Polarimetric Radar Observations of Hail Formation

Abstract

Analyses are made of the evolution of selected polarimetric radar data fields during periods immediately preceding the onset of near-surface hail indicators [high reflectivity and low differential reflectivity (Zdr)] in two nonsupercellular northeastern Colorado hailstorms. The primary data were obtained from the 11-cm-wavelength, dual-polarization Colorado State University (CSU)?University of Chicago and Illinois State Water Survey radar. In one of the storms, dual-Doppler wind field syntheses were available using additional velocity data collected by the CSU Pawnee S-band radar. In both events, linear depolarization ratio (LDR) values exceeding ?25 dB began to appear in the right flank of the 50-dBZ echo core region, within the 0° to ?20°C environmental temperature range, approximately 10 minutes prior to the onset of hail at the surface. Scattering calculations suggest that the LDR enhancement may have been caused by an increasing water fraction within the growing hailstones (spongy hail), or the development of a liquid water coat under wet growth conditions. Vertical structure of the Zdr fields was also examined. As hypothesized by Conway and Zrni?, it was found that the distinctness of the positive Zdr column associated with supercooled raindrops and incompletely frozen particles above the 0°C height varies from storm to storm.