Microwave radiative transfer with nonspherical particles

We present the results of radiative transfer calculations in the microwave region focussing on the effects-of multiple scattering by nonspherical hydrometeors. The use of microwave frequencies, for example 37 GHz, leads for raindrops, typical with 4 mm diameter, to a size parameter of 1.5 and requires an exact scattering solution rather than the Rayleigh approximation. The distortion of raindrop shapes from the spherical geometry then 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 four Stokes components which are important when the scattering of nonspherical particles is considered. In this case the third and fourth Stokes component do not decouple from the first-and second components. 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 an size dependent aspect ratio. These particles have a fixed orientation with their rotational axis aligned along the vertical. Results for nonspherical and spherical scattering will be presented. The differences between both methods, showing up to 15 Kelvin change in the polarization difference, depending on rain rate, frequency and viewing angle, will be discussed.