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Abstract Observations reveal extreme long‐range fire spotting occurred during California's Dixie Fire. Specifically, we describe the occurrence of remarkable 9, 12, and 16 km spotting events on 16 August 2021. Radar data reveal these spot fires are linked to bent‐over but deep convective plumes with plume tops reaching 10–12 km MSL. These plumes have characteristic lofting regions in the fire‐generated updrafts and pyrometeor fall out locations in the downwind subsiding portion of the plume. Infrared data indicate spot fires occur along the plume's central axis. The cross winds impacting the plume rise and pyrometeor transport were ∼15 m s−1, and the inferred transit time firebrands causing the longest‐range spot fire is ∼18 min. We also provide photographic evidence for large, partially burned pyrometeors at a range of ∼20 km from the fire and link these data to Ka‐band radar observations showing pyrometeor pulses and fall out over the observing site. The results of the study suggest that operational and research radars may be able to isolate periods conducive to long range spotting in near real‐time.
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Characterizing firebrands and their kinematics during lofting
Spot fires pose a major risk and add to the already complex physics, which makes fire spread so hard to predict, especially in the wildland urban interface. Firebrands can not only cross fuel breaks and thwart other suppression efforts but also directly damage infrastructure and block evacuation routes. Transport models and computational fluid dynamics tools often make simplifications when predicting spot fire risk, but there is a relative lack of experimental data to validate such parameterizations. To this end, we present a field experiment performed at the University of California Berkeley Blodgett Research Forest in California where we recorded the flame and firebrands emanating from a nighttime hand-drawn pile fire using high-frequency imaging. We used image-processing to characterize the fire intensity and turbulence as well as particle tracking velocimetry to measure ejected firebrand kinematics as they are lofted by the plume. We further collected embers that settled around the fire at varying distances and measured their size, shape, density, and settling distributions. We also examine existing physics-based time-averaged models of firebrand lofting and note discrepancies between such models, often used due to their speed and simplicity, and our experimental observations. Finally, we discuss some implications our observations could have on future modeling efforts by considering the time-dependent fire dynamics, intermittency in the plume turbulence, and in the firebrand generation rate. To the best of our knowledge, these are the first in situ observations of firebrand generation and lofting from representative fuels, addressing a major source of data gap and uncertainty in the wildland fire literature.
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- PAR ID:
- 10589353
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Physics of Fluids
- Volume:
- 36
- Issue:
- 10
- ISSN:
- 1070-6631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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