Assessing Fire Merging Behavior with Large Eddy Simulations

Multiple large fires can occur simultaneously as the result of natural disasters. For example, a single earthquake can cause numerous urban fires. During a wildfire, spot fires may be started by firebrands ejecting ahead of the main fire path. Whether or not these fires will merge into one mega-fire or persist individually is partially due to fuel loads and partially due to the background atmospheric boundary layer dynamics. Terrain and weather can also affect the probability of convergence among otherwise-separate fire events. Experimental studies with pools of burning liquids or with solid fuel beds suggest that the burn rate of conglomerated fires is larger than if they were burning independently. In contrast, the heat release of fires increases with more spacing between fires. While this behavior has been observed repeatedly in nature, controlled numerical experiments are lacking.

To explore these issues further, we employ the National Center for Atmospheric Research’s Weather Research and Forecasting model’s fire simulation package (NCAR WRF-FIRE). The basis of the Weather Research and Forecasting (WRF) model, coupled with a wildland fire-behavior module, has been used for numerous research case studies of large wildfires. WRF-Fire is a synthesis of WRF with a code that represents a surface fire behavior model. The semi-empirical surface fire behavior model calculates the rate of spread of the fire line based on WRF’s predictions of wind speed and direction as well as terrain slope and fuel properties. Any heat release resulting from the fire can affect the simulated weather in the vicinity of the fire as both fire line or post-frontal heat release is incorporated back into WRF’s dynamics. Several studies have highlighted the importance of simulating three-dimensional, time-varying weather and its impacts on the fire by coupling wildland fire models with numerical weather prediction models. Using idealized WRF-FIRE simulations in flat terrain, we seek to identify constraints on fire merging. We vary atmospheric stability, background wind speeds, and background wind direction veer.