Dual-Polarization Radar and Environmental Characteristics of Storms Producing Large Accumulations of Small Hail

Hail-bearing storms produce substantial socioeconomic impacts each year, yet challenges remain in forecasting the precise hail threat supported by a given environment and in using radar to accurately determine hail sizes. One class of hail threat is storms producing large accumulations of small hail (SPLASH). This study is an analysis of the environments and polarimetric radar characteristics of such storms. Thirteen SPLASH events were selected to encompass a broad range of geographic regions and times of year. Rapid Refresh model output was used to characterize the mesoscale environments associated with each case. This analysis reveals that a range of environments can support SPLASH cases; however, some commonalities included large precipitable water (exceeding that day's climatological 90th percentile values, surface-based CAPE <2500 J kg^-1, weak storm-relative winds (< 10 m s^-1) in the lowest few km of the troposphere, and a small component of the storm-relative flow orthogonal to the 0-6-km shear vector. Many of the storms were weak supercells that featured distinctive S-band radar signatures, including compact (< 20 km across) regions of high reflectivity factor (with maximum values in some cases exceeding 70 dBz), significant differential attenuation evident as negative differential reflectivity extending downrange of the hail core, and anomalously large specific differential phase (K_DP). K_DP values often approached or exceeded the National Weather Service’s operational color scale upper limit of 10.7 deg km^-1; reprocessing the level-II data revealed K_DP >17 deg km^-1, the highest documented in precipitation at S band. These high-K_DP regions are tracked over an hour during the SPLASH event, with radar signature characteristics quantified and compared across cases. Electromagnetic scattering calculations using the T-matrix method confirm that large quantities of small melting hail mixed with heavy rain can plausibly explain the observed radar signatures.