The contribution of turbulent plume dynamics to long-range spotting

Abstract

Spotting can start fires up to tens of kilometres ahead of the primary fire front, causing rapid spread and placing immense pressure on suppression resources. Here, we investigate the dynamics of the buoyant plume generated by the fire and its ability to transport firebrands. We couple large-eddy simulations of bushfire plumes with a firebrand transport model to assess the effects of turbulent plume dynamics on firebrand trajectories. We show that plume dynamics have a marked effect on the maximum spotting distance and determine the amount of lateral and longitudinal spread in firebrand landing position. In-plume turbulence causes much of this spread and can increase the maximum spotting distance by a factor of more than 2 over that in a plume without turbulence in our experiments. The substantial impact of plume dynamics on the spotting process implies that fire spread models should include parametrisations of turbulent plume dynamics to improve their accuracy and physical realism.

Drivers of long-distance spotting during wildfires in south-eastern Australia

“…above PBLH) from 1h before scan time was extracted from hourly time-averaged MERRA2 (GMAO 2015b). Wind speed at different heights in the troposphere can influence firebrand lofting by directly blowing firebrands downwind, or by producing a more tilted and turbulent plume with puffy updrafts capable of lofting and emitting firebrands greater distances (Thurston et al 2017).…”

“…Upper-level wind speed had weaker but still significant effects in the models. Wind at different levels can influence many aspects of wildfire behaviour, including plume development, plume turbulence and tilt, fire intensity, vorticity development, firebrand transport and ignition likelihood in receiver fuels (Koo et al 2010;Potter 2012b;Thurston et al 2017).…”

“…The Spot-number results suggest that high upper-level wind speed, together with unstable air (high lapse rate), increases the number of long-distance spot fires, potentially because of greater convection and turbulence in plumes enhancing firebrand dispersal (Thurston et al 2017). Although these variables indicated potential plume behaviour, further research matching observations of plume development and spot fire ignitions is needed to fully understand how plume behaviour affects longdistance spotting.…”

“…2. Firebrand transport: surface and upper-level winds, temperature and moisture, smoke plume dynamics and the aerodynamic attributes of the firebrand can influence the distance and direction of firebrand lofting and deposition (Albini et al 2012;Ellis 2013;Hall et al 2015;Thurston et al 2017). 3.…”

Physical parametrisation of fire-spotting for operational fire spread models: response analysis with a model based on the Level Set Method

“…In a recent study, Martin and Hillen (2016) also discuss the underlying physical processes for firebrands in detail and they derive a landing distribution based on these physical processes. Besides these statistical approaches, few numerical models based on Large Eddy Simulation (LES) (Himoto and 25 Tanaka, 2005;Thurston et al, 2017;Tohidi and Kaye, 2017) or Computational Fluid Dynamics (CFD) (Wadhwani et al, 2017), small world networks , cellular automata models (Perryman et al, 2013) also exist in the literature. Bhutia et al (2010) present one such study based on coupled fire/atmosphere LES for predicting the short range fire-spotting.…”