A re-examination of the synoptic and mesoscale meteorology of Ash Wednesday 1983

摘要

The fires on Ash Wednesday 1983 were some of the most destructive in Australia's history, with 75 lives lost, more than 2000 houses and 335 000 ha of rural land burnt, and more than 250 000 head of stock lost. Using archived objective analyses from the Australian National Meteorological Centre, the National Centre for Environmental Prediction (NCEP) reanalysis dataset, and a contemporary operational mesoscale numerical weather prediction model, the predictability and the dynamics of the wind structures over Victoria on that day are investigated, and a 40-year analysis archive used to assess just how meteorologically unusual the event was. It is shown that using the archived analyses to provide the initial state, and analysis lateral boundary conditions, that the timing of the wind change could be forecast extremely accurately. The strong post-frontal winds are shown to be associated with an unusually deep tropospheric trough system, and an analysis of 40 years of summer data from the NCEP reanalyses shows this system was in the strongest 0.1 per cent as measured by the intensity of the 850 hPa temperature gradient over Victoria. It is further shown that many of the other extreme fire weather events in that period were also associated with similarly unusually strong temperature gradients at 850 hPa, suggesting that this parameter has the potential to identify unusually severe fire weather events in medium-range, seasonal, and climate change numerical model forecasts.