Abstract
Atmospheric turbulent circulations in the vicinity of wildland fire fronts play an important role in the transfer of momentum into and out of combustion zones, which in turn can potentially affect the behavior and spread of wildland fires. The vertical turbulent transfer of momentum is accomplished via individual sweep, ejection, outward interaction, and inward interaction events, collectively known as sweep-ejection dynamics. This study examined the sweep-ejection dynamics that occurred before, during, and after the passage of a surface fire front during a prescribed fire experiment conducted in an open-canopied forest in the New Jersey Pine Barrens. High-frequency (10 Hz), tower-based, sonic anemometer measurements of horizontal and vertical wind velocity components in the vicinity of the fire front were used to assess the relative frequencies of occurrence of the different types of momentum-flux events, their contributions to the overall momentum fluxes, and their periodicity patterns. The observational results suggest that the presence of surface fire fronts in open-canopied forests can substantially change the sweep-ejection dynamics that typically occur when fires are not present. In particular, sweep events resulting in the downward transport of high horizontal momentum air from above were found to be more prominent during fire-front-passage periods.
Highlights
We examined the sweep-ejection dynamics [26] for the turbulent momentum fluxes that occurred in the vicinity of an advancing fire line during a management-scale prescribed fire experiment conducted at the Silas Little Experimental
Meridional,and vertical velocity components; and turbulent kinetic energy (TKE = 0.5 u0 + v0 + w0 ) at the 3, 10, and 19-m levels on the west tower are shown in Figure 4; these are representative of the general impact the advancing line fire had on atmospheric conditions at all the tower locations
More observational and coupled fire-atmosphere modeling research is needed to firmly establish the process-based connections between oscillatory behavior in the spread of some wildland fires and the periodicity patterns of momentum-flux sweep, ejection, outward interaction, and inward interaction events above fire fronts, the current study offers a plausible hypothesis that atmospheric sweep-ejection dynamics may be a factor in causing some of the observed variability in wildland fire spread
Summary
Fuel loading, fuel moisture, topography, the presence or absence of overstory vegetation, and ambient and fire-induced atmospheric conditions all contribute to the manner in which wildland fires spread across landscapes. Regarding the latter atmospheric-related factors, several observational studies of atmospheric interactions with wildland fires at spatial and temporal scales relevant to fire behavior and combustion processes have been carried out in recent years (e.g., [1,2,3,4,5,6,7,8,9])
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