Multiband Simulations of Multistream Polarimetric Microwave Radiances Over Aspherical Hydrometeors

A numerical precision assessment and simulation results of the Unified Microwave Radiative Transfer (UMRT) model incorporating aspherical frozen hydrometeors based on the NASA Goddard Space Flight Center (NASA/GSFC) OpenSSP database are presented. It is shown that UMRT maintains unconditional numerical stability and computational efficiency for absorbing and scattering clouds. UMRT requires symmetry of the transition matrix for the discretized planar‐stratified radiative transfer equation to realize numerically stable and accurate matrix operations as required by the discrete ordinate eigenanalysis method. UMRT has heretofore been restricted to spherical polydispersive hydrometeors. In this study, the necessary block‐diagonal structure of the full Stokes matrix for randomly oriented OpenSSP aspherical hydrometeors is shown to be maintained, albeit with small asymmetric deviations, which introduce small asymmetric components into the transition matrix that are negligible for most remote sensing applications. An upper bound of the brightness temperature error calculated by neglecting the asymmetric components of the transition matrix under even extreme atmospheric conditions is shown to be small. Hence, the OpenSSP hydrometeor database can be reliably used within the UMRT model. Block‐diagonal Stokes matrix elements along with other single‐scattering hydrometeor parameters were subsequently used in radiative simulations of multistream dual‐polarization radiances for a simulated hurricane event to demonstrate the inherent numerical stability and utility of the enhanced UMRT model. An intercomparison of computed upwelling radiances for a multiphase distribution of aspherical OpenSSP hydrometeors versus a mass‐equivalent Mie hydrometeor polydispersion for key sensing frequencies from 10 to 874 GHz shows the considerable impact of complex (vs. simple spherical) hydrometeors on predicted microwave radiances.