Assessing the Role 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 is an emerging topic that requires detailed numerical studies. The main objective of this study is to assess whether the forest canopy surrounding the well-known FireFlux~I experimental grass fire influences the near-surface turbulent atmospheric flow and the fire spread, and whether a coupled atmosphere-fire model, including an explicit tree drag force parameterization, can capture these fire-wind-canopy interactions. The results show that the coupled Meso-NH/BLAZE model at 10-m or finer resolution captures these effects before and during the passage of the fire front. Before the fire, a quadrant analysis shows the presence of sweep-ejection dynamics typical of forested environments in the coupled simulations, together with the development of a Kelvin-Helmholtz instability above the canopy interacting with a recirculation zone. During the fire, a quadrant analysis based on a wavelet transform to robustly extract velocity fluctuations shows a combined influence of sweep-ejection motions and fire-induced buoyancy on the fire spread and the fire-induced wind. The ensemble coupled simulations capture the influence of the canopy well and demonstrate that the observed signal is directly influenced by the canopy. However, the buoyancy-driven outflow is not captured in the coupled simulations, probably due to insufficient resolution, as this process is highly intermittent and involves much smaller coherent structures. There is therefore great potential for coupled atmosphere-fire models to perform attribution studies and provide insights into wildland fire behavior.