Tornado-Surface Interaction

West Virginia University Research Corporation, Morgantown WV

Investigators

Abstract

Our understanding of tornadoes and tornadic storms has increased significantly in the past two decades due in large part to extensive Doppler radar observations on different scales, mobile mesonet and other in situ observations, and numerical simulation on both the storm and tornado scales. One of the apparent lessons, however, has been the critical importance of physics on a large range of different scales, from full storm scale down to the few-meter deep inflow layer feeding the tornado corner flow, leaving the reliable prediction of tornado occurrence and behavior within a given storm as still a daunting task. This has made the gathering of more complete tornadic storm data sets on a large range of scales (as is being attempted in VORTEX-II, a large field observational program supported by National Science Foundation) a clear priority. However, this complexity also suggests a need to isolate and understand different pieces of the problem in more idealized studies. Intellectual Merit. Observations and simulation studies have demonstrated a great variety and complexity of tornado behavior near the surface and aloft. Much of this arises from the sensitivity of tornadoes to the properties of the near-surface inflow that feeds into the tornado corner and core flows. This study will focus on tornado-surface interactions and their effects on tornado intensification and structure. A principal new component will be to employ "immersed boundary" techniques to incorporate non-trivial surface geometry into an existing high-resolution large-eddy simulation (LES) tornado model. This will allow us to address several important issues with simulation studies for the first time: the effects of topographical features (such as small hills, ridges, valleys, or buildings) on near-surface tornado dynamics; the pressure and debris forcing on simple building structures for a variety of realistic tornado wind and debris fields; the potential importance to tornado dynamics of treating individual surface roughness elements rather than employing a simple surface roughness length approximation; and the lofting of isolated large objects (such as idealized vehicles) within tornadoes. In addition we will continue several of our ongoing theoretical and numerical studies of different facets of tornado and mesocyclone dynamics including: behavior and analysis of vortices far from axisymmetry; the interaction of vortices on different spatial scales; mechanisms for near-surface intensification of tornadoes; tornado-debris dynamics; and the analysis of tornado damage tracks and surface markings. Broader Impacts. A long-term goal of the research is better understanding of tornado occurrence and behavior in order to improve tornado prediction and increase public safety. The simulation of the forcing of tornado winds and debris flows on buildings, and of the effects of encountering buildings on tornado behavior should improve estimates of potential tornado damage in urban environments and aid engineer's attempts to design structures to withstand credible tornado conditions. Understanding the effects of topography on near-surface tornado behavior and intensification may also lead to strategies for reducing the likelihood of strong tornado damage in some environments. The improvements in LES of particle laden turbulent flows in different geometries developed for this project may find broader applications in other fields such as combustion, chemical processing or pollutant dispersal. The main educational component will be the in depth training of one PhD student. In addition, given the public fascination with tornadoes, the project will also promote science education and interest among the broader public through contributions to popular media.