Atmospheric turbulent structures and fire sweeps during shrub fires and implications for flaming zone behaviour

Background Wildfires propagate through vegetation exhibiting complex spread patterns modulated by ambient atmospheric wind turbulence. Wind gusts at the fire-front extend and intensify flames causing direct convective heating towards unburnt fuels resulting in rapid acceleration of spread. Aims To c...

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Veröffentlicht in:International journal of wildland fire 2023-01, Vol.32 (1), p.43-55
Hauptverfasser: Katurji, Marwan, Noonan, Bob, Zhang, Jiawei, Valencia, Andres, Shumacher, Benjamin, Kerr, Jessica, Strand, Tara, Pearce, Grant, Zawar-Reza, Peyman
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container_end_page 55
container_issue 1
container_start_page 43
container_title International journal of wildland fire
container_volume 32
creator Katurji, Marwan
Noonan, Bob
Zhang, Jiawei
Valencia, Andres
Shumacher, Benjamin
Kerr, Jessica
Strand, Tara
Pearce, Grant
Zawar-Reza, Peyman
description Background Wildfires propagate through vegetation exhibiting complex spread patterns modulated by ambient atmospheric wind turbulence. Wind gusts at the fire-front extend and intensify flames causing direct convective heating towards unburnt fuels resulting in rapid acceleration of spread. Aims To characterise ambient and fire turbulence over gorse shrub and explore how this contributes to fire behaviour. Methods Six experimental burns were carried out in Rakaia, New Zealand under varying meteorological conditions. The ignition process ensured a fire-line propagating through dense gorse bush (1 m high). Two 30-m sonic anemometer towers measured turbulent wind velocity at six different levels above the ground. Visible imagery was captured by cameras mounted on uncrewed aerial vehicles at 200 m AGL. Key results Using wavelet decomposition, we identified different turbulent time scales that varied between 1 and 128 s relative to height above vegetation. Quadrant analysis identified statistical distributions of atmospheric sweeps (downbursts of turbulence towards vegetation) with sustained events emanating from above the vegetation canopy and impinging at the surface with time scales up to 10 s. Conclusions Image velocimetry enabled tracking of ‘fire sweeps’ and characterised for the first time their lifetime and dynamics in comparison with overlying atmospheric turbulent structures. Implications This methodology can provide a comprehensive toolkit when investigating coupled atmosphere–fire interactions.
doi_str_mv 10.1071/WF22100
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Wind gusts at the fire-front extend and intensify flames causing direct convective heating towards unburnt fuels resulting in rapid acceleration of spread. Aims To characterise ambient and fire turbulence over gorse shrub and explore how this contributes to fire behaviour. Methods Six experimental burns were carried out in Rakaia, New Zealand under varying meteorological conditions. The ignition process ensured a fire-line propagating through dense gorse bush (1 m high). Two 30-m sonic anemometer towers measured turbulent wind velocity at six different levels above the ground. Visible imagery was captured by cameras mounted on uncrewed aerial vehicles at 200 m AGL. Key results Using wavelet decomposition, we identified different turbulent time scales that varied between 1 and 128 s relative to height above vegetation. Quadrant analysis identified statistical distributions of atmospheric sweeps (downbursts of turbulence towards vegetation) with sustained events emanating from above the vegetation canopy and impinging at the surface with time scales up to 10 s. Conclusions Image velocimetry enabled tracking of ‘fire sweeps’ and characterised for the first time their lifetime and dynamics in comparison with overlying atmospheric turbulent structures. 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Wind gusts at the fire-front extend and intensify flames causing direct convective heating towards unburnt fuels resulting in rapid acceleration of spread. Aims To characterise ambient and fire turbulence over gorse shrub and explore how this contributes to fire behaviour. Methods Six experimental burns were carried out in Rakaia, New Zealand under varying meteorological conditions. The ignition process ensured a fire-line propagating through dense gorse bush (1 m high). Two 30-m sonic anemometer towers measured turbulent wind velocity at six different levels above the ground. Visible imagery was captured by cameras mounted on uncrewed aerial vehicles at 200 m AGL. Key results Using wavelet decomposition, we identified different turbulent time scales that varied between 1 and 128 s relative to height above vegetation. Quadrant analysis identified statistical distributions of atmospheric sweeps (downbursts of turbulence towards vegetation) with sustained events emanating from above the vegetation canopy and impinging at the surface with time scales up to 10 s. Conclusions Image velocimetry enabled tracking of ‘fire sweeps’ and characterised for the first time their lifetime and dynamics in comparison with overlying atmospheric turbulent structures. 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Wind gusts at the fire-front extend and intensify flames causing direct convective heating towards unburnt fuels resulting in rapid acceleration of spread. Aims To characterise ambient and fire turbulence over gorse shrub and explore how this contributes to fire behaviour. Methods Six experimental burns were carried out in Rakaia, New Zealand under varying meteorological conditions. The ignition process ensured a fire-line propagating through dense gorse bush (1 m high). Two 30-m sonic anemometer towers measured turbulent wind velocity at six different levels above the ground. Visible imagery was captured by cameras mounted on uncrewed aerial vehicles at 200 m AGL. Key results Using wavelet decomposition, we identified different turbulent time scales that varied between 1 and 128 s relative to height above vegetation. Quadrant analysis identified statistical distributions of atmospheric sweeps (downbursts of turbulence towards vegetation) with sustained events emanating from above the vegetation canopy and impinging at the surface with time scales up to 10 s. Conclusions Image velocimetry enabled tracking of ‘fire sweeps’ and characterised for the first time their lifetime and dynamics in comparison with overlying atmospheric turbulent structures. Implications This methodology can provide a comprehensive toolkit when investigating coupled atmosphere–fire interactions.</abstract><doi>10.1071/WF22100</doi><tpages>13</tpages><orcidid>https://orcid.org/0000-0002-5572-9507</orcidid><orcidid>https://orcid.org/0000-0001-7505-8870</orcidid><orcidid>https://orcid.org/0000-0002-4876-2683</orcidid><oa>free_for_read</oa></addata></record>
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title Atmospheric turbulent structures and fire sweeps during shrub fires and implications for flaming zone behaviour
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