Recent work has demonstrated that under conditions of extreme fire weather, bushfires burning in rugged terrain can exhibit distinctly dynamic patterns of propagation, which can have a dramatic effect on subsequent fire development. Coupled fire-atmosphere modelling using large eddy simulation has been useful in shedding light on the physical mechanisms underlying these phenomena, for example highlighting the important role of fire-induced vorticity. In this paper we present new research that specifically investigates the environmental precursors necessary to drive dynamic fire propagation. This research extends our previous work using the WRF-Fire coupled fire-atmosphere model, to specifically consider the effect of wind speed and topographic aspect in generating the fire-induced vorticity necessary to drive dynamic fire propagation. Specifically, we examined the behaviour of simulated fires on leeward slopes under different wind speed and wind direction regimes.

The modelling results indicate the existence of environmental thresholds beyond which dynamic spread is likely to occur. The results also indicate that the transition from quasi-steady to dynamic fire propagation can be quite abrupt, requiring only minimal changes in wind speed and direction for onset. The propensity for dynamic interactions to produce erratic and dangerous fire behaviour 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.