Backscatter characteristics of nonspherical ice crystals: Assessing the potential of polarimetric radar measurements

The potential of ground-based polarimetric cloud radar measurements to impr ove information about cloud microphysics is evaluated. Using the discrete d ipole approximation (DDA), backscattering at 95 GHz (3.16 mm) has been comp uted for solid hexagonal columns and plates, hollow cylinders, and two type s of stellars. A randomly oriented compact column (a = 1) represents a firs t step toward considering polycrystals at millimeter wavelengths. Crystal s izes up to 2 mm in maximum dimension and random orientation with maximum pa rticle dimension in one plane have been considered, while the incidence ang le is varied from 0 degrees to 90 degrees. Thus the effect of shape, size, and orientation on radar observables as copolar reflectivity (Z(hh)), linea r depolarization ratio (LDR), and differential reflectivity (Z(DR)) as well as difference reflectivity (Z(DP)) can be investigated. Technical radar as pects such as the potential of scanning and cross polarization isolation re quirements are assessed as well as the benefit of alternative wavelengths c onsidered for cloud radar application (35, 140, 220 GHz). At 220 and 140 GH z, distinct resonance features are found, complicating the interpretation o f radar measurements at those frequencies. At 95 GHz the effects of shape o r orientational variation on Z(hh), are of the same order and much smaller than the effect of crystal size. On the other hand, LDR is governed by the crystal shape and orientation but almost independent of size. The results i ndicate that if crystals have a preferred orientation, LDR is a useful obse rvable for differentiating between the two major crystal types of columnar and planar shapes. However, it is shown that the corresponding cross-polar backscatter intensities are theoretically and technically difficult to achi eve. Z(DR) and Z(DP) involve only copolar intensities. If the lowest applic able antenna elevation angle is about 45 degrees for a ground-based measure ment, these parameters are promising for discriminating horizontally aligne d pristine crystals from randomly oriented ones or irregular aggregates.