On 31 May, 2013, a record-breaking 4.2-km wide tornado with winds >135 ms-1 struck central Oklahoma, just outside El Reno. On this day, the Rapid-Scan X-Band Polarimetric (RaXPol) radar collected an unprecedented dataset during tornadogenesis, acquiring data as low as <10 m above ground level. Additionally, a large number of storm chasers were present in the vicinity of the storm enabling a comprehensive visual survey of the supercell and tornado evolution from crowd-sourced still and video photography (The El Reno Survey Project, Seimon et al. 2015). Through a laborious process of spatio-temporal linking utilizing lightning flash frequency characteristics and Google Maps geolocations, all videos were synchronized to within 30 ms, creating a comprehensive visual database and enabling a detailed analysis of the storm from a variety of viewing angles and distances. The current project coupled the visual observations of the El Reno Survey with RaXPol observations and found that a condensation funnel in contact with the ground first appeared at 23:02:17 UTC. At this time, the only evidence of tornadic-strength rotation in the radar data was in the 0° elevation angle data. There was NO tornado vortex signature in ANY of the other radar data through 3.5 km. Using the traditional methodology of defining tornadogenesis as the time when a vertically continuous vortex in contact with the ground existed with a 40 m s-1 difference between inbound and outbound velocities, radar-based tornadogenesis time was approximately 23:04:15 UTC. Without the visual confirmation of a condensation funnel, the radar-based start time of the tornado would have been nearly 2 minutes later than actually observed. The coupled visual and near surface radar observations enable an analysis of the tornadogenesis process that has never before been obtained that provides a missing link in the story of tornado formation: the rotation associated with this tornado was clearly present at the surface first. Subsequently, rotation contracted aloft nearly simultaneously over the depth of the column for which data were collected (3.5 km).
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