Observations of Sweep-Ejection Dynamics for Heat and Momentum Fluxes during Wildland Fires in Forested and Grassland Environments

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  • 1 USDA Forest Service, Northern Research Station, Lansing, Michigan 48910
  • 2 University of California – Irvine, Department of Civil and Environmental Engineering, Irvine, California 92697
  • 3 San José State University, Department of Meteorology and Climate Science, San José, California 95192
  • 4 USDA Forest Service, Northern Research Station, New Lisbon, New Jersey 08064
  • 5 Michigan State University, Department of Geography, Environment, and Spatial Sciences, East Lansing, Michigan 48824
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Abstract

The vertical turbulent transfer of heat and momentum in the lower atmospheric boundary layer is accomplished through intermittent sweep, ejection, outward interaction, and inward interaction events associated with turbulent updrafts and downdrafts. These events, collectively referred to as sweep-ejection dynamics, have been studied extensively in forested and non-forested environments and reported in the literature. However, little is known about the sweep-ejection dynamics that occur in response to turbulence regimes induced by wildland fires in forested and non-forested environments. This study attempts to fill some of that knowledge gap through analyses of turbulence data previously collected during three wildland (prescribed) fires that occurred in grassland and forested environments in Texas and New Jersey. Tower-based high-frequency (10 or 20 Hz) three-dimensional wind velocity and temperature measurements are used to examine frequencies of occurrence of sweep, ejection, outward interaction, and inward interaction events and their actual contributions to the mean vertical turbulent fluxes of heat and momentum before, during, and after the passage of fire fronts. The observational results suggest that wildland fires in these environments can substantially change the sweep-ejection dynamics for turbulent heat and momentum fluxes that typically occur when no fires are present, especially the relative contributions of sweeps compared to ejections in determining overall heat and momentum fluxes.

Corresponding author address: Warren E. Heilman, USDA Forest Service, Northern Research Station, 3101 Technology Blvd., Lansing, MI 48910. E-mail: warren.heilman@usda.gov

Abstract

The vertical turbulent transfer of heat and momentum in the lower atmospheric boundary layer is accomplished through intermittent sweep, ejection, outward interaction, and inward interaction events associated with turbulent updrafts and downdrafts. These events, collectively referred to as sweep-ejection dynamics, have been studied extensively in forested and non-forested environments and reported in the literature. However, little is known about the sweep-ejection dynamics that occur in response to turbulence regimes induced by wildland fires in forested and non-forested environments. This study attempts to fill some of that knowledge gap through analyses of turbulence data previously collected during three wildland (prescribed) fires that occurred in grassland and forested environments in Texas and New Jersey. Tower-based high-frequency (10 or 20 Hz) three-dimensional wind velocity and temperature measurements are used to examine frequencies of occurrence of sweep, ejection, outward interaction, and inward interaction events and their actual contributions to the mean vertical turbulent fluxes of heat and momentum before, during, and after the passage of fire fronts. The observational results suggest that wildland fires in these environments can substantially change the sweep-ejection dynamics for turbulent heat and momentum fluxes that typically occur when no fires are present, especially the relative contributions of sweeps compared to ejections in determining overall heat and momentum fluxes.

Corresponding author address: Warren E. Heilman, USDA Forest Service, Northern Research Station, 3101 Technology Blvd., Lansing, MI 48910. E-mail: warren.heilman@usda.gov
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