Retrieving Cloud Ice Water Content Using Millimeter- and Centimeter-Wavelength Radar Polarimetric Observables

Scattering properties of a large collection of pristine ice crystals at millimeter and centimeter wavelengths are calculated using the generalized multiparticle Mie method. Millimeter- and centimeter-wavelength radar observables are also calculated by employing particle size distributions (PSDs) that ensure the bulk properties (e.g., ice water content and total number concentration) fall within physically realistic ranges. The relationships between radar reflectivity Z and ice water content ( IWC) are shown to be sensitive ( from one to two orders of magnitude in variability) to the PSDs used and are thus not recommended for IWC retrievals. The relationships between IWC and specific differential phase KDP are less dependent on PSDs. Simple relationships between IWC and KDP at different radar elevation angles and wavelengths are given. If only the general crystal type is known (i.e., planar vs columnar), IWC retrieval errors based on KDP mostly fall within 30%. If more detailed ice crystal type is known, the retrieval errors are reduced to mostly within 10%. These results are similar to earlier reports in the literature but are based on a more extensive collection of model ice crystals and electromagnetic-scattering computations at four wavelengths: X, Ku, Ka, and W bands. The applicability of KDP in retrieving IWC is limited by the measurement accuracy of KDP, which usually requires averaging over several kilometers in range. Given the same noise level, the shorter wavelengths may have relatively smaller fractional errors than the longer wavelengths in KDP-based IWC retrievals and are promising wavelengths for further investigation.