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.”