Near-surface Tornado Intensification

West Virginia University Research Corporation, Morgantown WV

Investigators

Abstract

One of the ultimate goals of tornado research is to predict tornado occurrence and strength as a function of the environments which give rise to them, a goal made extremely difficult by the range of physical scales and phenomena involved and the complexity of the interactions between them. The objective of this research project is an improved understanding of one critical ingredient of this problem: the phenomenon of near-surface intensification. Prior work by the Principal Investigators (PIs) and others has demonstrated that the turbulent interaction of a vortex with the surface can produce maximum wind speeds more than double those well above the surface and an order of magnitude greater intensification for short periods. Changes in the near-surface inflow can make the difference between extreme near-surface intensification or none at all. The PIs have identified a corner flow swirl ratio and a dimensionless time rate of change of low-swirl flux as two critical parameters governing this behavior. Near-surface intensification is expected to be important on both the mesocylone scale and the tornado scale, but much remains to be done to clarify its role in both tornadogenesis and determining tornado intensity, and to determine how to approximate this phenomenon in subgrid/surface parameterizations for storm scale simulations. This research will use and extend the PI's existing high-resolution, three?dimensional, unsteady, numerical model of tornado vortices. Controlled, limited-domain, numerical experiments will focus on identifying the dynamical relationships involved in near-surface intensification during transient vortex evolution. The approach complements observational case studies and storm model simulations and should prove useful in interpreting the results of both. The code will be modified to include buoyancy generated by latent heat released in the core and debris loading to allow initial estimates of their effects on the near-surface flow. Adding the funnel and debris clouds to the simulations will also foster more direct comparisons with field observations of tornadoes, such as videos and fine-scale Doppler radar measurements. Society will benefit through increased public safety, if improvements can be made in forecasting which radar detected mesocyclones will generate a tornado and what the damage potential might be. This research, which will include providing results to the research community through publications, meetings, graduate student education, and personal interaction, is designed to provide a necessary ingredient for such improvements. Also, detailed information on the tornado wind structure and debris transport should aid engineer's attempts to design structures to withstand tornado conditions.