Single-doppler analyses of low-level tornado structure

Date

2014-08

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Abstract

To this day, considerable uncertainty exists regarding the low-level wind fields and vertical structure associated with tornado vortices. Unfortunately, ground-clutter contamination and beam blockage make resolving the lowest levels of tornado vortices difficult for many research radars. Higher-frequency radars have proven especially useful in detecting important low-level aspects of tornado vortices. Owing to the higher sensitivity and finer angular resolution, these observations are critical in verifying and expanding upon our current understanding of tornado structure, which prior to now has relied primarily on theory and laboratory simulations.

This study utilizes TTUKa and Doppler on Wheels radar data to resolve the near-surface horizontal and vertical structure of tornadoes. Data from two specific cases, 14 April 2012 near Cherokee, OK, and 18 May 2013 near Rozel, KS, are analyzed. Horizontal and vertical cross sections of tornadoes in each case are captured using PPIs and RHIs, respectively. Vortex structure and evolution are also analyzed using the Ground-Based Velocity Track Display (GBVTD) technique. Through the GBVTD analyses, an in-depth look at axisymmetric radial profiles of tangential and radial velocity, angular momentum, and vertical vorticity are discussed. Finally, an attempt is made at calculating swirl ratio for the 18 May 2013 tornado.

Radial profiles of tangential velocity, vertical vorticity, and angular momentum during the 14 April 2012 case compare well with previous simulations based on the Burgers-Rott vortex model. A strong horizontal vortex is observed propagating towards the observed tornadoes, potentially influencing the radial profiles beyond the core of each tornado. The circulation of this vortex is counter-intuitive to what would be expected if caused by baroclinic vorticity generation along the inner edge of the forward-flank downdraft. It is proposed the vortex results from a rear-flank gust front wrapped around the tornado vortex. Strong near-surface outflow is also observed in RHI cross-sections of the second tornado, comparing well with subsequent GBVTD-derived radial velocity profiles.

A violent tornado observed on 18 May 2013 also resembles the Burgers-Rott vortex model in GBVTD-derived profiles performed roughly 80 m AGL. However, swirl ratios for this tornado range between 0.5 and 3.75, which based on previous studies and simulations, suggest a transition from single-celled to multiple-vortex structure. This is confirmed through visual observations by the TTUKa team, as well as GBVTD-derived radial profiles performed at 165 m AGL. A strong, sloping inflow layer is resolved in RHI cross-sections of this tornado, ranging in depth from 300 m to 80 m as one gets closer to the tornado center.

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Keywords

Tornado, Structure, TTUKa (doppler radar), Atmospheric, Science

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