An analysis of observed cold pools in VORTEX-SE supercells and quasi-linear convective systems

Date

2019-05

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Abstract

Many past severe weather field campaigns have focused on increasing our understanding of supercell dynamics within the traditional high CAPE and large vertical wind shear environments in the Great Plains of the United States. However, there is a lack of observational data of supercells and linear convective systems that occur in nontraditional environments (such as high shear and low CAPE) that are endemic to the southeastern United states. The Verification of the Origin of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) field project remedies this lack of in-situ data by observing supercells and QLCS circulations that occurred in Northern Alabama and Southern Tennessee during 2016 and 2017. During the VORTEX-SE field campaign, 16 Texas Tech University “StickNet” surface observing platforms recorded the equivalent potential temperature (θ_e) and virtual potential temperature (θ_v) deficits within the cold pools of supercell thunderstorms and QLCSs. These measurements were motivated by previous observational studies which showed tornadic supercells tend to have weaker temperature deficits and higher buoyancy in their flanking downdraft regions than nontornadic supercells. However, there are no studies that consider the cold pool deficits between tornadic and non-tornadic segments of QLCS events. This study compares 39 samples of potential temperature deficits and gradients taken within 15 km of tornadic and nontornadic mesocyclones and mesovortices during VORTEX-SE. No obvious differences in the thermodynamic deficits of the tornadic and nontornadic samples were found, likely due to the weakness of the produced tornadoes (≤EF1) and the small tornadic sample size (five cold pools). Comparison across storm mode did find some differences, with QLCS cold pools producing larger θ_v deficits than those observed in supercells. More importantly, our findings suggest that in a QLCS, the cohesiveness of a cold pool edge and the rate of buoyancy decrease may be important for tornadogenesis by virtue of the implied baroclinic vorticity generation. Two QLCS events exhibited the strongest θ_v gradients (dθ_v/dn) near tornadic mesovortices than near nontornadic mesovortices or other segments of the convective line. These results will help establish a climatology of Southeastern supercell and QLCS cold pool deficits and characteristics, components of which may shed light on mechanisms for tornadogenesis.

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Keywords

Supercells, Tornado, Cold pool, Thermodynamics, Quasi-linear convective system, Mesoscale convective system

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