Observations of Vorticity-Driven Lateral Spread in a Wildfire

Video footage of a recent California wildfire confirmed that dangerous fire spread can lead to unsurvivable foreground conditions. This process thus needs enhanced awareness across the wildfire sector. The fire moved sideways, downwind of a ridgeline, and formed dense, rapidly spreading spot-fires. Effective lateral rates-of-spread up to 20 km h−1 were measured. This is discussed in detail. A HPWREN camera system was installed on Santiago Peak in California. The Airport Fire, on two consecutive days, burned past the cameras by means of vorticity-driven lateral spread (VLS). This provided the most complete sets of time-series observations of VLS on a landscape-scale. Some remarkable measurements are derived from the observations. The overall lateral rate-of-spread averaged at 1.9 km h−1. Around plume touch-down events, that speed rose to 4 km h−1, but also peaked at 20 km h−1. The effective downwind spread of the overall fire envelope was 45 km h−1. A major spot-fire had a slope-affected headfire rate-of-spread of 15 km h−1 (equivalent to c. 2 km h−1 on flat ground) and a burn rate of 60 ha/h. The implications for fireground safety are extreme. An emphasis must be placed on predicting these events, as any burnover entrapments may well be unsurvivable. Avoiding a burnover requires good predictive capability, and observations such as these are critical for calibration.