Tornado Strength and its Dependence on Terrain

Author: Ian Gaida, Department of Physics

Mentor: Dr. Adam French, Department of Physics

There are two competing theories as to how the underlying terrain may affect the strength and longevity of a tornado. One is that as elevation rises, the tornado loses intensity due to lost angular momentum as the vortex is compressed. The other is that the rising land instead intensifies the updraft of the parent thunderstorm, increasing the tornado’s strength by stretching the vortex. In this study a tornado that passed over Spearfish Canyon on 29 June 2018 was analyzed to test these two theories. National Weather Service Doppler weather radar data were first analyzed to assess changes in the tornado’s parent thunderstorm as it crossed the northern Black Hills and Spearfish Canyon. This was done by measuring base reflectivity, wind velocity, normalized rotation, and vertically integrated liquid while tracking the position and ground elevation below the storm. The tornado reached its greatest intensity near its peak surface elevation of 6305 ft. and started to decline afterwards. All four of the storm values peaked around this height as well, implying the storm and tornado intensified as they ascended the Black Hills. As an additional test, numerical simulations of a generic supercell crossing a variety of terrain profiles were run with a high-resolution cloud model. This included level terrain, a bell-shaped hill, a plateau with a gradual descent, and combination bell-shaped mountain and plateau. Results consistently showed an increase in storm strength and low- and mid-level storm rotation on the uphill side, with the storm weakening or dissipating altogether on the downhill side. Evidence from both the tornado and the simulations suggest that the terrain’s impact on storm strength may affect tornado development more than any changes to the tornado’s angular momentum.