Numerical simulations of interactions between squall lines and supercells

Supercell thunderstorms and quasi-linear convective systems (QLCSs, or ``squall lines'') have both received a great deal of attention in the severe storms literature over the last 30 years, leading to a better understanding of both phenomena. However, each morphology has generally been studied independently of the other, leaving a void in the knowledge base regarding what happens when these convective structures are present in close proximity to one another, which is not uncommon. To this end, the present study is investigating mergers between isolated supercell thunderstorms and squall lines. Eight archetypal cases wherein at least one supercell merged with a squall line have been examined to find key commonalities evident from WSR-88D and other operationally available observations. One of the notable findings to emerge from this initial study was that in every case the characteristics of the supercell (e.g. strong radar reflectivity, mid- and low-level rotation) remained evident at least 1 hour (usually longer) after the merger, fundamentally altering the appearance of the squall line. This included several cases where a structure similar to a line-end vortex evolved post-merger.

Numerical simulations are being employed to assess the dynamic processes responsible for this observed behavior, particularly how rotation is sustained following the merger. Of additional interest is the impact that squall line-induced changes to the pre-line environment have on the supercell prior to the merger. These are hypothesized to be important in light of the observational analysis, but further investigation using numerical models will allow us to more definitively identify the role of these mesoscale environmental changes. A wide range of simulations are being run, including idealized experiments designed to test specific hypotheses under controlled conditions, as well as full-physics case-study simulations geared toward replicating actual merger events.