Local roughness effects on tornado dynamics

To estimate what damage a tornado measured over open plains might have produced had that tornado passed instead over a large structure or group of structures, one should consider how the presence of the structures themselves, or more generally any localized surface roughness elements, might have altered the tornado wind fields. The known sensitivity of the tornado cornerflow to properties of the near-surface inflow suggests that such effects may at times be significant. To explore this question, an extensive set of large eddy simulations of tornadoes is performed including simple block buildings, either singly or in arrays. Block dimensions, height, location and number are varied as well as tornado type and translation velocity. Simpler analytical models are used to help interpret the results and underlying dynamics. Quite different effects of roughness elements are seen depending on their distance from the tornado axis: far-field elements can either strengthen or weaken peak near-surface winds, while near-field elements generally only weaken them. For a modest-sized medium-swirl tornado, the presence of block structures can sometimes have dramatic weakening effects on the passing tornado. As a result, for example, the peak pressure forces per area experienced by a large building can be much smaller than those experienced by a smaller building approached by the same initial tornado. The effects of blocks structures on the mean flow in a large high-swirl tornado are less pronounced; however, the occurrence and intensity of secondary vortices within the tornado can be strongly altered by their presence. The variations in pressure forcing encountered by building faces in different scenarios are also considered. Important caveats to the study are that the blocks are treated as static and impermeable, no debris effects are considered, and only limited domain (~2 km) simulations are employed, with no feedback response on larger scales.