Tasmanians know all too well the impact of the 2013 Dunalley bushfire on the ground, but for the first time, research has revealed what happened above the town on that fateful day.
The time was 3.24pm on January 4, and the bushfire was already burning out of control. Several factors created the perfect storm.
Over the next 24 minutes, a violent column of moist hot air developed over the fire, combining with an unstable atmosphere.
It created what is known as a pyrocumulonimbus cloud, or a fire thunderstorm.
Photos captured from a distance what was an incredibly foreboding scene: a gigantic grey mass that many assumed was a smoke plume. It was, in fact, so much worse.
Facing a storm: how bushfires can create their own weather
In the wake of the bushfire, researchers accessed detailed data from the Bureau of Meteorology’s Koonya radar facility.
It emits radio waves and listens for an echo to be reflected back and is normally used to detect rainfall. But it can also detect ash particles.
Dr Grant Williamson from the University of Tasmania said this allowed researchers to examine the Dunalley cloud.
“It was clear it was a really good capture, that we could start to measure the volume of the plume and work out how high the smoke went,” he said.
The plume reached 15 kilometres at its peak, hitting the stratosphere.
Violent pyroconvection was recorded for about an hour. Two lightning strikes associated with the pyrocumulonimbus, known as PyroCB, hit the Tasman Sea.
When the plume eventually collapsed, an ember storm hit the town.
People took shelter where they could when the firestorm hit the small community of Dunalley.(Audience submitted: Tim Holmes)
About 10,000 hectares or 40 per cent of the total area burnt during the devastating Dunalley fires was associated with the January 4 PyroCB event.
“The thing about these PyroCB events is they are creating their own weather,” Dr Williamson said.
“The convection column is pulling in wind from all directions which makes the fire behaviour very unpredictable.”
University of Tasmania fire expert David Bowman said that didn’t mean PyroCBs themselves were entirely unpredictable.
“There’s a bit of a checklist you can have,” he said.
“You need heavy fuels, undulating terrain to create turbulence, you need wind to be moving the fire along, you need dangerous fire weather and you need an unstable atmosphere.”
Hobart at risk of firestorm
The PyroCB over Dunalley was the first known event of its kind in Tasmania but experts are warning it’s unlikely to be the last.
Mercy Ndalila was the lead researcher on the study.
“An important take-home message from this study is that south-eastern Tasmania is more vulnerable to extreme fire events,” she said.
“This is because of elevated fire weather within this region, which increases the risk of more PyroCBs in future.”
Scores of properties were destroyed when the firestorm hit the Tasman Peninsula.(Michael Goldsmith: Tasmania Fire Service)
Professor Bowman said Hobart was “quite likely” to be subjected to a PyroCB.
“Hobart has got all of those conditions: undulating terrain subjected to very strong winds and dry winds that come down the Derwent Estuary, we get the northerlies blowing down the Derwent Valley, heavy fuel loads like Mount Wellington and an unstable atmosphere because of the atmosphere with the cooler or warmer waters and land surface temperatures,” he said.
“If you start piecing all of this together we’re looking at unfortunately have to forewarn everybody that this is yet another risk factor we need to build in.”
There were 30 PyroCBs recorded on mainland Australia last summer, Professor Bowman said.
“These things are very dangerous and destructive,” he said.
“Even if they’re out in the wilderness or a forestry area, they’re still going to have tremendous impacts.”
Researchers say there are many factors that make Hobart vulnerable to a firestorm.(ABC News: Gregor Salmon)

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