QUARTERLY FOCUS
Satellites spy an early warning sign of deadly fire storms
Dr Graham Mills of the Bureau of Meteorology Research Centre speaks to delegates
at a meeting of the Bushfire Cooperative Research Centre in Melbourne.
Picture: Mike Rosel
THE extreme weather conditions that can turn an already dangerous bushfire into an explosive, deadly fire storm can now be better predicted, thanks to the work of a 30-year veteran of the Bureau of Meteorology’s research centre. Dr Graham Mills has found that weather satellite images provide an early warning of high-altitude, super-dry air which, if it descends to the ground, may contribute to radically increasing fire activity.
Graham, also a project leader at the Bushfire Cooperative Research Centre in Melbourne, said recently that meteorologists had always suspected that bands of very dry air in the upper atmosphere were associated with intense bushfires. “But,” he said, “I don’t think anybody understood how air from such a height could be involved in an explosive fire.”
Graham’s theory is that turbulence in the lower atmosphere taps into the dry air some four to five kilometres above ground and brings it to earth, rapidly reducing the humidity and further drying branches, twigs and leaves. “This rapid drying of the fuel (over two or three hours, in some instances) effectively lifts the fire threat,” Graham said.
His studies suggest that the lower-level turbulence or mixing required to bring the dry air to ground is more likely to occur in the afternoon, especially if it is a hot day. Existing fires, mountains and changes in wind speed and direction seem to enhance mixing. “The drying conditions are also linked with gustiness, which means that firefighters cop both barrels.”
Graham said that by pinpointing bands of super-dry air on satellite images, and identifying areas of strong atmospheric mixing through the use of computer models, “we now have the opportunity, in some cases, to give some hours of notice of potentially severe fire risk.”
2003 Canberra fires
Graham’s findings stem from his study of the conditions associated with the disastrous Canberra bushfires of 2003, when four people died and more than 500 homes were destroyed or severely damaged. A few hours before the peak of the fire, the relative humidity in Canberra and nearby high country locations such as Cooma dropped steeply from 15 per cent to about 3 per cent, a startlingly low figure. Graham found that, at the same time, a huge band of dry air 1000 kilometres long, 100 km wide and about 4 km deep stretched across NSW, though the satellite images were tricky to interpret.
Graham also checked his theory by studying the 2005 Wangary bushfire on the Eyre Peninsula, South Australia. Bushfires in the region claimed nine lives, injured 110 people and burnt more than 145,000 hectares. Again Graham identified a huge drop in humidity at Port Lincoln airport at the same time as two huge bands of dry air were visible on weather satellite imagery.
“At this point I remember the James Bond advice about apparently linked events,” Graham said. “Once is happenstance, twice is coincidence, three times is enemy action.”
Graham then set about studying six years of data from 22 weather stations as far apart as Amberley in Queensland, Hobart, the Northern Territory and south-west Western Australia. He found that of 232 “drying events”, 70 per cent were “pretty obviously” linked to bands of high-altitude super-dry air.
“I think it’s fitting,” said Graham, “that meteorologists, who always work hard to predict the wet end of the weather spectrum, should find something novel in predicting the opposite end of the spectrum.”
— Mike Rosel, Public Affairs group
Quarterly focus is updated on 1 January, 1 April, 1 July and 1 October.