Examining fire spread dynamics in canyon terrain through physics-based modeling: Mechanisms of fire line rotation and non-local fire behavior

Wildfire spread in complex terrain poses a major challenge for predictive modeling, as interactions between topography, wind, and combustion give rise to erratic fire behavior that caused fatalities among fire fighters. This study investigates the spread dynamics of a canyon fire exhibiting a characteristic fire line rotation, wherein the fire front progresses downslope along the canyon side-walls, perpendicular to the nominal wind direction. Using large-eddy simulations with a physics-based mesoscale solver, we model coupled fire–atmosphere–terrain interactions over kilometer-scale domains to resolve the three-dimensional flow and combustion structures governing fire spread. We consider a canyon terrain and compare it against two simpler configurations: a sloped ramp and a flat surface. Analysis of fire arrival times reveals that, despite identical ridge slopes, the canyon induces distinctly different spread behavior, resulting in oblique propagation along the canyon side-walls and intermittent progression in the valley. A detailed examination of flow field quantities attributes these phenomena to terrain-induced wind/slope misalignment and localized vorticity amplification, which persists after fire front passage and promotes extreme fire behavior. Furthermore, we demonstrate that the fire rate of spread in complex terrain is inherently non-local: individual sections of the fire line are influenced by neighboring segments, transient flow structures, and topographic features. Overall, our findings highlight the critical role of topography in modulating fire dynamics and provide physical insights into the mechanisms driving extreme fire behavior in canyon-like environments.