Effects of Topography on Tornado Dynamics: A Simulation Study

The properties of the near-surface inflow are known to be a critical factor in determining tornado structure and intensity, so it is natural to expect that topography might significantly impact tornado behavior near the surface. For many years tornado damage surveys over nontrivial terrain have supported this general conclusion but, given the number of factors involved and limited information available in individual cases, have not allowed systematic determination of the physical processes involved. In this work we use simulated tornadoes passing over idealized topographic elements to analyze some of the mechanisms by which topography can affect tornado behavior. A version of the "immersed boundary" method has been developed to allow the inclusion of topography in an existing 3D high-resolution large-eddy simulation model consistent with the turbulent surface layer treatment, subgrid model and staggered grids employed. A large set of simulations have been performed (over 250 to date) varying initial tornado swirl ratio, size, and translation speed, and topography shape (ridges, knolls, valleys, ridge pairs, ridges with gaps, etc.), height, length, width, orientation angle and surface roughness. Only modest-scale localized topographic elements have been considered, consistent with the limited (~2 km) domain tornado simulations performed; the potential effects of larger-scale topography on the parent mesocyclone or super cell have not been included.

A rich variety of changes in tornado path, intensity, and structure due to interaction with the terrain have been found in the simulations. The effects vary strongly with choice of tornado type, scale and translation speed, as well as topography shape, scale, alignment and surface roughness. The simulated vortices are sometimes deflected by slopes, sometimes attracted to slopes, sometimes stalled for a time over topographic features, sometimes detached from the surface. The vortices sometimes weaken, sometimes strengthen, heading either up or down slopes, often exhibiting large changes in corner-flow structure during their evolution. By comparing results from many simulations together with idealized analytical models, several distinct (often competing) physical mechanisms involved in the tornado-topography interaction have been identified and will be discussed in this presentation.