Observed Bulk Hook Echo Drop-Size Distribution Evolution in Supercell Tornadogenesis

Past studies of supercells have established the importance that the rear-flank downdraft (RFD) thermodynamic structure has on the likelihood for tornadogenesis. The ongoing dominant microphysical processes in the RFD are likely to be one of the most important determinants of the near-surface RFD temperature and moisture structure. Disdrometers obtain point measurements of drop size distributions (DSDs) to offer evidence of these microphysical processes, but such data are difficult to obtain given the hazardous weather in supercell RFDs (hook echo region) and provide poor spatiotemporal sampling of storms. Another approach is to infer bulk DSDs in supercell hook echoes using polarimetric radar data from the network of WSR-88D systems. Past studies using data from mobile, dual-polarization Doppler radars and specialty research radars have provided some evidence that tornadic supercell hook echoes contain smaller median rain drop sizes than seen in non-tornadic storms. In addition, in one case, median raindrop sizes plummeted in the 10 min prior to tornadogenesis, and in three cases, median raindrop sizes increased prior to tornado dissipation. However, a lack of data have prevented additional exploration into the possible connection between hook echo DSDs and the tornado life cycle.

This study details the development of a WSR-88D climatology for the study of several polarimetric aspects of supercell thunderstorms, including their use to estimate supercell DSDs. Novel approaches to case selection, data processing, quality control, and data analysis that will allow for streamlined studies of hundreds of cases are described. Also discussed will be preliminary results from our first attempt at exploiting the large number of supercell cases: an assessment of how bulk hook echo DSDs change at storm low levels, if at all, leading up to tornadogenesis for isolated supercells.