Rong-Shyang Sheu

Classification of clouds over the western equatorial Pacific Ocean using combined infrared and microwave satellite data

A new cloud classification scheme is presented that combines infrared and microwave satellite data. Because microwave radiation can penetrate deep into the cloud layer, this scheme is able to determine characteristics for both thin and deep clouds. Additionally, the new scheme can provide information on precipitation, which traditional infrared-visible cloud classification schemes have been unable to. The proposed cloud classification scheme utilizes the cloud top temperature obtained from infrared measurements and a microwave index that includes both emission and scattering signals. The following eight cloud classes are defined: warm nonprecipitating cloud, warm precipitating cloud, midtop nonprecipitating cloud, midtop precipitating cloud, thin high-top nonprecipitating cloud, deep high-top nonprecipitating cloud, anvil with stratiform precipitating cloud, and deep convective precipitating cloud. The classification scheme is validated by aircraft radar data obtained from Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. The new cloud classification scheme is used to investigate the clouds in the western equatorial Pacific Ocean warm pool region for the period from November 1992 to February 1993, allowing us to obtain for the first time the spectra of cloud coverage and precipitation distribution as a function of cloud types. The analysis shows that most of the cloudiness in this region is associated with warm nonprecipitating clouds. Precipitating pixels comprise only about 15% of total cloud pixels. Of the precipitating pixels, about 50% have cloud top temperatures warmer than −40°C. Using rainfall rates obtained from microwave satellite data, the relative contribution by each precipitating cloud type is investigated. It is found that the precipitating clouds with cloud top temperature warmer than −40°C contribute at least 23% of the total rainfall amount, while the remaining 77% of total precipitation is divided almost equally by anvil with stratiform precipitating cloud and the deep convective precipitating cloud.

The Operational Mesogamma-Scale Analysis and Forecast System of the U.S. Army Test and Evaluation Command. Part I: Overview of the Modeling System, the Forecast Products, and How the Products Are Used

Given the rapid increase in the use of operational mesoscale models to satisfy different specialized needs, it is important for the community to share ideas and solutions for meeting the many associated challenges that encompass science, technology, education, and training. As a contribution toward this objective, this paper begins a series that reports on the characteristics and performance of an operational mesogammascale weather analysis and forecasting system that has been developed for use by the U.S. Army Test and Evaluation Command. During the more than five years that this four-dimensional weather system has been in use at seven U.S. Army test ranges, valuable experience has been gained about the production and effective use of high-resolution model products for satisfying a variety of needs. This paper serves as a foundation for the rest of the papers in the series by describing the operational requirements for the system, the data assimilation and forecasting system characteristics, and the forecaster training that is required for the finescale products to be used effectively.

High-Resolution Satellite-Derived Dataset of the Surface Fluxes of Heat, Freshwater, and Momentum for the TOGA COARE IOP

Abstract An integrated approach is presented for determining from several different satellite datasets all of the components of the tropical sea surface fluxes of heat, freshwater, and momentum. The methodology for obtaining the surface turbulent and radiative fluxes uses physical properties of the atmosphere and surface retrieved from satellite observations as inputs into models of the surface turbulent and radiative flux processes. The precipitation retrieval combines analysis of satellite microwave brightness temperatures with a statistical model employing satellite observations of visible/infrared radiances. A high-resolution dataset has been prepared for the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) intensive observation period (IOP), with a spatial resolution of 50 km and temporal resolution of 3 h. The high spatial resolution is needed to resolve the diurnal and mesoscale storm-related variations of the fluxes. The fidelity of the satellite-derived s...

