Understanding Terrain Impacts on Tornado Flow through Tree-fall Analysis of the Joplin and Tuscaloosa-Birmingham Tornadoes of 2011 and through Numerical and Laboratory Vortex Simulations

Aerial imagery of tornado damage taken of the 22 May 2011 Joplin, MO and 27 April 2011 Tuscaloosa-Birmingham, AL tornado tracks was used to digitize the falling direction of trees (i.e., tree-fall) along each damage path. The digitized tree-fall was used to compute a normalized mean cross-section of observed tree-fall within the various life-cycle stages from each tornado. These mean patterns of tree-fall were subjectively compared to results from analytical vortex simulations of idealized tornado-induced tree-fall in an attempt to characterize mean properties of the near-surface flow as depicted by the model. A computationally efficient method of simulating tree-fall was also developed that uses a random distribution of critical tree-falling wind speeds based on the Enhanced Fujita scale. Results from these simulations suggest both tornadoes had a highly radial configuration of the near-surface wind field.

A few distinct tree-fall patterns were identified at various locations along the Tuscaloosa- Birmingham tornado track. Concentrated bands of intense tree-fall, collocated with and aligned parallel to the axis of underlying valley channels, extended well beyond the primary damage path. These damage patterns are hypothesized to be the result of speed-up caused by channeling of the near-surface inflow within valleys. Another distinct pattern of tree-fall, likely not linked to the underlying topography, may have been associated with a rear-flank downdraft (RFD) internal surge during the tornado's intensification stage. Here the wind field was strong enough to produce tornado-strength damage well beyond the visible funnel cloud. This made it difficult to distinguish between tornado- and RFD-related damage, and thus illustrates an ambiguity in ascertaining tornado damage path width in some locations.

Laboratory experiments were also performed, using Iowa State University's Tornado and Microburst Simulator, to better understand topographically-induced effects on a translating tornado-like vortex. Simulations performed with idealized 2-D models of a ridge and an escarpment reveal that the vortex track deviates in a sinusoidal manner. The evolution of vortex structure is consistent with behaviors related to vortex stretching and compression through the conservation of potential vorticity in an inviscid, homogeneous fluid. These results are compared to observations of tornado tracks from the 27 April 2011 tornado outbreak that show similar patterns of track deflection when crossing a significant rise or fall in elevation. Additionally, a 3-D section of the Tuscaloosa-Birmingham, AL tornado track was constructed to investigate the hypothesized flow-channeling effect. Preliminary tests imply channeling could only occur if the tornado had primarily radial near-surface flow that would already be aligned with the valley.