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ABSTRACT
Since maximum Mach numbers in an F5 tornado are expected to, at least, reach ~0.4, a study was undertaken to determine the major potential compressibility effects in such flows. Results from compressible large-eddy simulations of tornado corner flow dynamics are summarized. Comparison with previous incompressible simulations indicates that Mach number effects tend to be modest and may be estimated by an isentropic approximation. As the average maximum Mach number M within the tornado increases above one-half, the largest changes occur for "low-swirl" corner flows, those exhibiting a central vertical jet off the surface capped by a vortex breakdown, with the vortex breakdown found to occur at significantly greater heights as M increases. It also appears that the highest Mach numbers are most likely to occur during rapid transients in near-surface intensification that can sometimes occur during a tornado's evolution.
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1. Introduction
There has long been speculation about what maximum wind speeds are achievable in extreme tornadoes. A recent review (Davies-Jones et al. 2001) cites a limit on observations for F5 tornadoes of between 125 and 140 m s^sup -1^, with these values somewhat larger than the so-called thermo-dynamic speed limit based on convective available potential energy (CAPE) in the storm environment. This would appear to limit the maximum Mach number to <~0.5. Already this is high enough to suggest that compressibility effects may modestly alter the tornado flow structure, and there are reasons to question whether even higher Mach numbers could be reached. First, the observations involve considerable spatial and temporal averaging. second, Lewellen et al. (2000a) have shown that the interaction of a tornado with the surface can lead to significant near-surface intensification: low-level swirl velocities up to 2.5 times the maximum swirl velocity aloft and vertical velocities an additional factor of 1.4 larger yet. These velocities occur in the corner flow region, where the vortex core meets the surface and the surface layer inflow turns up into the central core of the tornado. Much higher near-surface intensifications appear possible in a particular unsteady process that we refer to as "corner flow collapse" (Lewellen et al. 200Ob; Lewellen and Lewellen 2002). The name refers to rapid reduction in vortex core radius near the surface...