Large-eddy simulations of bushfire plumes in the turbulent atmospheric boundary layer

Abstract

Spot fires are a hazardous phenomenon which can lead to unpredictable fire behaviour and accelerated fire spread. Spot fires occur when firebrands are lofted into strong ambient winds and ignite new fires downwind. Anecdotal evidence suggests that this lofting and transport of firebrands can be responsible for the ignition of spot fires at large distances, up to tens of kilometres ahead of the fire front. A thorough knowledge of the potential for lofting from a fire is therefore desirable in order to accurately predict the fires rate of spread and coverage. The extent to which firebrands are lofted and transported away from a fire is largely determined by both the intensity of the fire convective column and the strength of the ambient winds. Previous work on the response of fire plumes to background winds has principally relied on theoretical or highly idealised numerical models. Here we use high-resolution three-dimensional numerical simulations, performed with the UK Met Office Large-Eddy Model, to investigate the behaviour of bushfire plumes. We begin by simulating the dry, neutral atmospheric boundary layer for a range of wind speeds. Simulations are run to a quasi-steady state, ensuring that the flow displays realistic turbulence properties. Plumes are then produced by imposing a localised positive surface heat flux anomaly at the model surface. The sensitivity of the size, shape and intensity of the plume's updraft to the interaction between the plume and the turbulent atmospheric boundary layer is explored, with reference to the potential for spotting.