Microwave radiative transfer with nonspherical precipitating hydrometeors

We present the results of radiative transfer calculations in the microwave region focussing on the effects of multiple scattering by non-spherical hydrometeors. The use of microwave frequencies, e.g. 37 GHz, leads to a size parameter of 1.5 for raindrops with 4 mm diameter and requires an exact scattering solution rather than Rayleigh approximation. The distortion of raindrop shapes from the spherical geometry becomes significant and has to be taken into account when the scattering properties are calculated. The model developed uses the full Stokes vector to include the effects of cross-polarization terms between all Stokes components, which are important when the scattering of non-spherical particles is considered. The one-dimensional microwave radiative transfer model is based on the successive order of scattering method and assumes azimuthal symmetry. The shape of the hydrometeors is approximated by rotational symmetric ellipsoids with a size dependent aspect ratio. These particles have a fixed orientation with their rotational axis aligned along the vertical. Results for non-spherical and spherical scattering are presented. The differences between both methods, leading to 15 K change in the polarization difference depending on rain rate, frequency and viewing angle, are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.