Numerical Investigation of Effects of Terrain on Tornadic Winds By Using Coupled CM1 and CFD Simulation

To effectively reduce casualties and property damage induced by tornadoes, we need to precisely predict the paths and near-ground wind field of tornadoes. Tornado characteristics, in terms of path, flow characteristics, and intensity, are affected by terrain at the low boundary. To investigate the effects of terrain on tornadoes, this study employs a coupled CM1 (storm-scale Cloud Model 1) and fine-scale CFD model to generate a nonstationary, translating tornado vortex and then investigate the influence of terrain on tornadic fields near ground. Six terrains (a flat ground, the escarpment terrains with two slopes and hill terrains with two slopes in ASCE 7-22, and a real-world terrain of Carney, OK) are introduced in the coupled model to study the effects of terrain on near-ground tornadic wind field.

The tornado paths present a significant difference between the flat ground case and the cases that include a terrain, providing a clear evidence that a terrain significantly affects the tornado's path. It is noted that the complexity of the real terrain significantly increases the path deviation compared to the flat ground case.

This study further employs along-track analysis to measure maximum negative pressure on the terrain's surface, as well as maximum horizontal and vertical velocities at 10 meters above ground level (AGL). These metrics illustrate the terrain's impact on damage extent as categorized by the EF scale, indicating significant alterations in the spatial distribution of tornado intensity. The variation of the core radius, maximum horizontal velocity, and angular momentum along height among the cases with different terrain is also investigated. In conclusion, interaction between terrain and tornado at low boundary is complicated and alters the tornado behavior.