Turbulence in a wildland fire - a micrometeorological perspective

Modeling wildland fire behavior is a complex task given the interactions among a myriad of complex factors such as vegetation, topography, atmospheric conditions, fuel dryness, wind patterns and directions etc. Together, they influence the atmospheric turbulence and fuel combustion patterns which define the micrometeorological environment of a fire, which in turn affects fire front propagation. High performance computational models have progressed significantly in the last two decades in terms of modeling fire-atmosphere interactions but are fraught with large uncertainties associated with modeling fine (subgrid) scale turbulent processes. In order to address these shortcomings, a field scale experiment in the proximity of an eddy covariance tower was conducted in the FIREFLUX 1 campaign, where sonic anemometers measured three velocity components and temperature at four different heights above a grassland during a prescribed burn. Carbon dioxide and water vapor concentrations were measured at two different heights as well. It was found that turbulence intensity during fire propagation was four to five times greater than ambient values, and the turbulence associated with wind shear was as strong as the turbulence associated with buoyancy generated by the fire front. To further our understanding of how turbulence during the fire-front propagation is different from 'standard' atmospheric surface layer turbulence, a suite of techniques were employed, such as conditional analysis to identify sweep and ejection structures, frequency distributions to identify extreme events and deviation from Gaussian behavior, structure function analysis to understand production and inertial turbulence regimes, clustering analysis to understand the nature of intermittency, and wavelet analysis to detect coherent structures. Scalars such as heat, carbon dioxide and water were found to vary quite differently during fire propagation compared to their ambient states, while the statistical nature of momentum was relatively more well-behaved. In addition, these analyses can also be tied to the spatial tower-trough type structures associated with brushfires. Overall, unprecedented insights in to the behavior and structure of wildland fire turbulence were obtained by applying these micrometeorological analyses.