A fast, physically based model of firebrand transport by bushfire plumes

Abstract

Spotfires are a major problem in bushfire management, greatly complicate suppression efforts, and contribute to fires breaking control lines. Firebrands are often implicated in structure loss, and in extreme circumstances have been observed to ignite new fires over 30km ahead of the parent fire. Existing prediction techniques do not accommodate the problem of such long-range spotting, and the meteorological and fire conditions that lead to such events are not well understood. We present a computationally inexpensive, physically based model of ember transport within bushfire plumes, with four components: an integral plume model, a model of turbulence within the plume, a probabilistic model of ember transport by the plume, and a model of transport beneath the plume. The predicted ember landing distributions from this model compare satisfactorily to the explicit ember-transport calculations of Thurston et al. (2017) and to observations (Cruz et al., 2012). We examine the sensitivity of the simple model to its input parameters. The 90th percentile of spotting distance increases with increasing fire power and decreases with increasing ember terminal fall velocity and fire radius. Where there is a meteorological inversion or stable layer and the plume updraft is sufficiently strong to carry embers to that height, the stable layer substantially limits the transport distance, with increased height favoring longer-range transport. The effect of wind speed is complex, due to the competing factors of faster horizontal transport but suppressed vertical plume development with stronger winds.