Confirming Bottom-up Tornadogenesis in the 31 May 2013 El Reno Tornado

On 31 May, 2013, a record breaking 2.6 mile wide tornado with winds exceeding 300 mph struck central Oklahoma, just outside the city of 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 evolution of the storm and tornado 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, Google Earth, and storm chaser GPS logs, all videos were synchronized within 1 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 Project with the RaXPol radar observations and found that a condensation funnel in contact with the ground appeared at 23:02:54. At this time, the only evidence of tornadic-strength rotation in the radar data was in the 0° elevation angle RaXPol 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 difference between inbound and outbound velocities, radar-based tornadogenesis time was approximately 23:04:15. Without the visual confirmation of a condensation funnel, the radar-based start time of the tornado would have been approximately 1 minute and 20 seconds later than when it was actually observed. The coupled visual and near surface radar observations enable an analysis of the tornadogenesis process that has never before been obtained, providing a missing link in the story of tornado formation: the rotation associated with the 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). This project investigates the tornadogenesis process, presents the evidence supporting this claim, and provides a hypothesis for the mechanism for tornadogenesis in this case.