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Wind speed thresholds for vorticity-driven lateral fire spread.

Under conditions of extreme fire weather, bushfires burning in rugged terrain can exhibit highly atypical patterns of propagation, which can have dramatic effects on subsequent fire development. In particular, wildfires have been observed to spread laterally across steep, lee-facing slopes in a process that has been termed "fire channeling." Such erratic fire behavior is extremely dangerous for firefighting operations. Coupled fire-atmosphere modeling using large eddy simulation has indicated that the fire channeling phenomenon occurs in response to fire-induced vorticity on the fire's flanks in the immediate lee of a ridge line. Our research extends previous modeling, using the Weather Research and Forecasting (WRF) coupled fire-atmosphere model, WRF-Fire, to specifically consider the effect of wind speed in generating the fire-induced vorticity necessary to drive the lateral spread associated with fire channeling.

We simulated fires on leeward slopes under different wind speed regimes, with wind speeds characterised in terms of a reference wind speed U0. The topography in the model was an idealized triangular mountain with a north-south oriented ridge line and windward and leeward slopes of 20[degrees] and 35[degrees], respectively. The reference wind speed [U.sub.0] was prescribed values of 0,2.5, 5, 7.5,10, and 15 m [s.sup.-1].

Under the two lowest windspeed regimes the fire did not exhibit any atypical lateral spread, in stark contrast to the two highest wind speed regimes, in which the simulated fires readily exhibited significantly faster lateral spread. The results suggest the existence of a threshold wind speed, below which the prevailing winds are too weak to drive the vorticity-generating interaction between the wind, the terrain, and the fire's plume, so that no atypical lateral spread occurs. The model simulations further suggest that this threshold occurs for wind regimes characterized by [U.sub.0] = 5 m [s.sup.-1].

The simulated behavior of fires on leeward slopes, and the transition in fire propagation that can occur when prevailing winds are sufficiently strong, highlight the inherent dangers associated with firefighting in rugged terrain. The propensity for dynamic interactions to produce erratic and dangerous fire behavior in such environments has strong implications for firefighter and community safety. At the very least our findings provide additional support for the use of well-briefed observers in firefighting operations in complex topography.--Jason J. Sharples (University of New South Wales, Canberra), C. C. Simpson, and J. P. Evans. "Examination of Wind Speed Thresholds for Vorticity-Driven Lateral Fire Spread," presented at the 10th Symposium on Fire and Forest Meteorology, 14-18 October 2013, Bowling Green, Kentucky.

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Author:Sharples, Jason J.
Publication:Bulletin of the American Meteorological Society
Geographic Code:1USA
Date:Apr 1, 2014
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