Novel Observations of the 2013 El Reno Tornado: Confirming Ground-Up Tornadogenesis through Coupled Rapid-Scan Radar Data and Crowd-Sourced Storm Chaser Videography

The traditional paradigm of supercell tornado formation has, until recently, been thought to begin at mid-levels of the troposphere, where rotation starts to intensify. Through a somewhat slow process (O= ~10 minutes) often referred to as the ‘dynamic pipe effect’, or ‘top-down process’, the rotating air entering the bottom of the mid-level vortex converges, its rotation is amplified until it achieves cyclostrophic balance, and this process progresses toward the ground, forming a tornado when the rotating air reaches the surface. This hypothesis was originally observed in the late 1970’s in numerical and laboratory model simulations (e.g. Smith and Leslie, 1978), and in experimental Doppler radar data (e.g. Brown et al., 1977). Twenty years later, it was still believed that the dynamic pipe effect was responsible for ~67% of tornadoes forming from supercells (Trapp et al. 1999). However, with advances in radar observations over the last decade, particularly the development of rapid-scan radar, this process and its associated statistics have come into question. Several recent publications have concluded that the top-down process was not observed in the supercell tornadogenesis cases analyzed in the context of their studies (French et al. 2013, 2014; Houser et al. 2015). While these studies have clearly supported their claims and successfully refuted the top-down process, near-surface data have been missing from the datasets.

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).

References:

Brown, R. A., L. R. Lemon, and D. W. Burgess, 1978: Tornado detection by pulsed Doppler radar. Mon. Wea. Rev., 106, 29–38.

French, M. M., H. B. Bluestein, I. Popstefanija, C. A. Baldi, and R. T. Bluth, 2013: Reexamining the vertical development of tornadic vortex signatures in supercells. Mon. Wea. Rev., 141, 4576-4601.

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Houser, J. B., H. B. Bluestein, and J. C. Snyder, 2015: Rapid-scan, polarimetric, Doppler-radar observations of tornadogenesis and tornado dissipation in a tornadic supercell: The “El Reno, Oklahoma” storm of 24 May 2011. Mon. Wea. Rev., 143, 2685–2710.

Smith, R. K. and L. M. Leslie, 1978. Tornadogenesis. Quart. J. Roy. Meteor. Soc., 105, 107-127.