On the subsynoptic-scale meteorology of two extreme fire weather days during the Eastern Australian fires of January 2003

During January and February 2003 vast areas of Australia’s Alpine areas were burnt during a series of massive bushfires. In this paper the synoptic evolution and mesoscale circulation systems that led to extreme fire weather on 18 January, the day the fires devastated the Australian Capital Territory, and 30 January, the day the Alpine fires ‘broke out’ in the Victorian high country are discussed. In each case the extreme fire weather experienced was associated with a ‘cool change’ passage, but the strongest winds are shown to be associated with mesoscale wind maxima generated near the top of the mixed layer, which in these cases was 2000-4000 m deep. The surface structures of the cool changes were strongly affected by diabatic heating, and it is shown how these diabatic processes cause the structure of these changes to vary strongly over relatively short distances. On 18 January an additional feature was a series of progressive reductions in humidity at and in the areas surrounding Canberra. Reasons for this behaviour are discussed, and the role of a mid-tropospheric dry slot identified in water vapour imagery, and a possible contribution from cross-frontal circulations, are described.