Scattering from Ice Crystals at 94 and 220 GHz Millimeter Wave Frequencies

Polarimetric scattering from cloud ice crystals modeled as hexagonal columns, hexagonal plates and stellar crystals are calculated at 94 and 220 GHz frequencies using the finite difference time domain (FDTD) method. Two orientation models are considered. In the first model the long axes of columns and broad surfaces of plates and stellar crystals are parallel to the horizontal plane and randomly oriented on this plane. The second model assumes uniform random orientation in three dimensions (3-D). The ratio of the backscattering cross sections at 220 and 94 GHz exhibits a steady decrease with increasing size up to about 1000 μm (2000 μm for columns) for horizontally aligned ice crystals at side incidence as well as for 3-D random orientation. This dual frequency ratio (DFR) may be useful in gauging the size of ice crystals. DFR may also be useful in discriminating between ice crystal types since it shows major differences between columns and planar crystals. The linear depolarization ratio (LDR), i.e., the ratio of the cross-polar to co-polar backscattering cross sections, at vertical incidence for the horizontal alignment model is much higher for columns (over 20 dB) compared to plates and stellar crystals. As a result LDR also has the potential for discriminating columns from planar crystals. Furthermore, the normalized Mueller matrices for horizontally aligned columns at vertical incidence are shown to be diagonal and a function of only LDR. This same result applies to the 3-D random orientation model for all three crystal types.