Topographic and Surface Roughness Influences on Tornadogenesis and Decay

Over the past fifty years, meteorologists have made great strides in forecasting tornadoes, helping to protect life and property. Yet, there are certain aspects of these multifaceted atmospheric phenomena that remain elusive to our understanding, despite the advancements in observational and numerical technology. Much of the midlevel storm structure, dynamics and evolution are understood; however, it is what transpires at low-levels and at the surface that remains unclear. One question that has yet to be thoroughly investigated or definitely answered is: How do physical characteristics of the Earth’s surface affect a tornado? Can topography and land cover and the associated roughness impact a tornado’s genesis and dissipation locations? Some research has been conducted on this specific topic, but what has been done is limited in scope to specific states (e.g. Chavas 2018; Lyza and Knupp 2018; Kellner and Niyogi 2013). The purpose of the current study is to determine whether or not genesis and decay points of tornadoes across the continental U.S. are related to nearby topographic characteristics and/or surface roughness of the nearby land cover relative to the broader landscape. The results therefore relate tornadogenesis and dissipation points to physical surface features and characteristics.

A ten year dataset of tornadogenesis and decay point locations (time, date, genesis/decay latitude and longitude) is acquired from SPC storm reports (Severe Plot 3) encompassing the years 2008-2017.). To investigate links with ground cover, a proxy for surface roughness, genesis and decay locations for the entire continental United States are matched with ground cover categories available every 30 m² obtained from The National Landcover Database (NLCD 2006, NLCD 2011). Because the NLCD data are inherently qualitative, the native categories will be assigned quantitative values according to the EPA AERSURFACE user guide (EPA 2008). Elevation data are obtained through the Digital Elevation Models (DEMs) acquired from the USGS for comparison between local elevation and topographic ‘roughness’ (determined by the standard deviation of the elevation) at the tornado point versus the surroundings. The tornadogenesis and decay latitude and longitude data will be aggregated into an excel file, and imported into ArcGIS. Doing so enables the land cover and elevation data to be imported with tornado information layered on top. Two separate buffers are created in a circle surrounding the tornadogenesis and decay points: a 250 meter and 5 km radius buffer. Averages and standard deviations of surface roughness and elevation within each of these circles are calculated and compared to that of the tornadogenesis or decay point upon which they are centered. The purpose of this is to bridge the relationship between the tornado point locations and the surrounding landscape, specifically if a change in elevation or surface roughness has a noticeable link with genesis and decay points. After the numerical data have been tabulated, a two-tailed t-test will be utilized to test for statistically significant differences between surface roughness values or elevation between the genesis location and the averages at each radii.

• EPA, 2008: AERSURFACE User’s Guide. EPA-454/B-08-001. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711
  
  
• Hua, Z. and D.R. Chavas, 2019: The Empirical Dependence of Tornadogenesis on Elevation Roughness: Historical Record Analysis Using Bayes’s Law in Arkansas. Appl. Meteor. Climatol., 58, 401–411, https://doi.org/10.1175/JAMC-D-18-0224.1
  
• Kellner, O., and D. Niyogi, 2013: Land Surface Heterogeneity Signature in Tornado Climatology? An Illustrative Analysis over Indiana, 1950–2012. Earth Interact., 18, 1–32, doi:1175/2013EI000548.1.
  
• Lyza, A, and K. Knupp, 2018: A background investigation of tornado activity across the Southern Cumberland Plateau Terrain System of Northeastern Alabama