Topographic Effects on Wind Speed Estimation in Tornadoes

Differing effects of topography precluded introduction into the first version of tornado loading in the design standard. As it stands, observational cases defy generalization when considering topography (Houser et al., 2020). Changes in tornado intensity, size and path were noted from both numerical (Lewellen, 2012) and wind tunnel (Razavi and Sarkar, 2018) experiments. The sign of the changes in these studies varied, but the most significant changes occurred very close to the ground. A few studies suggest an increase in wind speeds in valleys due to channeling both in observation (Karstens et al., 2013) and numerically (Satrio et al., 2020).

The challenge of creating the wealth of realistic topographic cases in the simulations makes very specific conclusions through numerical simulation difficult. However, some general conclusions will be drawn based on numerical simulations of tornadoes in idealized terrain. For example, simulations using various topographic setups show increased variability in horizontal winds (Satrio et al., 2020).

Comparisons of these simulations in idealized terrain with actual tornado events will also be shared, including the 19 March 2018 Jacksonville, AL tornado, 3 March 2020 Cookeville, TN tornado and the 10 December 2021 which traversed through a heavily forested and complex terrain area in Land Between the Lakes (LBL). Parallels from the simulations were directly identified in both observed cases as treefall was found to fall predominantly parallel to the peaks and valleys. Rapid, sporadic expansion of the damage width characterized by unidirectional treefall was attributed to both speed-up effects in the LES and potential storm-scale momentum surges in observations.

Simple terrain including tornado translation over 2-d and 3-d hills have also been simulated computationally. Results will be shared and proposed speed-up effects will likely be suggested based on these and other simulations, both computational and experimental, for inclusion in the next version of the tornado loading standard (ASCE 7-28).