Ensemble Simulations with Coupled Atmospheric Dynamic and Dispersion Models: Illustrating Uncertainties in Dosage Simulations

Abstract Ensemble simulations made using a coupled atmospheric dynamic model and a probabilistic Lagrangian puff dispersion model were employed in a forensic analysis of the transport and dispersion of a toxic gas that may have been released near Al Muthanna, Iraq, during the Gulf War. The ensemble study had two objectives, the first of which was to determine the sensitivity of the calculated dosage fields to the choices that must be made about the configuration of the atmospheric dynamic model. In this test, various choices were used for model physics representations and for the large-scale analyses that were used to construct the model initial and boundary conditions. The second study objective was to examine the dispersion models ability to use ensemble inputs to predict dosage probability distributions. Here, the dispersion model was used with the ensemble mean fields from the individual atmospheric dynamic model runs, including the variability in the individual wind fields, to generate dosage probab...

Vertical stratification of tropical cloud properties as determined from satellite

A new retrieval scheme is developed to infer tropical cloud properties and vertical structure, including liquid and ice water content, cloud top and base, and cloud layering. The retrieval scheme utilizes a cloud classification scheme that uses both International Satellite Cloud Climatology Project (ISCCP) cloud top temperature and a microwave index from the special sensor microwave/imager (SSM/I). Different cloud classes have different allowed numbers of cloud layers. The retrieval scheme also incorporates findings from observational studies. When multiple liquid layers are judged to be possible, a “cloudiness likelihood” parameter is used to determine the priority for the presence of liquid layer at each level, based on the European Centre for Medium-Range Weather Forecasts analyzed relative humidity field. Cloud liquid water path is determined using a microwave satellite retrieval. In case of multiple liquid layers, the liquid water path is partitioned and assigned to each liquid layer in proportion to the adiabatic liquid water path in each layer. Depending on the cloud class, ice water paths are determined using one of the following methods: (1) ISCCP ice optical depth; (2) a microwave ice retrieval that uses ice scattering signals at high SSM/I frequencies; and (3) a residual method that infers ice from the difference between a “virtual” liquid water path derived from ISCCP total optical depth and the true liquid water path determined from SSM/I. The retrieved cloud layering is indirectly validated using cloud cooccurrence climatology from surface observations. The cloud base retrieval is compared with lidar measurements obtained during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment.

The Operational Mesogamma-Scale Analysis and Forecast System of the U.S. Army Test and Evaluation Command. Part II: Interrange Comparison of the Accuracy of Model Analyses and Forecasts

This study builds upon previous efforts to document the performance of the U.S. Army Test and Evaluation Command’s Four-Dimensional Weather Modeling System using conventional metrics. Winds, temperature, and specific humidity were verified for almost 15 000 forecasts at five U.S. Army test ranges using near-surface mesonet data. The primary objective was to use conventional metrics to characterize the degree to which forecast accuracy varies from range to range, within the diurnal cycle, with elapsed forecast time, and among the seasons. It was found that there are large interrange differences in forecast error, with larger errors typically associated with the ranges located near complex orography. Similarly, significant variations in accuracy were noted for different times in the diurnal cycle, but the diurnal dependency varied greatly among the ranges. Factor of 2 differences in accuracy were also found across the seasons.

Atmospheric humidity variations associated with westerly wind bursts during Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE)

Precipitable water determined from special sensor microwave/imager brightness temperatures is used to examine westerly wind bursts (WWBs) in the equatorial western Pacific. Six microwave satellite water vapor retrieval algorithms are tested for their performance in the tropics. The study of two major WWB events and several localized and short-lived wind perturbations in the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE) domain unveils the relationship between precipitable water field and anomalous wind regimes. WWBs are found to be accompanied by a drastic drop in the precipitable water amount. The decrease is partly explained by the transition between easterly and westerly wind regimes. A few days before the westerly wind is established, the transitional equatorward wind introduces the dry air from the subtropics. When the westerly wind eventually weakens, a transitional equatorward wind regime once again occurs and lasts for several days before the easterlies take over. The strongest WWB event during TOGA COARE is found to be consistent with midlatitude cold surge. This provides evidence for the midlatitude forcing of WWBs. While the interaction with higher latitudes explains the reduction in precipitable water in WWB events, it is not clear what region is responsible for feeding extra moisture to the western Pacific.

