92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Wednesday, 25 January 2012
Observation and Laboratory Experimentation of Tornadoes Translating Over Complex Topography
Hall E (New Orleans Convention Center )
Christopher D. Karstens, Iowa State University, Ames, IA; and W. A. Gallus Jr. and P. Sarkar

Although most tornadoes documented in the U.S. occur in areas with nearly flat terrain, tornadoes do occur in regions where the terrain is quite undulating. Yet, the understanding of the effects of topography on tornado structure and intensity remains extremely limited. Many of the tornadoes occurring on 27 April 2011 tracked through regions with complex topography, particularly in Alabama and northwest Georgia on northward through parts of the Appalachians. Shortly after the event, aerial orthophotos were taken by NOAA's National Geodetic Survey along significant stretches of the damage paths. These photos offer an unprecedented opportunity to study tornado damage, especially in areas with significant tree canopy and/or rough terrain that would make a ground survey nearly impossible. In this study, the orthophoto imagery is combined with ground survey information, conducted by the lead author in early May, to geospatially relate variations in the patterns and intensity of tree damage to the underlying terrain using a Digital Elevation Model (DEM). Preliminary analysis has revealed some interesting behavior. For example, trees in low-lying areas or valleys show convergent fall patterns and intense damage is noted. Trees along ridge lines, however, are often damaged to a lesser extent compared to adjacent areas. Composites of examples such as these will be presented. In addition, laboratory experiments were performed, using Iowa State University's Tornado and Microburst Simulator, to better understand terrain-induced effects on tornadoes. The simulator was configured to produce a low swirl- and high swirl-ratio tornado-like vortex that translates over idealized two-dimensional models of an escarpment and a hill. Preliminary results suggest that when translating to lower elevations, the vortex transitions toward a lower swirl-ratio configuration with a slightly narrower and more intense core. Measurements of surface pressure, three-dimensional winds, and derived swirl-ratios from these experiments are compared to control vortex simulations with a flat lower boundary, and to studies of straight-line boundary layer wind tunnel tests with similar topographic models. Information gained from the experiments is also compared to the observations of tree-fall patterns and varying damage intensity in regions where the modeled and observed terrain features are comparable.

Supplementary URL: