Turbulent heat transfer observed during a low-intensity forest-understory head fire

This study examines the impact of a low-intensity forest understory fire on turbulent heat transfer using sonic anemometer measurements at lower (3 m), mid (10 m), and upper (19 m) canopy from five in-situ towers within the burn plot and one upwind control tower. The fire induced up to a 50-fold increase in kinematic vertical turbulent heat flux, primarily driven by increased temperature perturbations. The maximum flux occurred at mid-canopy due to decreasing temperature and increasing vertical velocity perturbations from the lower to upper canopy. A double peak in turbulent heat flux during fire-front passage resulted from interactions between fire-induced turbulence and atmospheric flow, where a downdraft immediately following the fire front temporarily suppressed flux before the second peak emerged. The flux increase was largely due to intensified ejection events rather than a higher event frequency, indicating enhanced heat transfer efficiency. At mid and upper canopy levels, sweep events increased at all but one tower, while at the lower canopy, inward interactions increased and outward interactions decreased across all towers. The arrival of a sea breeze front, bringing cooler air and stronger winds, dampened fire effects. During the fire, spectral energy for temperature and heat flux intensified at higher frequencies, particularly in the lower canopy, flattening the spectral curve in the inertial subrange. Buoyancy-driven turbulence dominated heat transfer, though some mechanically generated turbulence was also present. These findings enhance understanding of fire-induced turbulence and heat transfer, aiding the development of fire behavior and smoke dispersion models for improved fire management.