An Alternative Representation of the Ice Canopy for Calculating Microwave Brightness Temperatures over a Thunderstorm

Abstract

Passive microwave brightness temperatures (TB's) at 92 and 183 GHz from an aircraft thunderstorm overflight are compared with values calculated from radar-derived hydrometer profiles and a modified proximity sounding. Two methods for modeling particles in the ice canopy are contrasted. The fist is a “traditional” approach employing Marshall–Palmer ice spheres. The second, or “alternative,” method partitions 20% of the ice water content into a Marshall–Palmer component for graupel and hail, and 80% into a modified gamma spherical particle size distribution function representing ice crystals. Results from the alternative approach are superior to those from the traditional method in the anvil and mature convective core. In the decaying convective region, the traditional approach yields better agreement with observed magnitude. Neither method, however, matches the geometry of the observed TB depression associated with the decaying convective core. This is likely due to the presence of graupel, which is not detected as a special signature in radar reflectivity, but does diminish TB's through scattering. Brightness temperatures at the relatively high microwave frequencies considered are shown to be very sensitive to the ice-particle size distribution.