Black Summer bushfire features that increased difficulties for disaster responders, such as unusual overnight intensity, have been investigated using advanced supercomputer simulations.

The research using two linked models combining bushfire behaviour and meteorology has examined why the Badja Forest, Green Valley Talmalmo/Corryong, Kangaroo Island, Stanthorpe and Yanchep bushfires were so extraordinary and challenging to firefighters.

“It's only possible to research the fire behaviour resulting from these interactions between the fire and the weather, such as extreme, local winds and rotating fire plumes through work like this,” research leader Dr Mika Peace says.

The project, involving Bureau of Meteorology researchers, is part of the Bushfire and Natural Hazards Co-operative Research Centre Black Summer program.

The bushfires were driven by interactions between the fire and the atmosphere, lead-up drought and heatwaves and local weather conditions, while strong above-ground winds, topography and the fire plume circulation accelerated surface fire spread at night.

“The conventional understanding of bushfire behaviour will tell you that fire activity will decrease overnight as the temperature drops, humidity rises and winds become lighter,” Dr Peace said.

“The modelling shows that very strong low-level winds descending to the ground behind the fire plume were a critical reason why the Badja Forest and Corryong bushfires burnt so fast overnight.”

In the case of coastal fires at Yanchep in WA and on Kangaroo Island, complex winds that changed the bushfire behaviour reflected heatwave conditions, temperature difference between the hot land and the cooler water and local topography.

Pyrocumulonimbus clouds, or fire generated thunderstorms, were a feature of the 2019-20 bushfires and the number recorded was an Australian record for one season.

The researchers say the five fires examined were not all associated with pyrocumulonimbus clouds, with other weather phenomenon also associated with extreme fire behaviour.