The sensitivity of fire-behavior and smoke-dispersion indices to the diagnosed mixed-layer depth

Diagnosing the spatial and temporal variability of meteorological quantities in the planetary boundary layer (PBL) is necessary for understanding and predicting how the atmosphere can influence fire behavior and for assessing the potential for smoke dispersion. When the PBL is well mixed, the vertical distribution of wind, potential temperature, and mixing ratio above a wildland fire can profoundly affect its evolution and behavior. The depth over which momentum, heat, and moisture can be mixed vertically within the PBL, hereafter referred to as the mixed-layer depth, is an important parameter for understanding and predicting when the atmosphere can influence a wildland fire. Furthermore, the distribution of wind within the mixed layer is an important consideration when assessing smoke dispersion. Discussions are ongoing among meteorologists and fire and smoke managers concerning the manner in which the mixed-layer depth is diagnosed, the sensitivity of fire-behavior and smoke-dispersion indices to various mixed-layer depth diagnostics, and the potential for sensitivities in the indices to affect fire- and smoke-management decisions. A quantitative assessment of these sensitivities can help inform the ongoing discussions and enable fire and smoke managers to anticipate whether the implementation of a given mixed-layer depth diagnostic could affect their ability to fulfill burn program requirements.

This study will investigate the sensitivity of fire-behavior and smoke-dispersion indices to the manner in which the mixed-layer depth is diagnosed in a well-mixed PBL. Above-ground observations of wind, potential temperature, and mixing ratio, which are necessary to diagnose the mixed-layer depth, are not commonly available in close proximity to most wildland fires. Nevertheless, mesoscale numerical weather prediction models may be used to simulate the required meteorological quantities at the locations of wildland fires. In this study, the WRF (Weather Research and Forecasting) model will be used to simulate the meteorological conditions associated with an eastern U.S. wildland fire event and to assess the sensitivity of fire-behavior and smoke-dispersion indices to diagnosed mixed-layer depth. The Ventilation Index, which explicitly includes the mixed-layer depth, and new fire-behavior indices that employ mixed-layer averaged wind, potential temperature, mixing ratio, and turbulent kinetic energy, will be investigated. The implications of the results of this investigation will be presented in the context of the aforementioned discussions concerning whether the adoption of various mixed-layer depth diagnostics is likely to affect operational fire- and smoke-management decisions.