Tornadogenesis and Early Tornado Evolution in the El Reno, Oklahoma, Supercell on 31 May 2013

Abstract

Abstract This study documents the formation and part of the early evolution of a large, violent tornado near El Reno, Oklahoma, based on data from a mobile, polarimetric, rapid scan, X-band, Doppler radar. The main circulation associated with the tornado formed near the ground initially, ~90 s prior to the development of the vertically coherent vortex, which built upward through a vertical column of at least 3.5 km in less than 20 s, the update time of the Doppler radar data. Strong but broad rotation from 500 m to 1.5 km AGL also preceded the formation of the tornado at the surface by several minutes. A precipitation-loaded downdraft was observed in the right-forward flank of the storm, which could have enhanced evaporative cooling and allowed for a faster rate of baroclinic generation of low-level horizontal vorticity, while descending reflectivity cores in the right-rear quadrant might have enhanced low-level convergence to the rear of or along the leading edge of the rear-flank gust front. The intensification of the tornado occurred in spurts, not steadily, perhaps owing to surges in momentum at the surface associated with the precipitation-laden downdrafts. The tornado was highly tilted even when it was intensifying, calling into question the importance of a vertical juxtaposition of the mesocyclone aloft and the tornado at the surface. In this case study, while the development of a weak-echo hole was evidence of rotation, the absence of one did not mean that there was not a strong vortex, owing to the lofting of debris.