95-GHz polarimetric radar signatures of pristine crystals mixed with aggregates and rimed crystals: Simulations and radar observations

Millimeter wave radars operating at 95 GHz are being used for the remote sensing of ice clouds from airborne platforms for cloud microphysical studies and for understanding the effects of clouds on the Earth's radiation budget. Two important goals are to identify ice crystal types and to estimate bulk parameters such as ice water content. Polarimetric radars at 95 GHz have the potential for doing both. Particle probe measurements in clouds show that ice crystal aggregates are observed more often than "pristine" or single type of ice crystals (e. g., hexagonal columns, hexagonal plates, stellar crystal, etc.). This paper focuses on polarimetric radar signatures of pristine ice crystals mixed with aggregates and rimed crystals. Simulation results from a detailed computational study of the radar signatures of these mixtures are compared with airborne 95 GHz radar measurements and in situ particle probe images. The measurement data is from field experiments conducted in 1997 in Wyoming. These are unique experiments that provide simultaneous 95 GHz polarimetric radar data with particle probe measurements. Results from simulations and experimental data for the reflectivity factor (Zh), differential reflectivity (ZDR), and linear depolarization ratio (LDR) agreed very well.

Pristine crystals and their mixtures with aggregates and rimed crystals show different clustering characteristics on the Zh - ZDR, Zh - LDR, and ZDR - LDR planes. An exception to this is the overlap of clustering patterns for columnar crystals and aggregates of stellar crystals and their rimed forms. A resolution for this problem is proposed based on computational results, which show that the clustering patterns of these crystals on the ZDR - ĉhv plane, where ĉhv is the correlation coefficient for copolarized signals, are different. These results are expected to be useful in improving ice crystal classification algorithms based on 95 GHz polarimetric radar observables.