Abstract
Nearly continuous wind retrievals every three minutes for an unprecedented 90-minute period were constructed during multiple mesocyclone cycles in a tornadic high-precipitation supercell. Asymptotic contraction rate analysis revealed the relationship between the primary and secondary rear-flank gust fronts (RFGF and SRFGFs) and the rear-flank downdraft (RFD) and occlusion downdrafts. This is thought to be the first radar-based analysis where the relationship between the near-surface gust fronts and their parent downdrafts has been explored for sequential mesocyclones. Changes in the SRFGFs were associated with surges in the RFD. During part of the mesocyclone lifecycle, the SRFGF produced a band of low-level convergence and associated deep updraft along the southwestern side of the hook echo region that ingested the RFD outflow and limited both entrainment into the RFD and reinforcement of low-level convergence along the leading edge of the primary RFGF. The second mesocyclone intensified from stretching in an occlusion updraft rather than in the primary updraft. This low-level mesocyclone remained well separated from the updraft shear region vorticity that was associated with a more traditional midlevel mesocyclone. However, the third mesocyclone initiated in the vorticity-rich region of the primary updraft zone and was amplified by stretching in the primary updraft.
Highlights
Despite observations that suggest there might be few kinematic differences between tornadic and nontornadic lowlevel mesocyclones [1, 2], more than half of low-level mesocyclones do not produce tornadoes [3]
It is important to find ubiquitous storm-scale flow structures associated with cyclically tornadic supercells that may be absent in nontornadic cyclic supercells or noncyclic tornadic supercells [7, 8]
Gust fronts were depicted at the Advances in Meteorology leading edge of the downdraft outflows with the rear-flank gust front (RFGF) propagating out beneath the mesocyclone as the rear-flank downdraft (RFD) expanded
Summary
Despite observations that suggest there might be few kinematic differences between tornadic and nontornadic lowlevel mesocyclones [1, 2], more than half of low-level mesocyclones do not produce tornadoes [3]. Surface observations from individual cycles suggest that tornadic supercells tended to have weaker cold pools regardless of whether they had strong or nonexistent forward flank convergence boundaries [6, 19,20,21,22] Another detail that has been added to the conceptual model has been the presence of a smaller-scale dynamically driven “occlusion” downdraft described as a separate downdraft from the primary RFD [23]. Internal downdraft outflows have been observed at the surface in close proximity to tornadoes with highly variable thermodynamic structure [30,31,32,33,34,35,36] These regions have been observed in radar-based analyses being manifested as a secondary, rear-flank gust front (SRFGF, [29, 37,38,39,40]) with rapidly evolving flow [41].
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