Intellectual Merit: Severe storms sometimes produce violent tornadoes leading to significant property damage and fatalities. Understanding what conditions lead to tornadoes and govern their evolution, structure and destructive potential near the surface are critical research goals, complicated by the range of phenomena involved and the difficulties of studying them in the field. This research focuses on one critical ingredient that is involved: the interaction of a vortex with the surface. Previous work by the Principal Investigators and others have demonstrated that this interaction can: lead to near-surface wind speeds more than double the maximums aloft for quasi-steady conditions or more than an order of magnitude greater for a class of transient evolutions producing a dynamic "corner flow collapse"; is highly sensitive to the properties of the near-surface inflow; and can be dramatically affected by the uptake of small-scale debris (e.g., dirt, sand).
Using an existing large-eddy simulation (LES) tornado model for controlled numerical experiments together with simpler analytical models, the Principal Investigator will extend the existing results in important ways for better understanding the dynamics of real atmospheric vortices. This will include: generalizing the near-surface-intensification analysis to asymmetric vortices such as translating mesocyclones, tornadoes and secondary "suction vortices"; more systematic study of asymmetric corner flow collapse; continuing to investigate how tornadoes respond to debris loading for more realistic conditions by using multiple debris species, tracking their pickup and deposition to produce simulated damage tracks and mapping differences between air and debris velocities; and, to the degree possible, comparing the results with available field observations.
Broader Impacts: The research will provide important ingredients to aid in improving predictions of tornado occurrence, behavior and destructive potential leading to increased public safety. Detailed information on wind structure and debris transport will aid engineers' attempts to design structures to withstand credible tornado conditions. Better understanding of the interaction of tornadoes with the surface may someday lead to strategies for reducing the likelihood of strong tornado damage in some environments. The improvements in LES of turbulent particle laden flows expected from this effort could have a wide range of application in other fields such as combustion, chemical processing or pollutant dispersal. The work will train one PhD student in depth. Given the public fascination with tornadoes it will also, through contributions to popular media, promote science education and interest among the broader public.