Performance of the hail differential reflectivity (HDR) polarimetric radar hail indicator

A series of poststorm surveys were conducted in the wake of hailstorms observed by the Colorado State University-University of Chicago-Illinois State Water Survey (CSU-CHILL) S-Band polarimetric radar. Information on hail characteristics ( maximum diameter, building damage, apparent hailstone density, etc.)was solicited from the general-public storm observers that were contacted during the surveys; the locations of their observations were determined using GPS equipment. Low-elevation angle radar measurements of reflectivity, differential reflectivity Z(DR), and linear depolarization ratio (LDR) were interpolated to the ground-observer locations. Relationships between the hail differential reflectivity parameter HDR and the observer-reported hail characteristics were examined. It was found that HDR thresholds of 21 and 30 dB were reasonably successful ( critical success index values of similar to 0.77) in respectively identifying regions where large ( similar to 19 mm in diameter) and structurally damaging hail were observed. The LDR characteristics in the observed hail areas were also examined. Because of sensitivities to variations in the hailstone bulk ice density, degree of surface wetness, and shape irregularities, the basic correlation between LDR magnitude and hail diameter was poor. However, when the reported hail diameters exceeded similar to 25 mm, LDR levels below similar to-24 dB were uncommon.