Assessment of Spatial-Temporal Heterogeneities in Quasi-Linear Convective System Cold Pools during the PERiLS Field Project

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2023-08

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Abstract

Mesovortexgenesis and its association with tornado likelihood in Quasi-Linear Convective Systems (QLCS) has emerged as an important field of exploration over the past few decades. The majority of prior studies have used idealized simulations or data assimilation to quantify theories of meso-γ-vortexgenesis and tornadogenesis in linear systems. Past studies also have shown a possible association of low-level horizontal vorticity along the leading edge of QLCS cold pools with cold pool strength through virtual potential temperature and pseudo-equivalent potential temperature gradients. However, observations of QLCS along-line heterogeneities are lacking, and, thus, this association has not been exclusively observed among different environmental regimes, such as varying shear/CAPE profiles. The Propagation, Evolution, and Rotation in Linear Storms (PERiLS) is the first large-scale, in-situ effort to bridge the gap between theory and observation. These efforts include (1) building a climatological record of QLCS cold pool intercepts, and (2) relating these cold pool properties (and implied baroclinic vorticity generation) to tornado likelihood. Texas Tech University’s focused role in PERiLS was the strategic placement of and collection of ground measurements by 24 StickNet probes. Two-thirds of the probes were dedicated to a 4×4 “StesoNet” grid, spaced 20-30 km apart to measure the background base state and identify along-line mesoscale heterogeneities of the QLCS cold pool, a spacing typically within the resolution of most operational model forecast products. The remaining eight probes were reserved for a rapid-deployment strategy, spaced 0.5-1.0 km apart and oriented parallel to the QLCS leading edge, to target the storm-scale structure of tornadic and nontornadic mesovortices at very-fine resolution. The measurement of along-line heterogeneities in QLCS cold pool strength at such small spatial scales has never been conducted in a coordinated field effort of this magnitude. This study will focus on four cases observed in the first year of PERiLS. Inferred low-level horizontal vorticity from various research and operational radars will allow for the identification of mesovortices along the leading edge of each cold pool. Tornado damage surveys from NOAA’s Damage Assessment Toolkit and Storm Events Database will be used to differentiate between tornadic and nontornadic mesovortices, further being categorized into pre-tornadic, time of tornado, and post-tornadic. Time-to-space conversions of StickNet data will help quantify spatiotemporal heterogeneities in cold pool strength, and will be compared across three different regimes for each observed QLCS segment: non-rotating, nontornadic, and tornadic. The main goals of this study are to (1) identify areas of low-level baroclinic vorticity generation by virtue of surface thermodynamic gradients, (2) correlate tornado likelihood with areas of higher baroclinity, and (3) continue to build upon the climatological record of QLCS cold pool intercepts in different shear/CAPE environments. Out of the four QLCS cases from the 2022 field campaign of PERiLS, 99 total cold pool intercepts by StickNets have been analyzed. Areas with sharper virtual potential temperature gradients were found near rotating segments of the leading edge. Analysis of these gradients infers higher areas of baroclinity, and thus implies greater low-level vorticity generation and possible areas of higher tornado likelihood, which builds upon previously found correlations between density gradients along the leading edge of QLCSs and tornadogenesis. In addition to the gradient analysis, other objectives are explored, including the weak observed cold pools in leading supercells, cold pool strength dependency of ‘hybrid’ mesovortices with the time-of-merger of supercells with the QLCS, and storm-relative wind analysis of three highly sampled mesovortex intercepts.

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Quasi-Linear Convective Systems (QLCS), Baroclinic vorticity, Cold pools, Tornadogenesis, Mesovortexgenesis, Mesocyclogenesis, Supercells, Convective Systems, Surface Observations, Propagation, Evolution, and Rotation in Linear Storms (PERiLS), Southeast

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