as significant phase shifts in the frequency distributions of the fine-scale surface fluxes calculated after input averaging. Application of a soil mosaic averaging technique, based on soil type and the soil moisture associated with each soil type, restores the characteristic mean values of the baseline area-averaged fluxes. This procedure also simplifies the surface flux distributions to dependence on the percentages of grid area covered by each soil type but without altering the basic multimodal character of the baseline distributions. It is demonstrated and explained why such a soil-type designation over the area forms the kernels for the surface flux distribution spectrums, around which the other input parameters act to disperse the frequency distribution properties of the aggregated surface fluxes.

Limitations in estimating surface sensible heat fluxes from surface and satellite radiometric skin temperatures

The objective of this study is to demonstrate that by use of simple physical techniques it is possible to obtain useful skill in retrieving sensible heat fluxes by the radiometric method but only on a site-specific basis. Sensible heat fluxes measured by Bowen ratio systems at two sites during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) 1987 and FIFE 1989 are compared to sensible heat fluxes estimated from radiometric measurements of skin temperature taken by pyrgeometers mounted 1.75 m above the surface. An experimental version of the biosphere-atmosphere transfer scheme (Ex-BATS), capable of reproducing statistically reliable estimates of the measured heat fluxes, is used to show that good agreement can be obtained between the model-diagnosed surface temperatures and the skin temperatures obtained from the pyrgeometers. However, because of biases in the radiometer values which are small in an absolute sense but large in terms of the differential temperatures between surface and atmosphere, use of such temperatures in modified bulk formula expressions gives rise to significant biases in the sensible heat fluxes. By reconciling radiometric estimates of the sensible heat fluxes and the fluxes measured by the Bowen ratio systems, a mean effective emissivity considered in the form of a site-specific calibration constant can be calculated for each of the two sites. Using the new emissivities, corrected radiometric surface temperatures are obtained that remove the biases at each site, demonstrating that the radiometric method is feasible but only on a site-specific basis. The corrected skin temperatures are then compared to the advanced very high resolution radiometer (AVHRR)-derived split window estimates of skin temperature obtained on 30 clear-sky days during the 1987 experimental period. The two data sets are found to be well-correlated but also with an underlying bias. Differences between the satellite estimated temperatures and the corrected skin temperatures are attributed to imperfections in the coefficients used in the split window equation and residual cloud contamination, as well as considerable differences in the scenes viewed by the pyrgeometers and satellite. These differences lead to significant errors in retrieving heat fluxes from the raw satellite skin temperature estimates. The estimates can be improved after applying a regression between the corrected radiometric skin temperatures obtained for the radiometers and the satellite-derived temperatures, although because of the residual cloud effects and scene mismatch, the final RMS error is of the order of 65 W m−2. An independent application of the regression fit between the radiometer temperatures and the AVHRR-derived temperatures derived from the 1987 data to sensible heat flux calculations for four clear-sky days in 1989 indicates poor skill, reinforcing the notion that for satellite applications the radiometric approach is also only feasible on a site-specific basis.

Modeling Carbon Sequestration over the Large-Scale Amazon Basin, Aided by Satellite Observations. Part I: Wet- and Dry-Season Surface Radiation Budget Flux and Precipitation Variability Based on GOES Retrievals

Abstract In this first part of a two-part investigation, large-scale Geostationary Operational Environmental Satellite (GOES) analyses over the Amazonia region have been carried out for March and October of 1999 to provide detailed information on surface radiation budget (SRB) and precipitation variability. SRB fluxes and rainfall are the two foremost cloud-modulated control variables that affect land surface processes, and they require specification at space–time resolutions concomitant with the changing cloud field to represent adequately the complex coupling of energy, water, and carbon budgets. These processes ultimately determine the relative variations in carbon sequestration and carbon dioxide release within a forest ecosystem. SRB and precipitation retrieval algorithms using GOES imager measurements are used to retrieve surface downward radiation and surface rain rates at high space–time resolutions for large-scale carbon budget modeling applications in conjunction with the Large-Scale Biosphere–A...

Relevance of Surface Energy Budget within Florida Sea-Breeze Front to Cross-Peninsula Rainwater Runoff Gradient

Abstract Analysis of surface latent heat flux measurements taken within the sea-breeze front of the coast of Florida during active thunderstorm periods demonstrates an important effect of the timing of coastal storms on the seasonal surface water budget. Historical records document a systematic cross-peninsula water runoff gradient across Florida, with total runoff greater on the east coast (Atlantic side) than on the west coast (gulf side). This situation persists even though convective rainfall tends to be greater in the summertime on the gulf side. In this paper, the authors examine the effect of the time of day that summer thunderstorms occur at a given location on poststorm evaporation of rainfall and place these effects into the context of the annual runoff at the coasts and seasonal rainfall in order to assess their possible significance. A surface water exchange analysis, based on datasets obtained during the 1991 summertime Convection and Precipitation Electrification Experiment, finds that part ...

The Importance of Short-Term Forecasting of Thunderstorms to Launch Operations at Cape Canaveral

The local meteorological events leading up to the launch of the space shuttle Atlantis on 2 August 1991 were captured in full-resolution GOES visible data being archived for the Convection and Precipitation/Electrification Experiment. The postponement of the launch on 1 August, and the successful lift-off on the following day provide a good example of the important role played by nowcasting and short-term forecasting at Cape Canaveral. In this brief article, we discuss the local weather conditions prior to, during, and after the launch and demonstrate the importance of short-term forecasting capabilities around the cape during launch operations.