Low-level mesovortices within squall lines and bow echoes. Part II: Their genesis and implications

Abstract

This two-part study proposes a fundamental explanation of the genesis, structure, and implications of low-level, meso-y-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such "mesovortices" are observed frequently, at times in association with tornadoes. Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs, especially when the environmental vertical wind shear is above a minimum threshold and when the Coriolis forcing is nonzero. As illustrated by a QLCSs simulated in an environment of moderate vertical wind shear, mesovortexgenesis is initiated at low levels by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a 30-min period, the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative and, more notably, planetary vorticity is stretched vertically: hence, the Coriolis force plays a direct role in the low-level mesovortexgenesis. A downward-directed vertical pressure-gradient force is subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line. In moderate-to-strong environmental shear, the simulated QLCSs evolve into bow echoes with "straight line" surface winds found at the bow-echo apex and additionally in association with, and in fact induced by the low-level mesovortices. Indeed, the mesovortex winds tend to be stronger, more damaging, and expand in area with time owing to a mesovortex amalgamation or "upscale" vortex growth. In weaker environmental shear-in which significant low-level mesovortices tend not to form-damaging surface winds are driven by a rear-inflow jet that descends and spreads laterally at the ground, well behind the gust front.