Observations of Mixed-Phase Precipitation within a CaPE Thunderstorm

Abstract

Various procedures for inferring hydrometeor characteristics from polarimetric radar data have indicated that regions with echoes exhibiting relatively high linear depolarization ratios along with relatively low differential reflectivity contain wet graupel or hail. Such particles could be found either in a melting zone below the 0°C level in a cloud or in a region of wet growth where the rate of supercooled cloud water accretion overwhelms the rate at which the latent heat associated with complete freezing can be dissipated. In subtropical clouds such as those studied in the Convection and Precipitation/Electrification (CaPE) project in Florida, at the ?5°C level or higher, neither condition is obtained. Yet similar polarimetric radar signatures were nevertheless observed at such levels during CaPE. Examination of in situ observations by the T-28 aircraft in the Florida clouds, along with results from previous laboratory and theoretical studies, suggests that the signature regions were characterized by raindrops formed at lower levels in the clouds in the process of freezing while being carried up in the updraft. This interpretation is supported by some model calculations of the process of raindrops freezing in a supercooled cloud where the particles continue to accrete cloud water. Complete freezing of millimeter-sized drops can take a few minutes after nucleation, and the particles can ascend 1 or 2 km in the updraft during that period.