The Mesoscale Meteorology of Extreme Wildfires over Southern Australia (Black Saturday)

On 7 February 2009 a series of fire complexes developed over the state of Victoria, Australia. The fires caused 173 fatalities and destroyed more than 2133 residences. The fires also decimated a number of townships, including Marysville and Kinglake. The combined effects of the fires are acknowledged as the worst on record and the day has since been named ‘Black Saturday'. The extremely hot, dry and windy conditions created a day of exceptional fire danger that was exacerbated by a variety of mesoscale phenomena. The mesoscale meteorology of Black Saturday is the focus of this study.

A numerical simulation is made with a nested version of the UK Met Office Unified Model in which the smallest domain has horizontal grid spacing equal to 444 m. The model reproduces the daytime maximum temperature to within a degree and the simulated coastal front arrives only about 30 minutes late, even though the model is initiated from a global analysis 27 hours earlier. The coastal front resembles an unsteady gravity current that propagates from the southwest into the extremely deep mixed layer over land. Once the coastal front reaches the mountains to the north and west of Melbourne it is distorted by the terrain, which produces considerable variability in its local direction of propagation.

The deep mixed layer and strong winds ahead of the coastal front promote the development of organized horizontal convective rolls that extended over much of the region. Cloud streets with similar orientation and spacing to the simulated rolls are also identified in satellite imagery, lending confidence in the representation of these structures in the model. The model also produces two nocturnal bores, one in the early morning ahead of the front and one in the evening that was coupled to the front. These bores were also observed and, remarkably, the simulated arrival of second bore is within about 10 minutes of the observations.

Assessing the fire danger using the McArthur forest fire danger index (FFDI) establishes the contribution of the mesoscale phenomena to local variations in fire danger. To the extent that the FFDI is related to the fire spread rate, it is possible that this spatial and temporal variability in the FFDI explains some of the spatial and temporal variability in the fire behaviour reported in extreme wildfires like Black Saturday. The horizontal convective rolls create organized bands of higher and lower fire danger, on horizontal scales of around 10 km. The localized descent that comprise part of the circulation of the encroaching cool air also create a coherent band of higher fire danger after the passage of the front. Finally, the passage of the second nocturnal bore coincides with observed increases in fire activity. Thus, mesoscale phenomena contribute very substantially to local variations in the fire danger.