21/12/2019

(AU) Scientists Fear Surge In Supersized Bushfires That Create Their Own Violent Thunderstorms

The Guardian - 

Pyrocumulonimbus (pyroCB) storms are feared due to the violent and unpredictable conditions they create on the ground
A pyroCB at Walhalla, Victoria, on 3 February 2019 . Photograph: Nicholas McCarthy
Scientists fear climate change will drive a surge in the number of supersized and dangerous bushfires that become coupled with the atmosphere and create their own violent thunderstorms.
Guardian Australia can reveal 2019 is likely to be a “standout year” for the number of bushfires that generate giant thunderstorm clouds known as pyrocumulonimbus, or pyroCBs.
PyroCB storms are feared by firefighters for the violent and unpredictable conditions they create on the ground.
PyroCBs are able to generate their own lightning strikes, mass downdrafts of air, gusty winds and even hail blackened with soot. The plumes generated from pyroCBs can influence the atmosphere at heights of up to 15km.
Embers still hot enough to start new fires can be shot out of a pyroCB at distances of 30km from the main fire.
The 411,000 hectare Gospers Mountain fire in the Blue Mountains, still burning out of control on Thursday, is likely the latest bushfire to have generated a pyroCB storm on 22 November.
Nicholas McCarthy, who has just completed a Phd at the University of Queensland on why bushfire thunderstorms form, said watching one develop was a “grounding experience”.
“Once you hear that first clap of thunder, you know there’s not a lot you can do,” he said. “There shouldn’t be anyone on the ground at that point. All of a sudden [the fire] loses a whole level of predictability.”



PyroCB fires can have devastating and dramatic consequences. Victoria’s deadly Black Saturday conflagration in February 2009 created its own lightning that caused fires 100km ahead of the main fire front.
Research scientist Rick McRae, who is the custodian of a register of Australian pyroCB events, said 2019 was a “standout year”.
In March, there were 15 pyroCBs detected in the Victorian high country, including a cluster of 12 in just four days – an unprecedented grouping on McRae’s register.
“This is the standout year but we are still analysing them,” he said.
Dr Andrew Dowdy, a meteorologist at the Bureau of Meteorology, has published several studies on pyroCB fires and their underlying conditions.
Research led by Dowdy and published in the journal Scientific Reports has found that adding more greenhouse gases to the atmosphere will make more dangerous conditions favourable to pyroCB events in the future, particularly for the southern parts of Australia.
Dowdy said an examination of conditions in the atmosphere and on the ground between 1979 and 2016 that are conducive to pyroCBs had already found a “statistically significant” trend in southern Australia.
He said: “We found that in summer in southern Australia and spring time, there have been large changes towards more dangerous conditions.”
Dr Simon Seemstra, manager of planning and predictive services at the New South Wales Rural Fire Service, said: “What’s happening now is that we are noticing an increase in incidence of these sorts of events. With a changing and heating climate, you are going to expect these effects.”
He said pyroCB fires were characterised by violent “mass flaming” that “puts lives at risk, whether those are firefighters or the community in the path of these events”.
The advice given to firefighters in NSW before and during a pyroCB event is to “make sure they have a safe refuge”.
He said: “It instills fear – the thought of a fire creating a thunderstorm that’s throwing embers and lightening in front of it. It creates a dangerous situation and we take them very seriously.”
In response to a rising number of pyroCBs, he said the NSW Rural Fire Service was using weather balloons to take atmospheric measurements during times it is feared that pyroCBs could form. New communications material for firefighters has also been produced.
Seemstra said one warning sign was a smoke column that rises to about 5km and forms a white top as moisture turns to ice. If the column continues to rise, a warning is sent to firefighters and communities on the ground.
As well as creating dangerous weather conditions, Seemstra said pyroCBs prevent the fire service from using aircraft either to take measurements, or to drop water or fire retardant.
Associate professor Jason Sharples, of the University of New South Wales, said pyroCBs needed two basic elements to form – an intense and expansive fire and an unstable atmosphere above it.
He said: “Think of it like a really bad thunderstorm with all the winds and the rain but then take out the rain, and put in embers.”
Dr Mike Fromm, of the US Naval Research Laboratory at the Department of Defence, told Guardian Australia that to identify pyroCB events scientists used satellites to examine images and also detect aerosol particles.
“They generally will punch a hole through the boundary between the troposphere and stratosphere,” he said. “They create a huge blanket of cloud that turns day into night.”
He said globally there was no evidence of a trend towards more pyroCBs but he also said the record was relatively short.
PyroCB storms typically lasted between one and eight hours, Fromm said. There was evidence of embers being shot out from the fire at distances of up to 30km.
“What we know about pyroCB conditions is summarised as ‘hot, dry, windy’,” he said. “If these factors intensify with climate change, it is reasonable to anticipate additional risk of such firestorms.
“The pyroCB is the most dramatic manifestation of fire weather and behaviour from the standpoint of the views from space.”
He said observations from the deadly Black Saturday fires of February 2009, where 173 people died, illustrated the dangers posed by pyroCBs.
“Black Saturday’s Kilmore East fire didn’t even exist until late morning of that fateful day; by mid-afternoon it generated a full-fledged, devastating pyroCB.”

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