Abstract
Micrometeorological observations were made during a prescribed fire experiment conducted in a region of complex terrain with grass fuels and weak ambient winds of 3 m s−1. The experiment allowed for the analysis of plume and turbulence structures including individual plume core evolution during fire front passage. Observations were made using a suite of in situ and remote sensing instruments strategically placed at the base of a gully with a 24° slope angle. The fire did not spread upwards along the gully because the ambient wind was not in alignment with the slope, demonstrating that unexpected fire spread can occur under weak wind conditions. Our observational results show that plume overturning caused downward heat transport of −64 kW m−2 to occur and that this mixing of warmer plume air downward to the surface may result in increased preheating of fine fuels. Plume evolution was associated with the formation of two plume cores, caused by vigorous entrainment and mixing into the plume. Furthermore, the turbulence kinetic energy observed within the plume was dominated by horizontal velocity variances, likely caused by increased fire-induced circulations into the plume core. These observations highlight the nature of plume core separation and evolution and provide context for understanding the plume dynamics of larger and more intense wildfires.
Highlights
Wildland fires can modify the atmospheric environment in the near vicinity of the fire front by the formation of fire-induced winds and indrafts [1]
While there has been an increase in understanding of the role fire–atmosphere interactions have on fire spread using coupled fire–atmosphere models [3,4], there is a continuing need for high spatial and temporal resolution observations of both plume structure and micrometeorology of the fire environment to better understand the physics of both fire spread and plume dynamics [5,6]
A number of field experiments have been conducted to address this need; few observations have been able to measure the structures within the plume core, which are required in order to account for changes in fire behavior associated with more complex environments [2]
Summary
Wildland fires can modify the atmospheric environment in the near vicinity of the fire front by the formation of fire-induced winds and indrafts [1]. Small-scale field experiments, defined generically as a burn unit with a horizontal scale of ~1 km or less, have been critical in observing key turbulent structures associated with the fire front and fire spread. Downdrafts of colder air into the canopy were observed to occur where larger fireline intensities were associated with increased downward momentum transfer With this increased downward motion, horizontal surface winds increased. These observations suggest a link between the plume core and the near-surface environment which likely influences fire spread While this has been suspected to occur in larger wildfires, small-scale field experiments allow for highly resolved observations to be made using a range of instrumentation optimally arranged to sample plume structures and fire behavior. The following sections of the paper discuss the experimental design and instrumentation, the results of the observations including turbulence statistics and the plume two-core structure observed by Doppler lidar, followed by the conclusions
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