Boundary-layer rolls and their potential impact on bushfires: a high resolution NWP case study

Two important processes that contribute to the spread rate of bushfires are wind direction variability and updraft phenomena: Wind direction variability in the atmospheric boundary layer contributes to lateral fire spread by broadening the fire front, which in turn increases the fire-front propagation speed. Updraft phenomena contribute to fire spread by lofting firebrands, which can ignite spot fires several kilometres ahead of the fire front.

Boundary-layer rolls are linear organised convective circulations that result in both surface wind direction variability and updrafts of a few metres per second, so are of potential interest for bushfire dynamics. Historically the dynamics of boundary layer rolls have been explored with high resolution, idealised large eddy simulations. However, current numerical weather prediction models are now reaching the horizontal resolution required to resolve boundary-layer rolls. Here we present results from a high resolution ACCESS case study of the meteorology of Black Saturday, 7th February 2009. The model was found to reproduce boundary-layer rolls, as observed by satellite and radar, with some sensitivity to the horizontal resolution of the simulation. The implications for fire danger and potential impacts on lofting of firebrands are also considered.