Assessing the Impact of Forest Canopy Drag on Experimental Fire Behavior Using Coupled Atmosphere-Fire Large-Eddy Simulations

Abstract

Wind plays a central role in wildland fire behavior, and understanding the interaction between the atmospheric surface layer and fire-induced dynamics in forested environments requires detailed numerical studies. This study investigates whether a forest, located upstream of the ignition area, can influence fire propagation downstream. Using the Meso-NH/BLAZE coupled atmosphere-fire model with an explicit tree drag parameterization, simulations of the FireFlux I experimental grass fire at 10-m resolution show that this intermediate resolution is sufficient to capture key canopy-related processes. The canopy generates a wake that enhances turbulent kinetic energy and momentum fluxes near the surface. This wake also presents dynamically coherent structures, possibly linked to Kelvin-Helmholtz instabilities, which interact unsteadily with a downstream recirculation. These interactions produce sweep-ejection motions, clearly identified through quadrant analysis, even far downstream from the forest canopy. During the fire, this sweep-ejection signature persists and reduces variability in fire-front positions near the canopy compared to the grass-only baseline simulation, likely due to the more isotropic nature of the canopy-generated wake compared to the free atmosphere. A wavelet-based approach is used to isolate transient fire-induced fluctuations, allowing for clearer detection of the canopy signature during the fire. This wavelet approach also reveals counter-gradient motions in the observations, superimposed on the canopy-induced signature, which are not reproduced in the simulations, likely due to unresolved fine-scale buoyant small-scale structures. These results highlight the ability of coupled atmosphere-fire models at intermediate resolution to explore complex fire-canopy interactions and to support attribution studies of observed fire behavior.