Rapid-Scan Mobile Radar Observations of Tornadogenesis
Abstract
Historically, the generally accepted hypothesis surrounding tornadogenesis in supercell thunderstorms is referred to as the 'top-down' hypothesis. According to this theory, strong rotation develops above the ground first, followed by progressively strengthening rotation at lower heights until the vertically rotating column intersects the ground, contracts, and forms a tornado. This paper provides evidence from a novel rapid-scanning mobile Doppler radar (RaXPol) that this hypothesis is likely incorrect. Instead, it appears that in many cases, tornadic-strength rotation develops either at near-ground levels first, or contemporaneously throughout the depth of the tornado-bearing layer. Herein, the spatio-temporal evolution of the tell-tale radar-based feature of a tornado, the tornadic vortex signature, is examined for five case studies of tornadogenesis. Of particular importance is an analysis of the genesis of the violent and deadly 31 May 2013 El Reno tornado, for which case data are collected every 16 s at heights <20 m above ground level (AGL). On this day, 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). By coupling the precise video documentation available from The El Reno Project with the RaXPol radar observations, it was determined that a condensation funnel in contact with the ground appeared at a time when, the only evidence of tornadic-strength rotation in the radar data was in the 0° elevation angle RaXPol data, at a height of < 20 m AGL. 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, providing distinct evidence that for this case, the tornado formed from the bottom-up. Furthermore, in the 5 datasets that were examined, NONE of the tornadoes formed following the top-down process.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A54H..25H
- Keywords:
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- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSESDE: 3314 Convective processes;
- ATMOSPHERIC PROCESSESDE: 3329 Mesoscale meteorology;
- ATMOSPHERIC PROCESSESDE: 3354 Precipitation;
- ATMOSPHERIC PROCESSES