MICROWAVE RADIATIVE-TRANSFER THROUGH HORIZONTALLY INHOMOGENEOUS PRECIPITATING CLOUDS

Recent advances in cloud microphysical models have led to realistic th ree-dimensional distributions of cloud constituents. Radiative transfe r schemes can make-use of this detailed knowledge in order to study th e effects of horizontal as well as vertical inhomogeneities within clo uds. This study looks specifically at the differences between three-di mensional radiative transfer results and those obtained by plane paral lel, independent pixel approximations in the microwave spectrum. A thr ee-dimensional discrete ordinates method as well as a backward Monte C arlo method are used to calculate realistic radiances emerging from th e cloud. Analyses between these models and independent pixel approxima tions reveal that plane parallel approximations introduce two distinct types of errors. The first error is physical in nature and is related to the fact that plane parallel approximations do not allow energy to leak out of dense areas into surrounding areas. In general, it was fo und that these errors are quite small for emission-dominated frequenci es (37 GHz and lower) and that physical errors are highly pronounced o nly at scattering frequencies (85 GHz) where large deviations and bias es up to 8 K averaged over the entire cloud were found. The second err or is more geometric in nature and is related to the fact that plane p arallel approximations cannot accommodate physical boundaries in the h orizontal dimension for off-nadir viewing angles. The geometric errors were comparable in magnitude for all frequencies. Their magnitude, ho wever, depends on a number of factors including the scheme used to dea l with the edge, the nature of the surface, and the viewing angle.