Development of 3-D Analytic Radiative Transfer Model Based on the Umrt Model and Horizontal Perturbation Series

A horizontally inhomogeneous unified microwave radiative transfer (HI-UMRT) model is being developed at the University of Colorado at Boulder to provide 3-dimensional (3-D) analytic forward radiative transfer solutions for 3-D inhomogeneous clouds in severe weather. The HI-UMRT model decomposes the 3-D differential RT equation into a series of azimuthal harmonics and iterative perturbation equations which are readily solved by the existing planar-stratified 1-D UMRT engine. The HI-UMRT solution inherits the properties of unconditional numerical stability, efficiency, and accuracy from the UMRT algorithm not only for spherical hydrometeors but for aspherical cloud ice particles based on the NASA/GSFC DDSCAT database. The HI-UMRT numerical results were produced and compared with 1-D UMRT simulations to evaluate the effects of cloud horizontal inhomogeneities on brightness temperature measurements.