Understanding how Complex Terrain Impacts Tornado Dynamics using a Suite of High-Resolution Numerical Simulations

Tornado structure is known to be sensitive to the characteristics of near-surface inflow, which in turn is affected by inhomogeneities in surface terrain. Many observational studies such as VORTEX-SE have identified potential influences of terrain on tornado dynamics, particularly in areas of complex terrain such as the southeast United States. However, investigating terrain effects observationally poses a number of challenges. While past studies have utilized damage analysis and numerical simulations to examine such effects, a comprehensive sensitivity study on the effects of various types of terrain on tornadoes has yet to be done. Examining the behavior of tornadoes that traverse terrain features may aid in understanding patterns of tornado strengthening / weakening and give insight on damage patterns left behind from complex near-surface flow structure in areas of complex surface terrain. In the present study, a large-eddy simulation (LES) model is utilized to simulate a medium-swirl tornado-like vortex moving over varying surface terrain. Terrain is implemented using an immersed boundary method within the LES model. A total of 30 different simulations with varying surface terrain are presented, including a control simulation with flat terrain for comparisons. All simulations are grouped into one of four terrain categories – 2D sinusoidal hills, 3D hills, valleys, and ridges – and within each of these categories, slight modifications to the terrain or translational velocity are implemented and the tornado’s response to these modifications is analyzed.

The study finds that as the vortex traverses the different terrain features, vortex structure becomes unsteady and asymmetric, especially at levels closest to the surface where friction plays the largest role. For 2D sinusoidal and 3D hill simulations, enhanced 10-m horizontal wind speeds occur in two distinct areas: i) in between adjacent hills as a result of flow channeling and ii) on the upslope portion of the hill which is a product of short-lived but robust secondary vortices. The secondary vortices are hypothesized to form as a result of stretching of pre-existing vertical vorticity, associated with terrain-induced convergence. Analysis of the pressure field at 10 m show that the addition of terrain into the LES model does produce predictable path deviations in the tornado track that repeat with respect to the terrain.

Composite analyses reveal that the near-surface core radius is widest (most narrow) as the vortex ascends (descends) the terrain. The valley simulations have the largest horizontal wind speeds and the ridge simulations have the strongest updrafts overall. For hill and sinusoid cases, the region between adjacent hills has the fastest horizontal wind speeds while the uphill has the strongest updrafts. Lastly, statistical calculations show overall horizontal and vertical wind speeds are a function of the overall characteristics of surface terrain such as slope steepness and areal extent of the terrain. The study concludes by assessing differences in tornado damage potential based on the areal extent and magnitude of along-track, 10-m horizontal and vertical winds.