Resolving the Effects of Canopy Structure on Fire-emitted Heat Transport using Large Eddy Simulations

We used the Regional Atmospheric Modeling System (RAMS)-based Forest Large Eddy Simulation (RAFLES) to investigate heat flux dispersion from a virtual low intensity fire. Of particular concern to air-quality management are the impacts of fires to air quality. For example, smoke from fires that occur in wildland-urban interface (WUI) areas can linger for relatively long periods of time and have an adverse effect on human health or reduce visibility over roads and highways in the vicinity of and downwind of these fires. Thus, understanding how the atmosphere interacts with these types of fires and the smoke they generate is crucial for fire and air quality management.

In the current work, we used RAFLES to investigate how the introduction of a certain amount of heat in forests of different structures influences turbulence and heat exchange between canopy and atmosphere. RAFLES simulates the effects of canopy structure on turbulent transport within and above canopies. It resolves flows inside and above forested canopies and other semi-porous barriers, while accounts for barrier-induced drag on the flow and surface flux exchange between the barrier and the air. Unlike most other models, RAFLES also accounts for the barrier-induced volume and aperture restriction via a modified version of the cut-cell coordinate system.

We started by simulating the canopy structure of a plot in the Pine Barrens, NJ Forest and prescribing a burn case based on a 2011 prescribed fire as shown in figure 1. We directly prescribed the heat flux to the bottom three grid layers of the simulation at rates proportional to an assumed fuel load distribution on the virtual forest floor. The mean flux rate was based on observations in the prescribed fire. Next, we changed the canopy structure and investigated the role of canopy structure on turbulence transport inside and above the canopy, and heat exchange between the canopy and the atmosphere during a low-intensity fire. We specifically tested homogeneous and heterogeneous canopy structures each with homogeneous and heterogeneous fuel loads.

According to our results, when moving towards a more homogeneous forest, the frequency of sweeps/inward interactions increases over the frequency of ejections/outward interactions at the top of the canopy, and thus, more heat and smoke get entrapped in a homogeneous forest than in a heterogeneous canopy. Hence, we would expect less smoke to be ejected to the boundary layer from the initial hot plume in a homogeneous than a heterogeneous forest.