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 lowlevel, meso-␥-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 QLCS 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 lowlevel 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.

Morphology and evolution of cold‐frontal misocyclones

“…These results for an NCFR in a weakly unstable environment are similar to the results of Trapp and Weisman (2003) for QLCSs in highly unstable environments. Their idealized simulations indicated that the strongest, most damaging straight-line winds occurred on the flanks of evolving mesocyclones.…”

“…The outer nest, or 'mother domain', covered a large area of the UK at 13.5 km grid point spacing; further nests, domains d02, d03, d04, were centred on the region of interest at 4.5, 1.5 and 0.5 km grid point spacing respectively (Figure 1(a)). The grid point spacing of 500 m on the innermost domain was considered sufficient to resolve the phenomena we are interested in -vorticity maxima with diameters of < 4 km -as they were robust features in our simulations with grid point spacing of 2 km or less and in simulations reported in the literature (Trapp and Weisman, 2003;Wheatley and Trapp, 2008). The very high-resolution domain d04 consisted of 202 × 148 grid points (101 × 74 km) in the horizontal plane.…”

The Bow Echo and MCV Experiment: Observations and Opportunities

“…Weisman and Trapp (2003) and Trapp and Weisman (2003) have further documented the ability of numerical models to generate intense surface mesovortices, which are perhaps analogous to rotating cells thought to produce damaging winds in some cases. However, sufficient observations have generally been lacking to confirm the dynamics inferred from such simulations.…”

On the Mesoscale Structure of Surface Wind and Pressure Fields near Tornadic and Nontornadic Cold Fronts

“…The horizontal vorticity may be associated with either the ambient vertical wind shear or buoyancy gradients across the gust front. The tilting mechanism has been shown to be responsible for vortex genesis in quasi-linear convective systems forming in environments containing substantial buoyant instability (e.g., Trapp and Weisman 2003;Weisman and Trapp 2003). Misocyclones may also form when horizontal vorticity associated with horizontal convective rolls is tilted into the vertical, where the rolls intersect the front (Atkins et al 1995;Marquis et al 2007).…”