Horizontal cross-section showing low-altitude mesovortices.

Figure 1: Horizontal cross-section, at z=0.25km of rainwater mixing ratio (colorfilled and contoured at 1 g kg-1 increments), storm-relative horizontal velocity vectors, and the -1 K perturbation temperature isotherm (bold blue line), valid at t = 5h. Bold arrows point to low-altitude mesovortices. Red box indicates 40X40-km portion of the domain plotted in Fig. 2. Tick marks are plotted every 10 km.

Horizontal cross-section showing wind velocity and magnitude.

Figure 2: Horizontal cross-section, at z=0.127 km of ground-relative horizontal wind magnitude (color-filled and contoured at 5 m s-1 increments) and storm-relative horizontal velocity vectors, valid at t=5 h.

Damaging winds associated with low-altitude mesovortices within bow echoes

It is well established that squall lines with outward-bowing segments--bow echoes-- often produce damaging "straight-line" winds at the ground. Some recent results from a study of low-altitude "mesovortices" within bow echoes suggest an unexpected and new paradigm for the production of such winds, as well as an equally surprising mechanism for the formation of mesovortices.

The well-known conceptual model attributed to T.T. Fujita shows damaging winds forced by intense downdrafts just behind the apex of the bow echo. Results from Morris Weisman's (NCAR) and my numerical cloud model similarly show a narrow strip of strong winds at the apex. But, given model environments of moderate to strong vertical wind shear, the most damaging winds--quantified in terms of duration and areal extent-- are actually associated with low-altitude mesovortices located more than 20 km to the northwest of the apex. Moreover, the swath of these winds expands with time, as individual mesovortices merge to form fewer, though larger, vortices.

Sensitivity experiments show that significant low-altitude mesovortices develop in simulated squall lines and bow echoes only when the environmental vertical wind shear is within a relatively narrow range of values and, surprisingly, when the Coriolis force term in the model is nonzero. Mesovortexgenesis is initiated by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a period of less than an hour, 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 genesis of low-altitude mesovortices!

This research was presented at the AMS Conference on Severe Local Storms in August 2002, and helps motivate objectives of an upcoming field program known as the Bow echo and Mesoscale Convective Vortex (MCV) Experiment (see companion article on page four).

by Jeff Trapp

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