In concentrating on this part of the flow we knowingly set aside the important question of tornadogenesis on the larger scale, that is, of how the storm-scale flow gives rise to and maintains the swirling, converging plume on the few kilometer scale that makes the occurrence of an intense tornadic vortex possible. As the place where the tornado vortex meets the ground, this region is of particular relevance, typically being the scene of the largest velocities, lowest pressures, sharpest velocity gradients, and greatest damage potential in the entire flow. In this work we focus on the last of these regimes and, in particular, on the tornadic corner flow where the boundary layer flow transitions into the core flow. In broad caricature we can identify three length scales of direct importance to the tornado structure: the storm scale of tens of kilometers, which ultimately drives the entire flow the outer tornado scale of a few kilometers in which the flow may be considered a converging, swirling plume 1 and the inner tornado scale of tens to several hundreds of meters characterizing the tornado core, boundary layer, and corner flow regions of the flow. Consequently one finds in the field that storms with seemingly quite similar structure on the kilometer scale may give rise to tornadoes interacting with the surface with quite different structures, or even to no tornado at all. More important, physical processes on a broad range of length scales from a few meters in the surface layer to many kilometers on the storm scale are all important and strongly coupled with each other. Correlating these studies with actual tornadoes in the field is more problematic here the flow is highly turbulent, and many physical parameters are involved. Controlled laboratory and numerical experiments of confined tornado-like vortices have produced similar ranges of flow structures when a single parameter such as the swirl angle of the incoming flow is varied. Categorizing and understanding this range of structures has been a long-standing goal of tornado research. For example, some tornadoes display thin, relatively smooth, almost elegant single funnels on the ground, while others are composed of many highly turbulent secondary vortices revolving about large central cores. Viewing any appreciable sample of the videotape records of actual tornadoes, one is impressed by the wide variety of flows that are evidenced, not only in the range of sizes and intensities, but in the different structures encountered. This sensitivity suggests that differences in the near-surface inflow layer may be a critical factor in determining whether an existing supercell low-level mesocyclone spawns a tornado or not. As S c decreases, the low-level vortex intensity rises to a maximal level where mean swirl velocities near the surface reach 2.5 times the maximum mean swirl velocity aloft further decreases force a transition to a much weaker low-level tornado vortex. The authors define a local corner flow swirl ratio, S c, based on the total flux of low angular momentum fluid through the corner flow and show that it parameterizes the leading effects on the corner flow of changes to the flow conditions immediately outside of the corner flow. Changes in the surface layer inflow or upper-core structure can dramatically affect the level of intensification and turbulent structure in the corner flow even when the swirl ratio of the tornado vortex as a whole is unchanged. This low swirl fluid arises initially from outside or below the larger-scale vortex or through frictional loss of angular momentum to the surface and forms much of the vortex core flow after it exits the corner flow region. A key ingredient of the corner flow dynamics is the radial influx of fluid in the surface layer with low angular momentum relative to that of the fluid in the main vortex above it. The most important physical variables considered are the tornado-scale circulation and horizontal convergence, the effective surface roughness, the tornado translation speed, the low-level inflow structure, and the upper-core structure. The goal is to explore some of the range of structures that should be expected to occur in nature within the tornadic “corner flow”-that region where the central vortex meets the surface. The results of high-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado with the surface are presented.
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