Coupled Influences of Topography and Atmosphere on Wildland Fire Behavior: Physics-Based Model Explorations

It has long been recognized that topography significantly affects wildland fire behavior. Many of the physical processes that control wildfire behavior are secondarily affected, including the coupling with the ambient atmosphere, the nature of the fire-induced flows, and the effectiveness of the thermal radiation to heat unburned fuel. Some of these effects can be predicted based on the slope of the topography at the location of the fire, others can be predicted with the inclusion of topography-influenced wind fields, but some require the application of a coupled fire/atmosphere model to be properly represented. Therefore, a physics-based coupled- atmosphere/wildfire model is used to examine interactions between topography and fire behavior. Specific attention is given to identifying the effects of topography that cannot be characterized through algebraic representation of the slope of the terrain at the location of the fire. Simulations are performed to look at issues including the differences between a fire burning on a hillside as opposed to a draw where the local terrain gradient is the same, and differences between a fire burning on a hillside versus the side of a canyon of the same slope when there is a crosswind. The focus of these simulations is to begin to identify some of the physical processes of fires that are heavily influenced by non-local topography, especially those that involve the interactive coupling between fire, atmosphere, and topography, and how these influences modify fire behavior. Idealized topographies are used for these simulations.