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Investigation of the aerodynamic effects of the forest canopy on the behavior of experimental fires

ED SDU2E

https://youtube.com/live/36i8CKUdrFc?feature=share

Wildland fires have demonstrated their destructive power over the past decade, with climate change exacerbating the flammability of fuels and the intensity of the fires that consume them. These fires result from multi-physics, multi-scale processes, ranging from local effects on ecosystems and populations to global impacts on atmospheric dynamics and climate. To better understand their spread and assess the associated risks, coupled fire-atmosphere simulations can be carried out at geographic scales. These numerical simulations simplify the fire by representing it as a fire front moving through heterogeneous fuels. Coupled with a microscale atmospheric model, they account for physical phenomena such as fire-induced surface heat fluxes, the development of smoke plumes, and surface air entrainment towards the fire front. While these simulations are essential for exploring these phenomena and addressing both scientific and operational objectives, they require precise calibration, particularly using data from instrumented prescribed burns. In this PhD thesis, the coupled fire-atmosphere model Meso-NH/BLAZE was implemented at decametric resolution under idealized atmospheric conditions to reproduce the experimental grass fire FireFlux I. The goal was to evaluate the impact of surrounding forest canopies on the experimental measurements acquired at two instrumented towers located within the burn plot and on the fire front's progression. To characterize the interaction between the turbulent wake of forest canopies and fire propagation, a quadrant analysis was conducted to detect coherent structures generated by instabilities. Turbulent fluctuations are typically decomposed using temporal averaging, but this approach was inapplicable here due to the transient nature of fires. A method based on wavelet transforms was therefore proposed to analyze atmospheric variables during the fire front's passage. The results show that a horizontal atmospheric resolution of 10 m in Meso-NH is sufficient to represent the vortices generated by the forest canopy. They also reveal that this forest wake alters atmospheric measurements in the grassland plot, both under fire-free atmospheric conditions and during fire propagation. Furthermore, atmospheric variability, which is a source of intrinsic variability in fire spread, is reduced under the influence of the canopy. However, the importance and spatial extent of this effect are often underestimated in the literature.

Jury

Sylvain DUPONT	INRAE	Reviewer
Ronan PAUGAM	Polytechnic University of Catalonia	Reviewer
Albert SIMEONI	Worcester Polytechnic Institute	Reviewer
Marie PARRENS	CESBIO	Committee member
François PIMONT	INRAE	Committee member
Rui SALGADO	Evora University	Committee member
Mélanie ROCHOUX	CERFACS-CNRS-CECI	Supervisor
Patrick LE MOIGNE	CNRM	Co-supervisor