Observations of Fire–Atmosphere Interactions and Near-Surface Heat Transport on a Slope

Abstract

A simple field experiment was conducted to measure and quantify fire–atmosphere interactions during a grass fire spreading up a hill under a moderate cross-slope wind. The observed fire intensity measured by passive radiometers and calculated sensible heat fluxes ranged between 90 and 120 kW m$^{-2}$. Observations from this experiment showed that convective heat generated from the fire front was transported downwind in the lowest 2 m and the highest plume temperatures remained in this shallow layer, suggesting the fire spread was driven primarily by the advection of near-ignition temperature gases, rather than by radiation of the tilted flame. Fire-induced circulations were present with upslope flows occurring during the fire-front passage helping to transport heat up the slope and perpendicular to the fire front. A decrease in atmospheric pressure of 0.4 hPa occurred at the fire front and coincided with a strong updraft core of nearly 8 m s$^{-1}$. These observations provide evidence that, even under moderately windy conditions, the pressure minimum in the fire remains rather close to the combustion zone and plume. The turbulence associated with the fire front was characterized by isotropic behaviour at 12.0 m above the ground, while less isotropic conditions were found closer to the ground due to higher horizontal variances associated with fire-induced flow at the fire front. From analysis of the turbulence kinetic energy budget terms, it was found that buoyancy production, rather than shear generation, had a larger contribution to the generation of turbulence kinetic energy, even during a highly sheared and moderate ambient wind.