Atmospheric turbulent structures and fire sweeps during shrub fires and implications for flaming zone behaviour
Marwan Katurji A * , Bob Noonan A , Jiawei Zhang
A B , Andres Valencia C , Benjamin Shumacher A D , Jessica Kerr B , Tara Strand C , Grant Pearce B E and Peyman Zawar-Reza A
+ Author Affiliations
- Author Affiliations
A School of Earth and Environment, University of Canterbury, Christchurch, New Zealand.
B New Zealand Forest Research Institute, Scion, Rotorua, New Zealand.
C Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand.
D Earth Observation Data Centre GmbH, Franz-Grill-Straße 9, 1030 Vienna, Austria.
E Fire Emergency New Zealand- FENZ, Wellington, New Zealand.
* Correspondence to: marwan.katurji@canterbury.ac.nz
International Journal of Wildland Fire 32(1) 43-55 https://doi.org/10.1071/WF22100
Submitted: 21 June 2022 Accepted: 28 October 2022 Published: 14 November 2022
© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing on behalf of IAWF. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)
Abstract
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.
Keywords: coherent structures, fire sweeps, fire turbulence, fire–atmosphere interactions, flaming zone, image velocimetry, surface-layer turbulence, UAV.
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