Sensitivity of atypical lateral fire spread to wind and slope.

This study presents new knowledge of the environmental sensitivity of a dynamic mode of atypical wildland fire spread on steep lee-facing slopes. This is achieved through a series of idealized numerical simulations performed with the Weather Research and Forecasting (WRF) and WRF-Fire coupled atmosphere-fire models. The sensitivity of the atypical lateral fire spread across lee slopes is tested for a varying background wind speed, wind direction relative to the terrain aspect, and lee slope steepness. The results indicate that the lateral spread characteristics are highly sensitive to each of these environmental conditions, and there is a broad agreement with the empirical thresholds calculated for lateral spread events observed in the 2003 Canberra bushfires. A theory to explain these environmental thresholds and their apparent interdependency is presented. The results are expected to have important implications for the management of wildland fires in rugged terrain.

Figure 4. Three-dimensional wind streamlines (m s−1 , as shown in scale) over the leeward slope at a time of 40 min for U0 = 15 m s−1, δ = 0∘ and θ = 35∘. The solid red filled area shows the extent of the fire area.