Large eddy simulation of atypical wildland fire spread on leeward slopes.

The WRF-Fire coupled atmosphere-fire modelling system is used to investigate the fire chan- nelling phenomenon through a series of idealised numerical simulations. Two and three-dimensional atmospheric simulations are used to investigate the atmospheric flow in the lee of a mountain with- out the presence of a wildland fire. In each simulation there is flow separation in the lee of the mountain, which is associated with the rapid deceleration of the flow at the top of the leeward slope. In 2D the leeward flow is dominated by the quasi-periodic formation of large-scale rotors, whereas in 3D the leeward flow is instead dominated by chaotic fine-scale features. The 3D wildland fire spread simulations are used to investigate the fire spread from an ignition region at the base of the leeward slope under varying fuel type and with the atmosphere-fire coupling both enabled and disabled. In the simulation with atmosphere-fire coupling enabled and a high fuel mass per unit area fuel type, the F fire spread closely resembles that expected during fire channelling. Specifically, the fire spread is initially dominated by upslope spread to the mountain ridge line, which occurs at an average rate of 2.0 km h-1 , followed by predominantly lateral spread, which occurs at a maximum rate of 3.6 km h−1 close to the ridge line. This intermittent rapid lateral spread occurs when updraft/downdraft interfaces, which are associated with strongly circulating horizontal winds at the mid-flame height, move across the fire perimeter in close proximity to the mountain ridge line. These updraft/downdraft interfaces are formed due to an interaction between the strong pyro-convection and the terrain-modified winds. Through these results, a new physical explanation of the fire channelling phenomenon is proposed.

Figure 10: Contour plots of the instantaneous fuel fraction remaining at times of 60, 90 and 120 min for the (a) FCF05, (b) FNF05, (c) FCF13 and (d) FNF13 simulations. White shading is applied to regions where the fuel fraction remaining is over 99% or under 1%. Terrain contour lines are given at 100-m intervals and the solid black lines represent the mountain ridge line and the base of the leeward slope. The fire ignition region is indicated by the dash-filled region. A subset of the full SFIRE model domain is shown in each plot.