The Influence of a Local Swirl Ratio on Tornado Intensification near the Surface

Abstract

The results of high-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado with the surface are presented. 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 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. 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. 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. 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. The authors define a local corner flow swirl ratio, Sc, 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. As Sc 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. 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.