Storm-Scale Polarimetric Radar Signatures Associated with Tornado Dissipation in Supercells

Recent studies have shown that numerous storm-scale processes important to the production of severe hazards, such as tornadoes, are often observable as polarimetric “fingerprints.” More recently, French and Kingfield (2019) investigated the potential for using characteristics and behaviors of the tornadic vortex signature (TVS), the radar representation of a tornado, for tracking the dissipation of an ongoing tornado. It was found that TVSs were observed to repeatedly undergo significant changes in the last two WSR-88D radar volumes they were identified in prior to dissipation, introducing the possibility that tornado dissipation could be skillfully “nowcasted.” However, the behaviors examined in these studies focused solely on the TVS, and not on storm-scale processes or other features within supercells that may portend tornado dissipation. In particular, there are several known polarimetric signatures in supercells that are thought to result from internal storm processes, some of which also may contribute to tornado maintenance.

This study focuses on the use of dual-polarization Doppler radar data from the network of WSR-88D systems to observe the evolution of various polarimetric “fingerprints” within a large sample of supercell storms as they relate to tornado dissipation. These “fingerprints” include an increase in hook echo median drop size; a decrease in Z_DR column height, area or volume; a decrease in the size and/or magnitude of the Z_DR arc; and a decrease in the separation of Z_DR-K_DP maxima. Unique methods for case selection, quality control, and data analysis, are described; these methods ensure each storm-scale process manifested through these polarimetric features are isolated and highlighted. Presented will be results from using the data to study changes in bulk hook echo DSDs leading up to tornado dissipation. Preliminary results from the additional three polarimetric signatures also may be discussed.