Hail Size Determination from Three-Body Scatter Spikes Using Dual-Pol Radar

Three-body scatter spikes (TBSS) have been identified for some time as a potential way to identify severe hail. It was initially suspected that a TBSS seen in the radar display of a thunderstorm with an NWS 88D radar was a certain indication of severe-sized hail (hail larger than 3/4 inches). However, three recent developments have altered this perception. First, the NWS changed the definition of severe hail from 3/4 inches to 1 inch. Second, the introduction of over-sampling (super-res data) for the NWS 88D radars has meant that far more TBSS and more subtle TBSS are now seen than previously detectable. Third, the introduction of dual-pol capability to the 88D radar has made subtle TBSS features even easier to detect. Consequently, TBSS or TBSS-like features can now be identified using an 88D for most severe hail-producing storms. It has also been noted from NWS severe storm warning verification efforts that many storms with a TBSS on radar, are not actually severe. Furthermore, there are many cases of storms with giant hail that do not have even a subtle indication of a TBSS. Currently, while a TBSS does indicate hail with a storm, there is no way to determine the size of the hail from this information. This is similar to the situation with the 88D Hydrometeor Classification Algorithm (HCA) which combines reflectivity and differential reflectivity to assess the presence of hail. The HCA is very precise in indicating the presence of hail, but there is currently no way to directly determine the size of the hail from dual-pol parameters.

In this work, we investigate the relation between the characteristics of hail spikes as seen in reflectivity and polarization parameters, to the size, type, and distribution of the scatterers in the storm. This is done through a theoretical treatment of radar scattering from distributions of hail and/or rain to predict what hail spikes should look like as a function of scatterers. We then compare predicted hail spikes with verified severe and non-severe storms selected from cases from the 88D network.