Atmosphere and fire interactions from the New Zealand experimental burn campaigns

Wildfires are increasing in intensity, frequency, and occurring earlier and later than normal on the seasonal timeline. Coupled atmosphere-flame-fuel dynamics makes wildfire a difficult phenomenon to understand and predict across its temporal and spatial spectrum of scales. This is especially true at the turbulence scale where rapid fluctuations of near-surface wind velocity and temperature within the atmosphere-fire boundary layer can control fire spread rates and extreme fire behavior. Appropriate observations from wildfires suitable for process-based investigations of coupled atmosphere-fire boundary layer dynamics do not exist, instead experimentally controlled fire burns are usually carried out. These experiments rely on repeated short-term wind driven fires that are usually restricted to certain meteorological regimes. Experimental design remains a challenging endeavor, which still lacks spatially coherent ambient and fire-induced atmospheric observations for understanding coupled dynamics. We have carried out several experimental fire burns in New Zealand for short stubble wheat and more dense and higher gorse shrubs. Our observations covered fuel properties, atmospheric turbulence, and flaming zone behavior. We have used uncrewed aerial vehicles carrying high speed infrared and visible video cameras, along with in-situ eddy covariance towers for ambient and fire-induced turbulence heat and momentum measurements. Some methodological highlights include the combination of image processing techniques, fuel density maps from aerial Lidar, and atmospheric turbulence structure analysis. We present a synthesis of research findings over the last two observational campaigns and introduce our new objectives for the upcoming crown fire forest canopy fire experiments. In addition, we discuss large eddy simulations of carefully designed numerical experiments allowing for a better understanding of the fire-atmosphere energetics at the atmospheric boundary layer scales.