A wildland fire is a complex, multi-scale, and multi-process dynamical system with its instantaneous state depending on chemical and physical processes occurring on a hierarchy of spatial and temporal scales. Observations of multiscale processes affecting the wildland fire are very difficult to make. These scales are as small as fuel particles during the combustion process (order of <10 cm), flame structures and heat transfer (order of one to tens of meters), and microscale/mesoscale meteorological processes (orders of 10-10 000 m) (Morvan, 2011;Sullivan, 2017aSullivan, , 2017b. As point measurements, micrometeorological in situ towers in experimental burns provide spatially limited information over a brief period of time as the fire approaches and then travels underneath the instrumented tower (
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.
Background. Understanding near-surface fire-atmosphere interactions at turbulence scale is fundamental for predicting fire spread behaviour. Aims. This study aims to investigate the fire-atmosphere interaction and the accompanying energy transport processes within the convective boundary layer. Methods. Three groups of large eddy simulations representing common ranges of convective boundary layer conditions and fire intensities were used to examine how ambient buoyancy-induced atmospheric turbulence impacts fire region energy transport. Key results. In a relatively weak convective boundary layer, the fire-induced buoyancy force could impose substantial changes to the near-surface atmospheric turbulence and cause an anticorrelation of the helicity between the ambient atmosphere and the fire-induced flow. Fire-induced impact became much smaller in a stronger convective environment, with ambient atmospheric flow maintaining coherent structures across the fire heating region. A highefficiency heat transport zone above the fire line was found in all fire cases. The work also found counter-gradient transport zones of both momentum and heat in fire cases in the weak convective boundary layer group. Conclusions. We conclude that fire region energy transport can be affected by convective boundary layer conditions. Implications. Ambient atmospheric turbulence can impact fire behaviour through the energy transport process. The counter-gradient transport might also indicate the existence of strong buoyancy-induced mixing processes.
This study presents two new remote sensing approaches that can be used to derive rate of spread and flaming zone velocities of a wildfire at very high spatiotemporal resolution. Time sequential image tracking from thermal or visible video collected on uncrewed aerial vehicles is used to estimate instantaneous spatial rate of spread of a surface fire. The techniques were developed using experimental wheat‐stubble burns carried out near Darfield, New Zealand, in March 2019. The thermal tracking technique is based on Thermal Image Velocimetry, which tracks evolving temperature patterns within an infrared video. The visible tracking technique uses colour thresholding, and tracks fire perimeter progression through time at pixel resolution. Results show that the visible perimeter tracking creates a higher mean rate of spread compared to thermal image velocimetry. The visible perimeter tracking provides rate of spread measurements for fire front progression whereas the thermal tracking techniqueis computationally more expensive, but can resolve velocities of thermal structures within the flaming zone and provides spatiotemporal rate of spread measurements. Both techniques are available as open‐source code and providevital scientific data for new studies concerning e.g. fire–atmospheric interactions or model validation. They may be adapted for operational purposes providing rate of spread at high spatiotemporal resolution.
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