Evolution of turbulent kinetic energy and the Haines Index during wildfires in the north central and northeastern U.S

Daily 24-72 hour fire-weather predictions for different regions of the U.S. are now readily available from the regional Fire Consortia for Advanced Modeling of Meteorology and Smoke (FCAMMS) that were established as part of the U.S. National Fire Plan. These predictions are based on daily MM5 model simulations of atmospheric conditions and fire-weather indices over specific modeling domains. Included in the suite of fire-weather indices provided by the FCAMMS is the well-known Haines Index (HI), an operational “mesoscale-type” index that characterizes the atmospheric risk of extreme fire behavior based solely on stability and moisture conditions in the lower to middle troposphere. However, there are other atmospheric variables that also influence the risk of extreme fire behavior, especially those that characterize conditions in the lower troposphere and atmospheric boundary layer where small-scale fire-atmosphere interactions are so important. One of those variables is atmospheric turbulence (i.e. wind gustiness), as measured by turbulent kinetic energy (TKE). Turbulent kinetic energy can be classified as a “boundary-layer-type” index, with its generation and dissipation dependent on wind shear and buoyancy conditions near the surface. Like the HI, predictions of TKE are available from the daily FCAMMS MM5 model simulations.

Using MM5 output from the FCAMMS – Eastern Area Modeling Consortium (EAMC), previous analyses of these indices have focused on determining whether large HI values typically occur with large near-surface TKE values in the north central and northeastern U.S., a potentially dangerous fire-weather condition. The analyses suggested that the simultaneous occurrence of high HI and high TKE values is relatively rare in this region. However, the analyses also suggested that significant wildfires in this region are often associated with values of the product of HI and near-surface TKE that exceed 15. Furthermore, under high HI conditions, buoyancy appears to be the primary mechanism for generating significant turbulence near the surface during the springtime wildfire season in this region.

Building on these previous analyses, this study examines the evolution of the HI and TKE throughout the atmospheric boundary layer during previous wildland fire episodes in the north central and northeastern U.S., based on output from the daily FCAMMS-EAMC fire-weather simulations. The behavior of the HI, boundary-layer TKE, and the product of the HI and near-surface TKE are examined to assess (1) the importance of boundary-layer turbulence in creating an environment even more conducive to erratic fire behavior than implied from high HI values alone and (2) the feasibility of combining predicted HI and boundary-layer TKE values in some fashion to anticipate the potential occurrence of extreme fire behavior.