The Effects of Varying Surface Roughness, Translational Velocity and Swirl Ratio on an Idealized Tornado

Tornado structure is known to be sensitive to the characteristics of near-surface inflow, which can be altered by the translational velocity of the tornado as well as the surface roughness. Many observational studies such as VORTEX-SE have identified potential influences of surface terrain on tornado dynamics, particularly in areas of complex terrain such as the Southeast United States. However, these effects are hard to study observationally due to temporal and spatial limitations of observations. While past studies have examined the effects of translation speeds and surface roughness characteristics, a comprehensive sensitivity study of this parameter space has yet to be done. Thoroughly examining the difference in tornado behavior when interacting with a smooth surface such as a flat, grassy field in contrast to a rough surface such as a residential area may help explain different tornado characteristics observed in the field. Surface stresses introduced by translation cause asymmetries in tornado structure, and thus varying tornado ground speeds may affect tornado dynamics. The goal of the current study is to provide a comprehensive sensitivity study of the impact of surface roughness and translation velocity to understand how these factors impact tornado dynamics. To accomplish this, a large-eddy simulation (LES) model is used to simulate an idealized tornado moving at different speeds and over surfaces of different roughness. Different boundary conditions are applied to create a low, medium and high swirl base flow, and then the surface roughness and translation velocity are varied.

Variables such as tangential velocity, radial velocity, and pressure are quantified at multiple heights to analyze the differences in tornado structure and intensity for several different combinations of translational velocities and surface roughness lengths. Moreover, the along-track EF-scale standard winds are plotted to evaluate the changes in tornado damage potential associated with variations of these parameters. Notable differences with varying surface roughness are found in the axisymmetric characteristics between simulations, especially pertaining to the corner flow regions of the tornado. One difference is the wind field is found to be more laminar in the lower surface roughness cases, while turbulent structures dominate the simulations with higher surface roughness. A second and more important distinction between the different simulations is how tornadic subvortices evolve in the model. The number of subvortices that rotate about the main circulation is shown to be dependent on the surface roughness length. Additionally, results from different swirl ratios will be compared to understand how representative these changes are across a spectrum of tornado flows.