Dual-polarization radar signatures of lump graupel at 94 GHz

Polarimetric scattering properties of lump graupel are computed at 94 GHz and compared with radar measurements. The model parameters in the computations are varied over the ranges reported in the literature. An oblate spheroidal shape is used with aspect ratio varying between 0.5 to 0.9. The density of the graupel ranges between 0.2 to 0.9 g cm-3, assuming an ice-air mixture. The oblate spheroids are oriented with their symmetry axes, on average, along the vertical direction with standard deviation of 100, 300, and 450. The equivalent volume diameter of the graupel particles is varied from 0.1 to 8 mm. The size distribution is based on a gamma model distribution developed for rainfall. Radar parameters are simulated using these models. Based on these models (and for 300 standard deviation of the canting angle) it is interesting to note that graupel with density less than 0.5 g cm-3 has ZDR in the range 0 to 1 dB and LDRv < -14 dB. The larger values of ZDR and LDRv occur for smaller values of the aspect ratio. When the density is about 0.5 g cm-3 or higher then ZDR begins to stretch into the negative dB values. At 0.7 g cm-3 ZDR ranges from -2.5 to +1.3 dB and LDRv reaches -8 dB. Measurements with the University of Wyoming's airborne 95 GHz dual-polarization radar system together with simultaneous two-dimensional particle probes (2D-C cloud and 2D-P precipitation probes) are compared with the model simulations. In one case the comparisons show good agreement for a density of about 0.6 g cm-3 and an aspect ratio of about 0.7.