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

Abstract

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 from the 1986-COHMEX data archive. Two types of vertically resolved radiative structure functions referred to as a “generalized weighting function” and an “emission source weighting function” are used to describe the process by which radiation originates and reaches the satellite radiometer. In addition, these weighting functions are subdivided into individual contributions by the various hydrometeor species generated by the cloud model. Along with the surface contribution and cosmic background radiation, these weighting functions provide a normalized description of where radiation originates and how it ultimately reaches the satellite. It is emphasized that this information provides an indepth understanding of how precipitation retrieval algorithms should be designed vis-à-vis the passive microwave problem.