Multiparameter Radar and Microwave Radiative Transfer Modeling of Nonspherical Atmospheric Ice Particles

Multiparameter radar and polarized microwave radiative transfer models for arbitrarily shaped particles are developed. These models are used concurrently to study the radar/radiometry properties of atmospheric ice crystals with regard to particle shape and bulk density. The ice crystals modeled are horizontally oriented hexagonal plates, columns, and needles. The ice water content is fixed at 0.1 g/m3 and a realistic size distribution is used. The radar modeling is done for S through K-band, and the passive microwave calculations are at 37, 85, and 157 GHz. The electromagnetic scattering properties of the ice crystals is computed using the discrete dipole approximation. The modeling results show that particle shape is important for both multiparameter radar and passive microwave radiometry. Radar reflectivity and upwelling microwave brightness temperatures depend strongly on the individual particle volume, which in turn depends on the ice crystal shape. Radar differential reflectivity is high for the plates and lower for needles and columns. Linear depolarization calculations indicate that oblate ice crystals such as plates can be distinguished from prolate crystals such as columns. At 85 and 157 GHz, significant polarization brightness temperature differences are calculated for plates and columns. The particle bulk density strongly affects the radar and radiometer observables. Recent measurements taken of ice crystals using the C-band polarimetric DLR (German Aerospace Research Establishment) radar and the National Center for Atmospheric Research’s CP-2 radar are discussed to show that the model results have practical applicability. Modeling with equivalent volume spheres is shown to approximate poorly the ice crystal results. The implications of the results for future observations and instruments and for combining radar and radiometer measurements are outlined.