Examination of wind speed thresholds for vorticity-driven lateral fire spread

Recent work has demonstrated that under conditions of extreme fire weather, bushfires burning in rugged terrain can exhibit highly atypical patterns of propagation, which can have a dramatic effect 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 channelling’. Fire channelling, in turn, has been associated with serious escalation in fire activity and the development of pyrocumulonimbus storms. Coupled fire-atmosphere modelling using large eddy simulation has indicated that the fire channelling phenomenon occurs in response to fire-induced vorticity on the fire’s flanks in the immediate lee of a ridge line. In this paper we extend previous modelling, using the WRF-Fire coupled fire-atmosphere model, to specifically consider the effect of wind speed in generating the fire-induced vorticity necessary to drive the lateral spread associated with fire channelling. We examine the behaviour of simulated fires on leeward slopes under different wind speed regimes, which are characterised in terms of a reference wind speed U0. The topography is taken to be an idealised triangular mountain with a north-south oriented ridge line. The windward and leeward slopes are taken to be 20 and 35, respectively, and the height of the mountain is approximately 1 km. Initial and boundary conditions are taken in the form of a vertical wind profile that has a uniform horizontal (westerly) wind field of constant speed U0 at 200 m or above, and decays quadratically for heights below 200 m, to zero at the surface level, maintaining its westerly direction throughout. Moisture is assumed to be absent throughout the profile and potential temperature is assumed to be a constant 300 K. The reference wind speed U0 is prescribed values of 0, 2.5, 5, 7.5, 10 and 15 m s. The simulated fire spread under each of the wind speed regimes was examined for evidence of the occurrence of rapid lateral spread across the leeward slope. Under the two lowest wind speed 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 characterised by U0 ≈ 5 m s. The modelling results are also discussed in connection with some recent laboratory-scale fires examining the same effect. The fire behaviour in both cases was found to be qualitatively consistent, though issues surrounding the transferability of the results across the different spatial scales involved prevented a more quantitative comparison. The simulated behaviour of fires on leeward slopes, and the transistion 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 behaviour in such environments has strong implications for firefighter and community safety. At the very least the research findings provide additional support for the use of well-briefed observers in firefighting operations in complex topography.