Effects of Finescale Debris on Near-Surface Tornado Dynamics

Abstract

Debris clouds provide an important visual signature of tornadoes and can potentially significantly affect the wind structure, damage potential, and Doppler radar measurements of tornado wind speeds. To study such issues, the dynamics of finescale debris have been added to an existing high-resolution large-eddy simulation model of tornado dynamics. A so-called “two-fluid” or “Eulerian–Eulerian” approach is employed, together with a surface layer model for lofting and depositing debris. In this paper the debris implementation is described, three critical dimensionless parameters governing tornado debris effects are identified, and sample results from a large set of simulations of tornadoes with idealized debris are presented. The results demonstrate that the accumulation of small-scale debris within the surface layer and corner flow can significantly alter the wind speeds and flow structure of the tornado vortex within a few hundred meters of the surface. They suggest that the total mass of the debris cloud can reach tens of thousands of tons. Near the surface, the debris mass loading can be well above 1, the peak mean velocities can be reduced by as much as half, and the total momentum (air plus debris) can either significantly increase or decrease. Local air and debris velocities can differ significantly and in a nontrivial fashion, thereby complicating the interpretation of Doppler radar measurements of tornado structure. Debris fluctuations, centrifuging, negative buoyancy, and angular momentum transport are all significant mechanisms for the debris effects. A negative physical feedback reduces the sensitivity of the results to changes in the parameterization of the surface debris fluxes. The realistic simulation of tornado debris clouds and surface damage tracks should prove useful in identifying the dynamics governing their observed counterparts.