2.1
Low-Level Mesovortices within Squall Lines: Vortexgenesis and Association with Damaging Surface Winds
Robert J. Trapp, NOAA/OAR/NSSL, Boulder, CO; and M. L. Weisman
The formation and implications of low-level, meso-gamma scale vortices within quasi-linear convective systems (QLCSs) like squall lines and bow echoes are investigated. Such “mesovortices” are observed frequently, at times in association with tornadoes, and can significantly impact QLCS predictability.
Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs only when the environmental vertical wind shear is within a relatively narrow range of values, 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 horizontal crosswise 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. Negative vertical pressure gradients are subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line.
The QLCSs evolve into bow echoes whose strongest low-level winds surprisingly are found >=20 km to the northwest of the bow-echo apex rather than just behind the apex, as typically conceptualized. In other words, what are regarded as the most damaging “straight-line” winds are associated directly with low-level mesovortices. The swath of these winds expands with time, owing to a mesovortex amalgamation, or “upscale” vortex growth.
Session 2, Mesoscale Convective Systems II
Monday, 12 August 2002, 1:30 PM-3:00 PM
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