The Effects of Land Cover Type on Tornado Intensity in the Southeastern USA

While storm scale mechanisms such as rear flank downdraft internal surges and descending reflectivity cores are known to be associated with changes in tornado intensity, tornadoes also intensify and weaken without direct correlation with a known storm-scale or tornado-scale feature. Thus, it is plausible that non storm-related features could be affecting the tornado. One external mechanism that could possibly influence the intensity of a tornado is the ground surface over which the system is traversing; it is possible for surface roughness to potentially contribute to weakening or intensification of tornadoes. The purpose of this research is to determine if there is a relationship between frictional changes across land cover transition areas and tornado formation/decay and/or changes in intensity.

According to the basic dynamics governing rotational flow fields, friction disrupts the cyclostrophic balance achieved between the pressure gradient force and the centrifugal force. This disruption allows for rotating winds to converge closer to the axis of rotation by causing the inward-directed PGF to exceed the outward-directed centrifugal force, ultimately resulting in increased rotational velocities, according to the conservation of angular momentum. Since different types of land cover have unique frictional values, it is possible that changes in land cover type could impact how a vortex behaves as it translates across a heterogeneous surface.

The chosen domain of the Southeastern U.S. ensures a large number of tornado events will be available to study and most importantly, houses a multitude of land cover types over relatively small geographic areas. Historical storm report data, damage surveys (if available) and level II WSR-88D radar data will be acquired from various archival resources for select tornadic storms within the domain. As a proxy for tornado intensity, the difference between the maximum inbound and the maximum outbound radar velocities (ΔVmax) will be calculated. Changes in tornado intensity will be related to land cover type at locations along the tornado path where radar data are observed. Tornado widths will be approximated from radar observations when available, and a region of influence will be extended to either side of the central point of the tornado, representing the inflow region directly affecting the tornado.    

The most recent land cover data will be extracted from the National Land Cover Database and will be analyzed in combination with the velocity data using a GIS framework. Tornado intensity changes and genesis/decay locations will be spatially analyzed against surrounding land cover types and land cover change locations. Known surface roughness parameters from the Environmental Protection Agency will be assigned for each land cover type; thereby making it possible to derive a quantitative method to statistically evaluate the relationships. Statistical tests will be applied to investigate possible relationships between tornado intensity change (genesis/decay) locations and land cover type transition zones (land cover type and land cover transition zones). Preliminary results will be presented.