Effects of Forest Overstory Vegetation on Near-Surface Atmospheric Turbulence during Surface Fire Events

The local atmospheric environment surrounding a wildland fire is typically complex and highly turbulent. The presence of forest overstory vegetation during fire events can add to this complexity and affect the near-surface atmospheric turbulence regimes that develop in response to the fires. These turbulence regimes can feed back on the behavior of wildland fires and play a significant role in the local dispersion of fire emissions. Recent prescribed fire field experiments incorporating in situ instrumentation for measuring turbulent circulations before, during, and after the passage of fire fronts have yielded new insight into how the presence or absence of forest overstory vegetation can influence overall turbulence energy and anisotropy in the turbulence fields in front of and behind fire fronts. In this study, high frequency (10-20 Hz) sonic anemometer measurements of horizontal and vertical wind speeds at multiple heights during prescribed surface fires conducted in the New Jersey Pine Barrens (forest overstory vegetation present) and at the Houston Coastal Center near Galveston, TX (no forest overstory vegetation) were used to assess the differences in the local fire-induced atmospheric turbulence regimes that developed during the fires. Analyses of turbulent kinetic energy (TKE) and TKE component time series, turbulence anisotropy variability, and the spectral components of the high frequency three-dimensional winds for the different surface fires suggest that the presence or absence of forest overstory vegetation can have a significant impact on the characteristics of directional turbulent mixing before, during, and after fire front passage.