Ice Water Path Estimation and Characterization Using Passive Microwave Radiometry

Microwave emission emerging from a precipitating cloud top and lying in a radiometer's field of view represents the culmination of a complex interaction between emitted microwave radiation and its ongoing extinction through overlapping regions of liquid, melting phase, and ice. The encounter with the ice region represents the final interaction between the upwelling microwave radiation and the cloud constituents. Hence, an ice phase characterization perhaps represents a more inherently retrievable property from a combination of scattering-based channels above 37 GHz than the underlying rainfall. Model computations of top-of-atmospheric microwave brightness temperatures TB from layers of precipitation-sized ice of variable bulk density and ice water content (IWC) are presented. The 85-GHz TB is shown to depend essentially on the ice optical thickness, while the possibility of using the 37- and 85-GHz brightness temperature difference ΔTB to estimate the integrated ice water path (IWP) is investigated. The results demonstrate the potential usefulness of using scattering-based channels to characterize the ice phase and suggest a top-down methodology for retrieval of cloud vertical structure and precipitation estimation from multifrequency passive microwave measurements. Radiative transfer model results using the multiparameter radar data initialization from the Cooperative Huntsville Meteorological Experiment (COHMEX) in northern Alabama are also presented. The vertical behavior of the simulated multifrequency TB, albedo, and extinction is presented along with the associated multiparameter radar measurements during the cloud lifecycle. Ice water path values estimated from the radar measurements are compared with the above theoretical computations for the corresponding TB values and show agreement for values of IWP less than 1 kg m−1. Above this, assumptions in the form of the ice-size distribution fail to adequately characterize the ice scattering process. Brightness temperature TB warming effects due to the inclusion of a cloud liquid water profile are shown to be especially significant at 85 GHz during later stages of cloud evolution.