Meteorological aspects of the Margaret River fires of November 2011

Abstract

Early in the morning of Wednesday 23 November 2011, a fuel-reduction burn near Margaret River in southwest Western Australia increased dramatically in activity. The fire escaped control lines and burnt southwards along the coastal fringe, subsequently destroying 32 homes, including the historic Wallcliffe House, and damaging others. Satellite imagery showed a smoke plume of vastly greater extent than other fires in the area. Here, we present a preliminary analysis of the meteorology of the event, based on very high resolution (400 m) simulations with the Bureau of Meteorology's ACCESS numerical weather prediction (NWP) system and available observations. We find that several mesoscale features likely contributed to the fire becoming or remaining active overnight. The first of these was a region of marked near-surface drying over and around the fire ground, contrary to the normal diurnal cycle and due to the advection of dry air from inland regions to the northeast of the fire. The second was the development of strong mountain-wave activity and downslope winds over the fire during the Tuesday night and extending into Wednesday morning, caused by a combination of a strengthening pressure gradient as a high pressure system passed to the south of the region, nocturnal cooling, and the local topography. The third was the development of a pronounced low-level jet, which provided a source of strong momentum for the mountain waves to bring to the surface. During the day, while the fire was heading rapidly southwards under the influence of strong hot northerlies, the observed increase in activity around midday may have been due to a lengthy, coherent wake shed from hills to the north of the fire. The essential dynamics of mountain waves are quite well known, although their occurrence on topography as low as here presents some forecasting difficulties, since the common assumption of inviscid flow cannot be made when the “mountain” is contained entirely within the boundary layer. Nevertheless, strong nocturnal downslope winds form a recognised part of the climate of both Perth and Adelaide, and likely occur in other regions also. Their influence on fire behaviour in Australia does not seem to have been widely documented before, although Sharples (2009) identifies one case. We expect that similar winds will occur again and recommend that managers of lee-slope fires exercise appropriate caution in the future, especially overnight and in the morning.