Turbulent Heat Transfer Observed during a Low-Intensity Forest Understory Head Fire

Abstract 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 midcanopy 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, although some mechanically generated turbulence was also present. These findings enhance the understanding of fire-induced turbulence and heat transfer, aiding the development of fire behavior and smoke dispersion models for improved fire management. Significance Statement This study investigates fire-induced turbulence and heat transfer during a low-intensity forest understory fire, using unprecedented sonic anemometer data from multiple in situ towers at lower, mid-, and upper canopy levels, along with an upwind control tower. The findings reveal up to a 50-fold increase in vertical turbulent heat flux, peaking at midcanopy, and highlight complex fire–atmosphere interactions. Key dynamics include a strong downdraft trailing the fire front, producing a double peak in heat flux, a substantial rise in ejection event contributions across the canopy, increased inward interactions in the lower canopy, and interactions with an incoming sea-breeze front. These insights enhance fire behavior and smoke dispersion models, informing more effective fire management strategies.