A comparison of turbulence dynamics during wildland fires in forested and grassland environments

Fire-induced turbulence and the feedback into the fire, following changes in ambient conditions, differ for forested (sub-canopy) and grassland environments. Even so, studies pertaining to the turbulence dynamics in each scenario are relatively isolated. A detailed understanding of the differences necessitates an examination within an integrated contextual framework. In this study, we synthesize observations from multiple experimental surface fires: two sub-canopy backing fires and one sub-canopy heading fire in the New Jersey Pinelands, along with a grassland heading fire at the Houston Coastal Center. We highlight the major differences in the relative strength and role of the streamwise and cross-stream turbulent eddies in fire spread and the relative importance of some terms of the TKE budget over others, based on fire intensity and the surface-fire environment. In the sub-canopy burns, turbulent eddies are strongest near the canopy top: high streamwise turbulent flux accompanies low cross-stream turbulent flux and vice versa. In the grassland fire, both streamwise and cross-stream eddies strengthen simultaneously until a certain height before starting to weaken with height, informing a vertical length scale for the fire influence. Moreover, the forward sweep from streamwise eddies assists in the fire spread by pushing hot gases toward unburnt fuel. In the sub-canopy fires, shear production and buoyancy production are more substantial near the canopy top for more intense fires, while their magnitudes decrease with decreasing fire intensity. In all sub-canopy fires, buoyancy production seems to dominate shear production at the mid-canopy height, becoming the key mechanism for vertical transport of TKE at that height. In the grassland fire, shear production dominates buoyancy production near the surface and is insubstantial beyond a certain height relative to the flame length, while buoyancy production increases with height, becoming substantial further away from the surface. Turbulent transport terms are also active in both environments. For intense sub-canopy fires, there is a loss in TKE due to its expulsion to the boundary layer aloft via the transport term. This is compensated by a reversal process comprising an in-flux of TKE into the canopy via the transport term. In the grassland fire, the transport term mimics this behavior until a certain height. Given that the system of equations governing fire physics is very complex, model developers can avail of the knowledge of the relative significance of the respective turbulent fluxes and TKE budget terms in each environment, to adjust their model complexity accordingly.