The Effects of Translation and Surface Roughness on Tornado Structure and Flow

Tornado structure is known to be sensitive to the characteristics of near-surface inflow, which can be altered by the translational velocity of the tornado as well as the surface roughness. However, these effects are hard to study observationally due to a temporal and spatial limitation on resolution for radar data and the fact that environmental parameters cannot be controlled. The impacts of different translational speeds and surface characteristics on tornadoes have not been studied extensively. However, the importance of the influence of complex terrain on tornado dynamics and tornadogenesis likelihood is significant, especially in areas of the southeast US where terrain tends to be more complex than in the Great Plains. Findings from this study would aid in understanding and forecasting tornado behavior. This study uses a large-eddy simulation (LES) model to simulate an idealized tornado moving at different speeds and over surfaces of different roughness. The LES model has a domain of 0.5 km in the horizontal and 2 km in the vertical. Translational velocity of the tornado is implemented by a moving boundary at the surface while surface roughness is implemented using a logarithmic law for surface stress at a specific roughness length. Boundary conditions at the top are such that a continuous updraft is forced. Variables such as tangential velocity, radial velocity, peak pressure drop, etc. are quantified at multiple elevations to analyze the differences in tornado structure and intensity for different combinations of translational velocities and surface roughness (including a scenario with no movement and no roughness). It is expected that while a tornado with no movement and no surface roughness would display very symmetric characteristics, varying movement and roughness act to disrupt the symmetry and affect the intensity of the tornado. Plans for implementing more complex terrain types will also be discussed.