Comparisons of WSR-88D Dual-Pol Hail Size Discrimination Algorithm Output with Physical Hail Measurements and In Situ Impact Disdrometer Data

The upgrade of all the National Weather Service WSR-88D to dual-polarization capability ushered in improved hail detection capability. The hydrometeor classification algorithm (HCA) was the first operational product to leverage the capability to more accurately distinguish hail. The algorithm was an improvement to previous hail detection methods, which solely relied on reflectivity factor at horizontal polarization. The hail size discrimination algorithm (HSDA) became operational in 2017 and stratifies detected hail into three classes: small hail (< 2.5 cm), large hail (2.5-5 cm), and significant severe hail (> 5 cm). The use of a rapidly deployable network of hail impact disdrometers and the physical hail measurements made during the Insurance Institute for Business & Home Safety’s field research program are ideal data sources to explore the performance of the hail size discrimination algorithm. This study examines six supercell thunderstorms sampled in 2017-2018. Hail swaths were produced using the HCA/HSDA output and compared to the in situ and physically measured maximum hail sizes. The HCA/HSDA product performed well in capturing the boundaries of the swath of hail. Size estimation, however, remains challenging. The algorithm did not perform well in detecting very large hail from the mature supercells where size sorting occurred.