Discrimination of Mixed- versus Ice-Phase Clouds Using Dual-Polarization Radar with Application to Detection of Aircraft Icing Regions

Dual-polarization radar measurements and in situ measurements of supercooled liquid water and ice particles within orographic cloud systems are used to develop probabilistic criteria for identifying mixed-phase versus ice-phase regions of sub-0 degrees C clouds. The motivation for this study is the development of quantitative criteria for identification of potential aircraft icing conditions in clouds using polarization radar. The measurements were obtained during the Mesoscale Alpine Programme (MAP) with the National Center for Atmospheric Research S-band dual-polarization Doppler radar (S-Pol) and Electra aircraft. The comparison of the radar and aircraft measurements required the development of an automated algorithm to match radar and aircraft observations in time and space. This algorithm is described, and evaluations are presented to verify its accuracy. Three polarization radar parameters, the radar reflectivity factor at horizontal polarization (Z(H)), the differential reflectivity (Z(DR)), and the specific differential phase (K-DP), are first separately shown to be statistically distinguishable between conditions in mixed-and ice-phase clouds, even when an estimate of measurement uncertainty is included. Probability distributions for discrimination of mixed-phase versus ice-phase clouds are then developed using the matched radar and aircraft measurements. The probability distributions correspond well to a basic physical understanding of ice particle growth by riming and vapor deposition, both of which may occur in mixed-phase conditions. To the extent that the probability distributions derived for the MAP orographic clouds can be applied to other cloud systems, they provide a simple tool for warning aircraft of the likelihood that supercooled water may be encountered in regions of clouds.