The Relationship between Polarimetric Signatures and Rotation Derived from Doppler Velocity in Supercell Thunderstorms

Recent dual-polarization radar studies have indicated that signatures unique to supercell thunderstorms may provide added information concerning dynamic and kinematic processes. Particle size distribution information afforded by these variables lends insight into hydrometeor size-sorting, used to uniquely infer in-situ storm-modified flow. Specifically, enhanced storm-relative mean wind and strengthened storm rotation related to tornadogenesis have been observed through development of an arc-like shape in differential reflectivity (Z_DR), referred to as a Z_DR arc, as well as through the spatial separation of maximum regions of Z_DR and specific differential phase (K_DP) at low levels and in extended columns. While these signatures and their implications have been approached through individual or small-sample case investigation and modeling studies, current literature lacks in-depth investigation of a broader sample of storms within the context of other metrics of storm rotation and intensity. Additionally, comparative studies of these signatures and their application in varied regions, particularly from the perspective of operational S-band radar, are not well represented.

Current work analyzes tornadic and non-tornadic supercell storms from multiple climatic regions using data from the Next-Generation Radar (NEXRAD) network of polarimetric Weather Surveillance Radars - 1988 Doppler (WSR-88DPs). Specific signatures of low-level Z_DR arc and the separation of maximum regions of Z_DR and K_DP are examined relative to storm-scale rotation provided by azimuthal shear derived from Doppler velocity. This work also uniquely integrates supporting lightning analysis for insight into updraft behavior and inherent storm intensity for added context.

The primary objective of this study is to assess the prognostic potential of these signatures as they relate to low- to mid-level rotation, its association with updraft strength, and storm intensity. To this end, preliminary results are in agreement with earlier findings that the signatures explored here often coincide specifically with the onset of tornadoes and act as a good diagnostic of the low-level conditions important to tornadogenesis processes. While polarimetric variables may reinforce velocity indication of tornado development in this manner, they display poor prognostic value, providing minimal to no lead time. With respect to broader storm rotation, the strongest displays of these signatures in tornadic storms follow enhancements in mid-level rotation tied to the updraft. This reinforces convective intensification implied by strengthening rotation, but again suggests no added prognostic value. Also consistent with earlier work, the signatures are shown to develop in non-tornadic supercells though they are less persistent in time or weaker in magnitude. In addition to documented differences between tornadic and non-tornadic storms, results suggest that thresholds of parameter values associated with these signatures and their temporal evolution vary nontrivially between climatic regions. Combined, these early results have implications for the utility of these signatures in operations as well as the generalized application of related thresholds and spatiotemporal trends.