Effects of debris on near-surface tornado dynamics

It is well recognized that strong tornadoes can transport substantial quantities of debris at high velocity and that this increases their damage potential significantly; however, there has been little study, to date, addressing whether debris loading can significantly affect the fluid-dynamic structure of the tornado itself. The debris cloud occupies only a small fraction of the tornado and the volume fraction of debris within the cloud (even at its heart) is likely very small, suggesting little impact. Our previous large-eddy simulation studies of tornado dynamics, however, have demonstrated that the properties of the near-surface inflow (where debris loading will generally be highest) critically affect the structure of the tornado corner flow (the region of highest expected winds, where the tornado core meets the surface). Given the large density ratio between, for example, dirt and air, large debris mass loadings are possible within the near surface and corner-flow regions even for small volume fractions. Not only can the debris loading result in large changes in the effective total fluid density in these regions: it provides an important additional mechanism for angular momentum transport as well through the outward centrifuging of debris.

To address this issue we have added debris dynamics to our high-resolution, unsteady LES tornado model, approximating the debris as a second fluid, of variable density, exchanging momentum with the primary fluid (air) through a drag force depending on debris-particle properties. The surface debris flux is dependent on the air and debris flow just above the surface, as well as on assumed surface properties. Preliminary simulations indicate that for some conditions debris can build up within the cornerflow so that the debris momentum can dominate the air momentum near the surface and the tornado flow structure can be dramatically altered. The addition of debris to the simulations allows two secondary objectives to be explored as well: the correlation of a tornado's visual appearance with its underlying flow structure, and a representation of the difference between air velocity and debris velocity fields (which could be of importance in interpreting fine resolution Doppler radar measurements of tornadoes).