Evolution of the Tornado and Near-Tornado Wind Field of the Selden, Kansas Tornadic Supercell on 24 May 2021 Using a Rapid-Scan, X-Band, Mobile, Doppler Radar

Abstract

On 24 May 2021, a supercell in Selden, KS produced tornadoes which were probed by RaXPol, the University of Oklahoma’s rapid-scan, mobile, X-band, polarimetric Doppler radar. According to the National Weather Service, the primary tornado was on the ground for 30 minutes and attained a maximum width of 850 yards as a multiple vortex tornado. Throughout this time, the tornado produced EF-1 damage in and around Selden. RaXPol was deployed twice on this tornado; RaXPol moved once as the tornado was approaching. During the first deployment, data from shortly after tornadogenesis to right after a transition to multiple vortex phase were collected. After rapidly redeploying to a safer location, data were then recorded from the end of the tornado’s multiple vortex phase through its dissipation. While the cyclonic tornado was ongoing during the second deployment, a strong anticyclonic vortex or marginal tornado at the southern end of the Rear Flank Gust Front (RFGF) passed directly over RaXPol. Along with a high-definition video taken of the cyclonic tornado from the first deployment location, the high-resolution data collected by RaXPol throughout the lifespan of both vortices provide a rare opportunity to correlate structural features of the cyclonic tornado, anticyclonic vortex, and cyclonically rotating parent supercell to changes in tornado movement and behavior. Using the data captured by RaXPol, a thorough analysis of the tornado pair and the parent supercell was completed and cross sections through both vortices were constructed through their respective Doppler velocity couplets roughly perpendicular to the radar beam. Ultimately, the RaXPol data help to construct a detailed narrative of the Selden tornado pair.

RaXPol observed several important phenomena concerning the Selden tornado pair. Numerous significant shifts in the track of the primary cyclonic tornado were identified, and the tornado as well as its parent low-level mesocyclone were observed to occlude with time leading up to the tornado’s dissipation. During analysis, various Rear Flank Downdraft momentum surges (RFD surges) were identified, and they were found to be of critical importance to the occlusion and track behavior of the Selden tornado. The transition of the Selden tornado into and out of a multiple vortex phase were also observed, and the initial transition of the tornado into its multiple vortex phase was found to be the result of a temporary inflow disruption associated with RFD surges. While the primary cyclonic tornado was ongoing, RaXPol observed two independent anticyclonic vortices with different behavior, including a ‘rogue’ anticyclonic vortex and the marginally tornadic anticyclonic member of the Selden tornado pair. The ‘rogue’ anticyclonic vortex appeared very near the Selden tornado for a brief time, and numerous ideas for its origin and nature are discussed. The marginally tornadic anticyclonic vortex was found to form when a potent RFD surge impinged on the southern end of the RFGF, and the vortex was observed to occlude in a similar manner as the primary Selden tornado leading up to its dissipation.