Detection and Sizing of Hail Using the Differential Phase

Current WSR-88D’s algorithm for hail detection is based on three radar variables: reflectivity (Z), differential reflectivity (Z_DR), and correlation coefficient (CC). We consider adding differential phase (DP) to those three variables to better detect hail and estimate its size.

Our study reveals two characteristics of DP in hail thunderstorms. The first manifests as area of enhanced DP located in reflectivity cores. The enhanced DP is produced by the differential phase upon scattering off hailstones (so called phase delta). This is mostly caused by hail with sizes up to about 3 cm for which the current hail detection algorithm based on Z, ZDR, and CC produces the probability of hail up to about 0.7. In such cases, the DP unambiguously indicates hail.

The second type of DP characteristic in thunderstorms is its enhancement in areas at the far edge of a reflectivity core relative to radar. This DP type indicates unambiguously big and giant hail of sizes larger than 4 cm. The origin of this DP type is discussed and the amplitude of the enhanced DP is related to the hail size. From this study, a strong potential of DP for hail detection and sizing is inferred.