Extended Abstract (200K)12.8
Near-surface intensification during unsteady tornado evolution
D. C. Lewellen, West Virginia University, Morgantown, WV; and W. S. Lewellen
The intensification of a vortex flow in the region where the core meets the surface (the corner flow) relative to its strength aloft, is an essential ingredient of the destructive potential of a tornadic supercell thunderstorm. In earlier work we identified a swirl ratio based on the volume flux of low swirl fluid passing through the corner flow as a critical parameter in determining the structure and intensification found within a quasi-steady corner flow. In the present work we systematically consider strongly time-varying corner flows, again utilizing results from high resolution large-eddy simulations. In these cases the time rate of change of the volume flux of low swirl fluid passing through the corner flow (suitably non-dimensionalized) is found to be a key additional parameter in determining the greatest intensifications achieved and the structure of the corner flow at that time. A particular range in this parameter results in much larger near surface velocity intensifications than can be sustained for quasi-steady conditions. If the low swirl flux entering the corner flow drops either too quickly or too slowly, however, then large peak intensifications may not be encountered; in the former case because the core near the surface has insufficient time to constrict far enough while conditions are favorable; in the latter because a central downdraft forced by a growing vertical pressure gradient reaches the surface before the most favorable conditions are reached. The volume flux of low swirl fluid passing into the corner flow is easily changed by perturbations of the near surface flow conditions, even far from the vortex center, for example by a downdraft partially blocking the low swirl inflow at some radius.
On the mesocyclone scale these results suggest a possible route by which the rear-flank downdraft can directly promote tornadogenesis, and emphasizes how modest differences in near surface flow, and perturbations thereof, can represent the difference between an intense vortex on the surface or a negligible one. On the tornado scale these results are important ingredients in explaining the rapid changes in tornado intensification and structure often encountered during their evolution.
Supplementary URL: http://eiger.mae.wvu.edu
Session 12, tornadogenesis
Thursday, 15 August 2002, 8:00 AM-10:00 AM
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