Turbulence spectra measured during fire front passage

Abstract

Four field experiments were conducted over various fuel and terrain to investigate turbulence generation during the passage of wildland fire fronts. Our results indicate an increase in horizontal mean winds and friction velocity, horizontal and vertical velocity variances as well as a decreased degree of anisotropy in TKE during fire front passage (FFP) due to fire-induced winds. Vertical velocity and temperature variances observed during FFP approached the local free convection prediction when represented as a function of stability parameter z/L under very unstable conditions. The results of our wavelet spectral analysis show increased energy in velocity and temperature spectra at high frequency during FFP for all four cases; we hypothesize this is caused by the shedding of small eddies generated from the fire front. Additionally, spectral energy of velocity components at low frequencies may be affected by cross-flow intensity, topography, presence of canopy layer, and degree of fire–atmosphere coupling. When the velocity spectra are normalized using the friction velocity u* following Monin–Obukhov scaling, the velocity spectra observed during the FFP collapsed into a fairly narrow band in the inertial subrange, suggesting that as far as inertial range is concerned, the friction velocity u* is a valid scaling parameter that can be used for wildfire application. When the temperature spectra are normalized by T*, the temperature spectra observed during the FFP did not show any systematic behaviors predicted by the similarity scaling due to the extreme surface heating environment of fires.