Columns of super-dry air that reach the earth’s surface from
high altitudes could be responsible for extreme fire behaviour in
some of the worst bushfires in Australia ’s history.
In work undertaken through the Bushfire Cooperative Research
Centre, Dr Graham Mills, from the Bureau of Meteorology, has found
that under certain conditions columns of dry, fast moving,
high-altitude air descend, causing a rapid loss of humidity at
ground level and very strong gusty winds. If positioned over a
bushfire, this phenomena can cause it to erupt.
These “dry slots” are a relatively common occurrence
near jet-stream level, where jet aircraft fly, but need some link
to reach the surface. This link is often provided by just the
conditions that already create extreme fire danger.
Mills looked closely at satellite images of the bushfires in
Canberra on 18 January 2003.
On that day a cold front was pushing across southeastern Australia
towards the hot air masses and gusty winds already fanning the
going fires. Columns of hot air were rising up to 6km to mix with
the drier air aloft. The strong vertical mixing then brought the
much drier and fast moving air rapidly to the surface.
“If there is a going fire, then the dry slot is potentially
one of the triggers to set the fire going uncontrollably. Suddenly,
the fuel is drying out more quickly, the wind is stronger and
gustier, and the fire can take off explosively,” Mills said.
The research also links the dry slot phenomena with three other
major fire events: the South Australian Eyre Peninsula fire in
January 2005; the day of the lightning ignitions in Victoria and
NSW on 8 January 2003 that lead to the prolonged alpine fires; and,
the early 2006 fires on the central NSW coast.
Mills said the research is now being developed as a basis for a
forecasting tool for fire authorities. “You have got several
hours of warning if you can see one of these things moving towards
a fire. When a dry slot moves closer, we can give the fire agencies
some warning that the fire behaviour may soon become more
explosive.”
He said the forecasting tool is not only useful for extreme events
like the summer alpine fires of 2003 but also for smaller bushfires
and for hazard reduction burns.
"Beware the dry air aloft" has long been the mantra of experienced
fire managers but little was understood about its behaviour and
origins.
"We didn't really know where it was except that it was up there
somewhere. Now we know exactly where it comes from and what sort of
explosive impact it can have on a bushfire."
An article on this research appears in the Australian
Meteorological Magazine.
