Reconstruction of near-surface tornado wind fields from forest damage patterns in complex terrain

Measurements via in situ probes and both stationary and mobile Doppler radar platforms in flat, open terrain have dramatically advanced our knowledge of low-level tornadic flow fields. Practical limitations of radars and other measurement platforms, however, preclude detailed observations of ongoing tornadoes in the mountains, leaving unanswered questions about tornado behavior and characteristics in steep topography. An effort to address these questions expands upon previous work that compares actual tree-fall patterns in flat forested landscapes following a tornado to those simulated by a Rankine vortex model coupled with a tree model. This method iteratively compares simulated and observed damage patterns for various vortex parameters, ultimately yielding a reasonable representation of the characteristics of the near-surface wind field when the tornado passed over the forest. The recent 27 April 2011 tornado outbreak spawned two long-track tornadoes that passed across heavily forested landscapes and mountainous regions. One EF-4 tornado passed over the western portion of the Great Smoky Mountains National Park in southeastern Tennessee and an EF-3 tornado crossed the mountains of northern Georgia in the Chattahoochee National Forest. High-resolution aerial photographs of each tornado track enable an analysis of the location and orientation of the damaged trees, while select ground surveys sample the species and physical characteristics of the trees shown in the aerial imagery. The tree-fall patterns allow an estimation of the near-surface wind field for each tornado over the rugged topography using an adaptation of the coupled vortex and tree model. Comparisons between topography and observed tree-fall directions indicate that complex terrain strongly influences the character of the near-surface tornadic wind field.