It was an image that punctuated an already horrific fire season. As campaign-level fires raged across the state, the Licola bushfire, east of Melbourne, burned with such ferocity that it was visible on the Bureau of Meteorology’s (BOM) radar.
The subsequent thundercloud that formed from the smoke and flames caused more than 1,200 lightning strikes, sparking further fires and creating unpredictable conditions for frontline firefighters, aircraft and emergency management teams.
This is what happens when a bushfire creates its own weather.
Often, before a bushfire creates its own weather, it becomes plume-dominated – where the energy in a rising convection column is more powerful than the wind near ground level driving the fire. It doesn’t take much for this to occur; it doesn’t even require a big fire.
Hilly terrain, lots of dry fuel, fire size, heat and instability in the air all contribute to creating a plume-dominant fire.
If conditions are right, the intense heat of the fire coupled with rising air generates pyrocumulus cloud in the upper portions of the convection column, recognisable by its crisp, white, fluffy, cotton-wool-like appearance. Sometimes pyrocumulus cloud will become large enough to generate lightning. When that happens, it’s called pyrocumulonimbus – a thunderstorm created by a bushfire.
A STORM CREATED BY FIRE IS UNPREDICTABLE
Several things can happen when fires become plume dominated. Updrafts and wind flow within the plume, coupled with the extra heat from the convection column will cause the wind to rise faster, drawing in more air at ground level and increasing the fire intensity. This then releases even more convective energy.
If a plume-dominated fire evolves to create its own storm activity, it becomes self-sustaining unless forces outside the plume change.
Lightning can start new fires kilometres ahead of the main fire front. Spot fires are also likely to break out as embers are swept up in the swirling winds. There is potential for rain to form within pyrocumulonimbus clouds, producing downbursts and wind strong enough to change fire behaviour.
“At its biggest, the plume on the Licola fire would have reached 12 kilometres into the atmosphere,” CFA Fire Behaviour Analyst Musa Kilinc said. “As we saw, this led to dangerous fire behaviour, particularly when lightning was generated from within the structure.
“Bunyip was different in that the plume was going up and down all day depending on the weather conditions and the rate of heat release from the fire. At its peak it reached 10 to 12 kilometres into the atmosphere, and didn’t really dissipate until the evening when conditions moderated.”
RISKS OF FIGHTING A PLUME-DOMINATED FIRE
“The unpredictability of plume-dominant fires makes them dangerous for firefighters to control by conventional means,” Musa explained.
“As a result, these fires are generally uncontrollable by ground or aerial resources.
Suppression actions and options at the head of the fire tend to be severely restricted until there’s a major decline in the fuel, weather, or topographic conditions.”
Musa said it is often hard to see the smoke plume when working close to the fire edge.
“Entrapment becomes a huge issue for firefighters if conditions suddenly change. Lookouts should monitor and report on weather conditions, any signs of outflow winds from the pyrocumulus or nearby thunderstorms, any spot fires, and any changes in the smoke column, and warn crews of risks.
“To ensure firefighter safety we use LACES: lookouts, awareness, communication, escape route and safety zone.”
Plume-dominated fires and the formation of pyrocumulonimbus clouds are difficult to predict.
“With climate change, the potential for more frequent plume-dominant fire behaviour is very real,” Musa said.
“The flammability of our vegetation, climate change, our weather patterns and the nature of our topography also means we’re prone to fires like this in Victoria.
“Researchers in fire agencies, the Bureau of Meteorology, CSIRO and universities around the world are working together to better understand the complex variables that lead to plume-dominated fires and the formation of pyrocumulonimbus clouds.
“Ultimately this research is about ensuring the safety of our firefighters and emergency services personnel, and the communities they protect.
“It’s a big challenge because while we can foresee the weather to some extent, where and when a fire will break out, and whether or not we will get on top of the fire is not something we can easily control.”