Wildfire evolution in the convective boundary layer (Formerly P5.1)

The interaction between wildland fires and local winds is important to the evolution of wildland fires. Because wildland fires occur in the atmospheric boundary layer (ABL), circulations in the ABL may significantly affect how wildland fires evolve. We used the University of Utah Large-Eddy Simulation model coupled to a fire spread parameterization to simulate grass fires in a fully turbulent convective boundary layer (CBL). In order to capture the interaction of large CBL eddies with the narrow but intense fire lines that are characteristic of grass fires, we used a domain 3.2 by 3.2 km in horizontal extent, with a horizontal grid size of 10 m. Fires were ignited after the CBL was fully turbulent.

In order to efficiently generate fire behavior statistics, we ignited multiple grass fires of the same size at the same time but at different locations in the domain. The results demonstrate that fire spread in the CBL is not deterministic. The figure shows the surface heat flux and near-surface winds 5 minutes after igniting 8 fires. The fire lines are obvious. In this case, the burned area varies by a factor of 2 among the 8 fires. The results suggest that a probabilistic (ensemble) prediction of fire spread is warranted. We also conjecture that sudden increases in fire intensity may be a result of unpredictable but favorable alignment of a fire with a boundary layer eddy.