Investigating the fire channelling phenomenon using the WRF model.

Sharples, J., C. Simpson, J. P. Evans, M. McCabe and R. McRae
Annual conference of the Australian Meteorological and Oceanographic Society (AMOS) 2012, Sydney Australia, 31 Jan – 3 Feb 2012.


Observations of major fires have revealed a number of instances of atypical fire propagation, which occur in connection with steep, lee-facing slopes under conditions of extreme fire weather. Characteristics of the atypical spread include: rapid lateral propagation of the flank; downwind extension of the flaming zone; and the upwind edge of the flaming zone constrained by a major break in topographic slope. Instances of such atypical spread have been referred to as fire channelling events (Sharples et al. 2011).
Research has indicated that the fire channelling phenomenon results from the interaction between a fire and a lee-rotor. Some of the firebrands circulated within the rotor are incorporated into the synoptic flow above the rotor and are deposited downwind where they ignite spot fires that grow and coalesce, thereby explaining the extensive flaming zones associated with fire channelling events. Less clear, however, is the mechanism driving the lateral spread, which proceeds across the lee-slope in a direction perpendicular to the synoptic winds. Based on the available evidence it is possible to rule out a number of prospective mechanisms such as vertical momentum transport associated with directional wind shear. Combustion tunnel experiments have also indicated that the lateral spread only requires the presence of a fire and a lee rotor.
To better understand the physical mechanism driving the lateral spread, and the complex dynamics underlying the fire channelling phenomenon more generally, the WRF model is being implemented at the landscape scale to conduct a number of idealised simulations. Initial simulations involve an idealised triangular ridge with the effect of fire represented as a surface heat flux emanating from the lee slope. The simulations will investigate the effects of stratification in the atmosphere and will ultimately incorporate actual terrain data. The presentation reports on progress with this work.

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