Multiscale Local Forcing of the Arabian Desert Daytime Boundary Layer, and Implications for the Dispersion of Surface-Released Contaminants

Abstract Four 6-day simulations of the atmospheric conditions over the Arabian Desert during the time of the 1991 detonation and release of toxic material at the Khamisiyah, Iraq, weapons depot were performed using a mesoscale model run in a data-assimilation mode. These atmospheric simulations are being employed in a forensic analysis of the potential contribution of the toxic material to so-called Gulf War illness. The transport and concentration of such surface-released contaminants are related strongly to the planetary boundary layer (PBL) depth and the horizontal wind speed in the PBL. The product of the PBL depth and the mean wind speed within it is referred to as the ventilation and is used as a metric of the horizontal transport within the PBL. Thus, a corollary study to the larger forensic analysis involves employing the model solutions and available data in an analysis of the multiscale spatial variability of the daytime desert PBL depth and ventilation as they are affected by surface forcing fr...

The Operational Mesogamma-Scale Analysis and Forecast System of the U.S. Army Test and Evaluation Command. Part III: Forecasting with Secondary-Applications Models

Output from the Army Test and Evaluation Command’s Four-Dimensional Weather System’s mesoscale model is used to drive secondary-applications models to produce forecasts of quantities of importance for daily decision making at U.S. Army test ranges. Examples of three specific applications—a sound propagation model, a missile trajectory model, and a transport and diffusion model—are given, along with accuracy assessments using cases in which observational data are available for verification. Ensembles of application model forecasts are used to derive probabilities of exceedance of quantities that can be used to help range test directors to make test go–no-go decisions. The ensembles can be based on multiple meteorological forecast runs or on spatial ensembles derived from different soundings extracted from a single meteorological forecast. In most cases, the accuracies of the secondary-application forecasts are sufficient to meet operational needs at the test ranges.

Satellite retrieval of tropical precipitation using combined International Satellite Cloud Climatology Project DX and SSM/I Data

A new precipitation retrieval scheme for use in the tropics is developed using a combination of visible (VIS)/IR and microwave satellite observations. The new scheme combines the advantages of both the ample coverage of VIS/IR sampling and the physical directness between precipitation and microwave radiances. A VIS/IR algorithm is “trained” using Special Sensor Microwave/Imager (SSM/I) derived values of precipitation. Cloud top temperatures and visible optical depths provided by the International Satellite Cloud Climatology Project (ISCCP) level DX analyses are “binned” by every 10 K and every 10 unit optical depth, respectively. Lookup tables for the probability of rain and mean rainfall rate are constructed for each cloud top temperature/optical depth cell whenever the optical depth is available (daytime). For the nighttime, the tables are based only on cloud top temperature. The instantaneous rainfall rates are obtained by multiplying the mean rainfall rate by the probability of rain, both characterized by the cloud top temperature and/or visible optical depth associated with the satellite pixel. The satellite retrieval always retains results from optimal parameters, which means SSM/I results are used whenever SSM/I samplings are available; otherwise, VIS/IR results are used during daytime, while IR-only results are used during nighttime. The final precipitation product can have resolution as high as 3 hours in time and about 0.3° in space. However, averaging in space and/or time improves the statistics of the derived rainfall rates, since the statistics of the algorithm are more fully utilized. The satellite-derived rainfall product compares well with ship rain gauge data in terms of rain/no rain determination, and agrees well with daily rainfall accumulation derived from a large-scale moisture budget. The satellite retrieved rainfall rates are biased high relative to radar observations. In the absence of satisfactory data with which to validate the satellite-derived rainfall rates, it can be concluded that there is useful information in the ISCCP data set that relates to precipitation and that the inclusion of visible optical depth information improves the daytime retrieval of precipitation relative to IR-only methods.