AN ALTERNATIVE REPRESENTATION OF THE ICE CANOPY FOR CALCULATING MICROWAVE BRIGHTNESS TEMPERATURES OVER A THUNDERSTORM

Passive microwave brightness temperatures (T(B)'s) at 92 and 183 GHz f rom an aircraft thunderstorm overflight are compared with values calcu lated from radar-derived hydrometeor profiles and a modified proximity sounding. Two methods for modeling particles in the ice canopy are co ntrasted. The first is a ''traditional'' approach employing Marshall-P almer ice spheres. The second, or ''alternative,'' method partitions 2 0% of the ice water content into a Marshall-Palmer component for graup el and hail, and 80% into a modified gamma spherical particle size dis tribution function representing ice crystals. Results from the alterna tive 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 magnitu des. Neither method, however, matches the geometry of the observed TB depression associated with the decaying convective core. This is likel y due to the presence of graupel, which is not detected as a special s ignature in radar reflectivity, but does diminish T(B)'s through scatt ering. Brightness temperatures at the relatively high microwave freque ncies considered are shown to be very sensitive to the ice-particle si ze distribution.