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

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 raindrop sizes than those seen in non-tornadic storms. In addition, in one case, median raindrop sizes plummeted in the 10 min prior to tornadogenesis. However, a lack of data have prevented additional exploration into the possible connection between hook echo DSDs and the tornado life cycle.

In this presentation, results from the analysis of a large sample of tornadogenesis and tornadogenesis failure cases will be discussed. The cases originate from a recently-developed WSR-88D climatology designed to study several polarimetric aspects of supercell thunderstorms, including their use to estimate supercell hook echo DSDs. Temporal evolution of storm low level differential radar reflectivity (ZDR), median drop size (D0), specific differential phase (KDP), and total number concentration (NT) for the time prior to tornadogenesis and tornadogenesis failure will be presented. Near storm environment analysis will be shown for a subset of cases to investigate potential differences between boundary layer moisture. Bootstrap statistics are used to test for statistical significance of any differences between case types. Based on study results, realistic prospects of using DSD proxies for future real-time tornado warning scenarios will also be discussed.