Abrupt surface drying and fire weather Part 1: overview and case study of the South Australian fires of 11 January 2005

On the day of a severe bushfire on the lower Eyre Peninsula, South Australia (11 January 2005), an extreme reduction in near-surface humidity was observed. The meteorology of this event showed similarities to that observed at Canberra Airport on the day of the devastating fires of 18 January 2003, both in the abrupt surface humidity reduction to extremely low levels, and in the presence of a dry band in the middle troposphere in 6.7 μm wavelength ‘water vapour channel’ satellite imagery. As fine fuels respond to changes in atmospheric humidity on time-scales of the order of an hour, and as fire behaviour becomes increasingly more extreme as fuels become very dry, understanding and forecasting such unusual surface drying events may be important to fire managers. The relation between the lowering of the surface humidity, exchanges of mid-tropospheric dry air with the surface, and the synoptic dynamics of the atmosphere that allowed this process to occur on that day are described.