Observational Analysis of Left- and Forward-Flank Boundaries in Supercells

Airmass boundaries are the loci for tornadogenesis, but their overall importance to tornado formation remains unclear.  The TORUS (Targeted Observation by Radars and UAS of Supercells, 2019, 2022) and TORUS-LItE (TORUS Left-flank Intensive Experiment, 2023) field campaigns were focused on data collection aimed in part to clarify the role played by boundaries in the generation/amplification of near-surface rotation in supercells.  In this presentation, results from analysis of in situ observations of the left/forward-flank (LF/FF) of supercells targeted during TORUS and TORUS-LItE, complemented by analysis of surface observations from VORTEX2 (the second Verification of the Origins of Rotation in Tornadoes Experiment), will be presented.

In the first part of this presentation, analysis is presented of mobile mesonet observations from VORTEX2, TORUS, and TORUS-LItE aimed to assess the frequency, location, thermodynamic characteristics, and kinematic characteristics of LF/FF boundaries. A total of 228 boundary identifications are cataloged. The vast majority of supercells (87%) have a LF/FF boundary but, due to storm-to-storm and temporal variability, spatial patterns in these boundaries are difficult to discern.  Pseudo-vertical vorticity (the along-transect horizontal shear) is largest near and on the cool side of identified boundaries. Tornadic supercells consistently show larger pseudo-vertical vorticity along LF/FF boundaries than non-tornadic supercells.  Density gradients across these boundaries are smaller for tornadic storms but the magnitude of these gradients scale directly with the magnitude of pseudo-vertical vorticity.

During TORUS 2019 and TORUS-LItE uncrewed aircraft systems (UAS) were used to collect (above ground) in situ kinematic and thermodynamic observations across LF/FF boundaries. Results from analysis of three supercells (11 June 2019, 26 May 2023, and 12 June 2023) observed by UAS in TORUS and TORUS-LItE are presented herein.  Multiple LF/FF boundaries are often detected in each supercell but are typically separated by less 1000 m.  As such, these resemble outflow surges more than separate LF and FF boundaries.  Lateral gradients in vertical motion, distributed over scales of several hundred meters, made appreciable contributions to horizontal vorticity. Thus, horizontal vorticity estimates based solely on vertical shear might underestimate its magnitude. Analysis of the 26 May 2023 case reveals a likely streamwise vorticity current (SVC) embedded in the head of a density current-like feature. It is hypothesized that the positive hydrostatic pressure perturbations that would otherwise lead to a gust front designation for these LF/FF boundaries is masked by the dynamic pressure reductions associated with significant horizontal vorticity.