Discrimination between Rain and Snow with a Polarimetric Radar

Abstract

Polarimetric signatures of snow precipitation for six Oklahoma snowstorms are examined. The available data consist of specific differential phase KDP, differential reflectivity ZDR, cross-correlation coefficient ρhv, and radar reflectivity factor Z. These data were obtained with the 10-cm-wavelength Cimarron polarimetric weather radar. The data suggest that in pure snow the average values of KDP and ZDR do not follow a systematic trend with change of the radar reflectivity factor if Z < 35 dBZ; this is not the case in rain. Precipitation is qualified as snow if the average ZDR is less than 0.2 dB for Z < 35 dBZ. The presence of a bright band with a pronounced ρhv minimum and ZDR maximum is a good discernible feature for discriminating between snow and rain. Thus, a localized deep minimum of the cross-correlation coefficient delineates the transition region between snow and rain in the horizontal direction if sufficiently large snowflakes are generated in the transition area. Otherwise, a sharp change of ZDR can be used to localize the position of the snow–rain line.