Examination of wind speed thresholds for vorticity-driven lateral fire spread.
Sharples, J.J., C.C. Simpson and J.P. Evans
Piantadosi, J., Anderssen, R.S. and Boland J. (eds) MODSIM2013, 20th International Congress on Modelling and Simulation. Modelling and Simulation Society of Australia and New Zealand, December 2013, pp. 263-269. ISBN: 978-0-9872143-3-1.
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−1 .
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−1.
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
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
Figure 2: Fuel fraction after 120 minutes of elapsed time, simulated using different reference winds speeds:
(a) U0 = 0 m s−1 ; (b) U0 = 2.5 m s−1 ; (c) U0 = 5 m s−1 ; (d) U0 = 7.5 m s−1 ; (e) U0 = 10 m s−1 ; (f)
U0 = 15 m s−1 .
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Last updated 31st January 2013