Thunderstorms in arid and semi-arid regions like the U.S. intermountain west are often associated with dry microbursts. These microbursts are caused by evaporating precipitation in dry environments causing strong and difficult-to-observe cold outflow boundaries associated with rapid changes in wind and turbulence. A particularly dangerous situation can occur when microburst outflow winds are terrain-channeled into and within canyons during ongoing wildfire events, creating complex tactical challenges for firefighters and emergency managers. Given the dangers to firefighters by unpredictable microbursts and outflow boundaries within canyons, this paper quantifies the canyon-enhancement of wind and turbulence from microburst outflow boundaries using idealized large eddy simulations from the Weather Research and Forecasting (WRF) model. A series of simulations were conducted with the center of microburst downdrafts were placed 1.3 and 3.3 km upwind of a series of canyon types differing in length and slope angle. These canyon simulations are compared to microburst outflow boundary characteristics in flat terrain deriving topographic multiplier and differences in horizontal winds (wsp), upward vertical velocity (w), and turbulence kinetic energy (TKE). The increase in these variables is larger when the microburst is closer to the canyon and for steeper canyon walls generating increase in wsp by 4.5-6.6 m s-1, w by 1.9-8.4 m s-1, TKE by 1.4-6.6 m2 s-2. The topographic multiplier for horizontal winds is 0.1-0.2 times higher within the long-distance canyons compared to the short-distance canyons.
Branko Kosović, National Center for Atmospheric Research, Boulder, Colorado
Luchetti, Nicholas, National Weather Service, Raleigh, North Carolina
Friedrich, Katja, Atmospheric and Oceanic Sciences (ATOC), University of Colorado Boulder, Boulder, Colorado</p
