A study of the influence of forest gaps on fire-atmosphere interactions

Much uncertainty exists regarding the possible role that gaps in forest canopies play in modulating fire-atmosphere interactions in otherwise horizontally homogeneous forests. This study examines the impact of forest gaps on fire-atmosphere interactions using the ARPS-CANOPY model, a version of the Advanced Regional Prediction System (ARPS) model with a canopy parameterization. A series of numerical experiments are conducted with a stationary low-intensity fire, represented in the model as a line of enhanced surface sensible heat flux. Experiments are conducted with and without forest gaps, and with gaps in different positions relative to the fireline. For each of the four cases considered, an additional simulation is performed without the fire to facilitate comparison of the fire-perturbed atmosphere and the background state. Analyses of both mean and instantaneous wind velocity, turbulent kinetic energy, air temperature, and turbulent mixing of heat are presented in order to examine the fire-perturbed atmosphere on multiple time scales. Results of the analyses indicate that the impact of the fire on the atmosphere is greatest in the case with the gap centered on the fire, and weakest in the case with the gap upstream of the fire. It is shown that gaps in forest canopies have the potential to play a substantial role in the vertical as well as horizontal transport of heat away from the fire. Results also suggest that in order to understand how the fire will alter wind and turbulence in a heterogeneous forest, one needs to first understand how the forest heterogeneity itself influences the wind and turbulence fields without the fire.