CAREER: CONIFER: Role of Canopy Turbulence in Wildland Fire Behavior

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Wildfires have become increasingly more destructive with changing patterns of climate and land-use change. A small number of very large fires have become extremely high-intensity infernos and have been causing the majority of the damage. However, the understanding of the physical processes governing wildland fire behavior has remained limited. Although some research progress has been made in understanding how fires spread on grasslands, several aspects of fire behavior in forested environments are still not well understood. Under certain circumstances, surface fires occurring on a forest floor can ascend vertically and damage the forest canopy. Moreover, if the conditions are favorable, the fire can spread horizontally tree-to-tree across the canopy, becoming a much higher intensity crown fire. Understanding the physical and dynamic meteorology of wildland fires is consequently of utmost importance and necessary to develop mitigation strategies such as prescribed burns and effective fuel management practices such as forest thinning. To assess the effectiveness of such management efforts, it is, therefore, crucial to understand wildland fire behavior inside the forest canopy environment. This project will characterize the mechanisms of turbulent wind flow in the forest canopy during wildland fire events, which will result in (i) advances in fire behavior modeling, (ii) developing decision support tools for fuel treatments and prescribed fires, and (iii) improved models of air quality impacts from wildland fires. Moreover, the project will train a cohort of early-career scientists and students in the fundamentals of wildfire science.In a vegetated environment, the fire occurs below the forest canopy, which means that canopy-induced turbulent eddies interact with the fire and the fire-atmosphere interaction is modulated by the fundamental nature of canopy turbulence. The forest canopy acts as an extended sink of momentum for the air above, thereby imposing a range of length and velocity scales. Moreover, the presence of shear-induced instabilities at the canopy-atmosphere interface results in intermittent coherent structures that penetrate the sub-canopy airspace and aid in canopy-atmosphere interaction. The physics of turbulent flow in vegetated canopies itself is an active area of research to date. Standard atmospheric boundary layer formulations are inadequate in explaining canopy-atmosphere exchange of mass, momentum, and energy. This is rendered further complicated by the presence of the fire, which generates additional shear and buoyancy-induced motions. Using lab and field experimental datasets, this project will explore the following questions: (i) how do the coherent structures imposed by the canopy influence the convective and radiative regimes that regulate fire spread? (ii) how do the unique nature of turbulent transport and turbulent diffusivity in the canopy impact the redistribution of turbulent energy during wildfires? and (iii) how does the intermittent nature of canopy turbulence impact bursting motions associated with fire spread? This project will alter the current fuel-centric perspective, where the vegetation appears as mere fuel to be consumed by the flames, to a new paradigm, where the canopy itself actively modulates the wind and the fire through interactions spanning multiple scales.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.