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Abstract Tornadoes continue to pose a significant risk to communities, making it essential to understand the near-surface winds impacting structures. This study aims to validate the use of cycloidal debris swaths (loop-like patterns of debris deposited on the surface) for estimating near-surface winds in tornadoes. Ground surveys of cycloidal debris swaths confirmed previous findings that these patterns primarily consist of organic debris deposition such as cornstalks from nearby fields. However, the possibility of scouring cannot be ruled out, and the exact formation mechanisms of cycloidal debris swaths remain unknown. Cycloidal debris swaths occur in both violent and weak tornadoes, typically outside of the peak growing season and in terrain with limited flow impedance, where debris is available to be lofted and deposited on the surface. Aerial imagery of cycloidal debris swaths was analyzed to measure the height and width of each individual loop. The measurements were used in an equation derived by Fujita to estimate wind speeds for the 10 December 2021 western Kentucky tornado near Princeton, Kentucky, and the 9 December 2023 Clarksville, Tennessee, tornado. These estimates were compared to other wind speed estimates from nearby anemometry, damaged structures, an overturned shipping container, and treefall. The findings of this study show that wind speed estimates from cycloidal debris swaths are consistent with wind speed estimates from other estimation methods. This study’s validation efforts and technological advancements (availability of aerial imagery and GIS software) support the use of the cycloidal debris swath method for estimating near-surface tornado wind speeds. Significance StatementWith the recent improvements in remote sensing technology and imagery availability, this study revisits a Fujita method that uses cycloidal debris swaths to estimate tornado wind speeds. Findings show that wind speed estimates derived from cycloidal debris swaths align with other estimation methods, validating their use for future tornado assessments. These findings allow for a better understanding of near-surface winds impacting structures and can help improve building codes and create more resilient communities. 

Abstract A simulated vortex within a large-eddy simulation is subjected to various surface terrain, implemented through the immersed boundary method, to analyze the effects of complex topography on vortex behavior. Thirty simulations, including a control with zero-height terrain, are grouped into four categories—2D sinusoidal hills, 3D hills, valleys, and ridges—with slight modifications within each category. A medium-swirl-ratio vortex is translated over shallow terrain, which is modest in size relative to the vortex core diameter and with no explicitly defined surface roughness. While domain size restricts results to the very near-field effects of terrain, vortex–terrain interaction yields notable results. Terrain influences act to increase the variability of the near-surface vortex, including a notable leftward (rightward) deflection, acceleration (deceleration), and an expansion (a contraction) of the vortex as it ascends (descends) the terrain owing to changes in the corner flow swirl ratio. Additionally, 10-m track analyses show stronger horizontal wind speeds are found 1) on upslope terrain, resulting from transient subvortices that are more intense compared to the control simulation, and 2) in between adjacent hills simultaneous with strong pressure perturbations that descend from aloft. Composite statistics confirm that the region in between adjacent hills has the strongest horizontal wind speeds, while upward motions are more intense during ascent. Overall, valley (ridge) simulations have the largest horizontal (vertically upward) wind speeds. Last, horizontal and vertical wind speeds are shown to be affected by other terrain properties such as slope steepness and two-dimensionality of the terrain.