An Observational Analysis of Potential Terrain Influences on Tornado Behavior

Myths surrounding the relationship between tornado behavior and significant terrain features have abounded in American culture. Many of these myths have been dispelled directly by tornado events that did not fit the preconceived notions of the local population. Although a large push has been made to dispel the remaining myths about tornado “protection,” the exact ways in which terrain may or may not impact tornado behavior, including genesis, intensification, weakening, and dissipation, remain unclear. Limited observational and numerical modeling research has been performed on this problem, with observational analyses often limited to individual cases. One of the most critical steps to understanding how terrain might influence tornado behavior is the documentation of repeated behaviors observed with tornadoes that can potentially be linked to underlying terrain variations.

In this presentation, we establish four repeatedly-observed behavioral modes that tornadoes appear to exhibit in the presence of terrain using a data set consisting of 72 cases. These four modes include: 1) formation/intensification on downslopes and dissipation/weakening on upslopes, 2) formation/strengthening on a plateau upslope/edge and weakening/dissipation on a plateau downslope/edge, 3) following a valley, and 4) following the edge of a plateau. Many of the 72 cases analyzed exhibit behaviors fitting multiple modes. In compiling these events, we analyze numerous sources of data, including multiple operational and research radars, National Weather Service and other ground and aerial surveys, and satellite data, to trace specific tornado tracks and to analyze changes in larger-scale storm characteristics, such as rotational characteristics of parent mesocyclones or mesovortices. Select case studies are utilized to illustrate examples of each interaction mode. The identification of these four potential terrain influence modes on tornado behavior will serve as a baseline for hypotheses leading to future observational, theoretical, and numerical modeling studies of severe storm interactions with terrain.