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Rare firestorm weather events are becoming the new normal

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Massive bushfires have been generating their own weather systems, often causing further devastation (Screenshot via YouTube)

As our country heats up, pyrocumulonimbus firestorms are becoming an increasingly frequent weather phenomenon, writes Nick Goldie.

LOCAL AUTHORITIES are urgently warning motorists to stay clear of floodwaters. Just a couple of days ago, the approaching fires were the real threat. The wise person remembers that the fire season is by no means over.

One of the effects of this extreme bushfire season has been an increase in the number of pyrocumulonimbus events. These are known as pyroCbs, or simply firestorms.

A pyroCb event occurs when a fire is sufficiently large and intense to create a plume which punches up to and even into the stratosphere, taking with it smoke, ash, heat, water and quite probably causing wind storms, cold downdrafts, lightning, even the uncanny black hail which Canberra residents in 2003 found so disturbing.

Not many years ago, pyroCbs were quite rare, an interesting, if scary, scientific curiosity.

At a fire weather conference in Bowral, NSW, in 2011, Dr Mike Fromm of the U.S. Naval Research Laboratory told me that he had been on a research flight in a NASA DC8 when, high in the troposphere, he smelt smoke. It was not burning aircraft fittings, he said, but gum leaves.

He was smelling the smoke from an Australian bushfire. The smoke had travelled around the world at a stratospheric level. And this, said Dr Fromm, was grit in the eye of U.S. surveillance satellites, which had to be able to see what they were doing.

In December 2019, a young firefighter was killed at Jingellic, on the Victoria-NSW border and two others were injured when a freak wind picked up their fire truck and flipped it on to its roof.

RFS Commissioner Shane Fitzsimons said it was:

“...a fire tornado or the collapse of a pyro-convective column that had formed above the main fire front. That's resulted in cyclonic-type winds that have moved across the fire ground and has literally lifted up a ten or 12-tonne fire truck."

A fire tornado is a different beast to the relatively common fire whirl or fire willy-willy which rises, whirling, out of the flames. A fire tornado, correctly speaking, is anchored to the cloud above a fire and descends, whirling. It can also lift off the ground, as happened in Canberra and return to ground level further along its track. There have been two scientifically recorded fire tornadoes: Canberra and perhaps at the 2018 Carr Fire near Redding in California, which also resulted in a fatality. A fire tornado can lift a car off a road or flip a fire truck on to its back.

PyroCb events have certain requirements, otherwise, they remain “just” severe bushfires. The problem is not to be wise after the event, but to use the information that we have to make accurate predictions. When a fire is at catastrophic levels, it’s no place for a volunteer firefighter to be.

Fire researchers have been trying to find a reliable way of predicting a pyroCb event and the focus of much of their attention has been what’s going on above our heads – a pyroCb doesn’t happen unless the atmosphere above a fire is unstable – where a “parcel” of air is warmer than the atmosphere surrounding it, is less dense and therefore rises. This leaves a gap and the surrounding air rushes inwards or even downwards.

This atmospheric behaviour is very important for fire agencies in the field, but it is not well understood. Other key elements of fire behaviour such as “deep flaming” need to be recognised; that is, where a fire changes from being a fire line to a broad area of flames. This can be caused by two fire lines meeting, by a change of wind direction, by the topography or even by careless use of a driptorch.

A little-understood concept is that of “ruggedness”; it is no coincidence that many of Australia’s (and America’s) most destructive wildfires have been in rugged, mountainous areas. The fires are not just difficult to access, but they are fierce and accompanied by fierce winds off the terrain.

Until recently, pyroCb events were rare in Australia, despite our history of widespread bushfires. This is changing. Jason Sharples, Professor at the University of New South Wales in Canberra and Rick McRae of A.C.T. Emergency Services have been keeping track of pyroCb events, and the list is scary. Satellite technology since 1978 has allowed detailed records of atmospheric events. The elusive pyroCb occurred just twice (with two more “possibles”) between 1978 and 2001, but since 2001, according to Sharples, another 78 pyroCb events have been logged, with no less than 18in March of 2019.

At least another 30 pyroCbs have been recorded in Australia since September 2019, with a further 15 possible storms still being investigated.

Professor Sharples, himself a volunteer firefighter, says that when hot, dry and windy conditions align with atmospheric instability, it creates the perfect condition for pyroCb events.

“You can’t fight them,” he says. “We have to work out the best way of predicting the monsters and staying out of their way.”

Nick Goldie is a science journalist and volunteer firefighter who lives in rural NSW. He is currently writing a book on extreme fire events with fellow volunteer firefighter, mathematician and Professor Jason Sharples and  A.C.T. Emergency Services researcher Rick McRae.

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