Published 09/07/2018
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Coupled atmosphere-wildland fire modelling

D.Veynante , C.Lac , B.Cuenot , J.Baptiste , F.Bosseur , C.Mari , P.Le , D.Cariolle , J.Balbi

Canadian forest , coupled with an extension of the atmospheric fluid dynamic model of Clark , Clark and Hall and more recently with the Weather Research & Forecasting Model meso-scale mode. While these efforts are effective at simulating the coupled effects at the scale of a large fire with a high degree of fire front precision, the use of the Rothermel model should be subject to caution as the effects of wind and slope to the rate of spread are expressed as coefficients that are experimentally fitted to wind values as if the fire was not there.
A tight interaction exists between the development of a wildfire and the local meteorology near the front. The convective effects induced by the fire heat release can modify the local wind circulation and consequently affect the fire propagation. In this study we use a meso-scale numerical model in a Large Eddy Simulation configuration coupled to a simplified physical front tracking wildfire model to investigate the differences induced by the atmospheric feedback in propagation speed and behaviour. Simulations of typical experimental configurations show a good response of the coupled fire-atmospheric model. Numerical results matches qualitatively observed values for fire induced winds and convection. Both numerical models already have operational usage and might ultimately be run to support decisions in wildfire management.
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The main originalities of this combination reside in the fact that Meso-NH is run in a Large Eddy Simulation configuration and that the RoS model used in ForeFire provides a physical formulation to take into account the effect of wind and slope. In the proposed RoS model, wind and slope effects are explicitly taken into account by calculating a flame tilt angle with a vector method and therefore may be more appropri-ate for use in a coupled configuration. In order to provide a simplified formulation of the RoS under different conditions, it is also assumed that the radiant factor decreases with the surface/volume ratio of the flame and that the gas flow speed and direction in the flame are given by the vectorial sum of the wind speed with a given vertical gas velocity due to buoyancy and combustion.
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