Modeling the Region of Strongest Winds in a Tornado

Results from high resolution, fully 3-dimensional, unsteady simulations of the tornado-scale wind field are presented, with emphasis on the corner-flow region where the turbulent interaction of the tornado with the surface produces maximum wind speeds. Simulations have been made for a range of values of such variables as a swirl ratio, the effective surface roughness, the tornado translation speed, the upper level outflow/inflow boundary conditions, and low level inflow structure. Results are discussed in terms of a corner-flow swirl ratio, Sc, which incorporates much of the influence of the other variables on the surface intensification of the tornado. Variations which increase this corner-flow swirl ratio increase or decrease the surface intensification of the vortex depending upon whether it is below or above a narrow critical range. Within this critical range, swirl velocities close to the surface reach 2.5 times the maximum swirl velocities aloft, while at substantially lower or higher values of Sc, little vortex intensification occurs. At very low values what does occur, occurs well off the surface, and at high values of Sc, most of the intensification occurs within secondary vortex eddies, rather than the mean vortex. Thus, modest differences in the near surface layer inflow can mean the difference between strong, little, or essentially no intensification of the vortex near the surface. Turbulence in the tornadic corner-flow region appears to be generally dominated by chaotic secondary vortices which transport angular momentum inward at small radii near the surface, thus enhancing the surface intensification of the velocities in the tornado.

Attempts are made to relate model results to some of the tornado-scale observations obtained during VORTEX and to speculate about what needs to be done next to better understand the role surface interaction plays in the intensification of a tornado, with the goal of discovering which observations might be most helpful in recognizing tornadogenesis at an early stage of development. Based on the demonstrated strong sensitivity of the maximum winds in our simulated tornado to flow properties of the near surface inflow layer, we speculate that the near surface inflow layer may provide a critical ingredient in deciding whether an existing supercell low level mesocyclone will spin up to a tornado or not.