Retrieval of ice cloud parameters using a microwave imaging radiometer

Based on the radiative transfer theory, the microwave radiance emanating fr om ice clouds at arbitrary viewing angles is expressed as an analytic funct ion of the cloud ice water path (IWP), the particle effective diameter (D-e ), and the particle bulk density (rho(i)). Thus, for a given particle densi ty, the earth-viewing measurements at two frequencies (e.g., 340 and 89 GHz ) can provide an estimate of D-e and IWP for submillimeter-size particles. This physical retrieval is rested using data from the Millimeter-wave Imagi ng Radiometer (MIR). A comparison among MIR, radar, and infrared sensor mea surements shows that the MIR frequencies are affected primarily by thick ic e clouds such as cirrus anvil and convection. Over highly convective areas, the measurements from 89 to 220 GHz are nearly identical since the scatter ing by large ice particles aloft approaches the geometric optics limit, whi ch is independent of wavelength. Under these conditions, only the lower MIR frequencies (89 and 150 GHz)are used to retrieve D-e and IWP. In general, the MIR-derived D-e displays a reasonable spatial distribution comparable t o the radar and infrared measurements. However, the magnitude of the IWP re mains highly uncertain because of insufficient information on the ice parti cle bulk density.