Bushfire insights uncovered with new fire-atmosphere modelling

New research is offering insights into the destructive Black Summer bushfires in 2019-20, which were at times driven by complex interactions between the fire and the atmosphere that produced extreme local fire behaviour.

Cutting edge research led by Dr Mika Peace from the Bureau of Meteorology and the Bushfire and Natural Hazards CRC has used advanced super-computer simulations that combine bushfire behaviour and meteorology to investigate why the Badja Forest (New South Wales), Green Valley Talmalmo/Corryong (NSW/Victoria), Kangaroo Island (South Australia), Stanthorpe (Queensland) and Yanchep (Western Australia) bushfires were so extraordinary and challenging to firefighters.

“The research uses two linked models to help us understand the processes driving these challenging and destructive fires – one which simulates the fire and one which simulates the weather, so by combining them we can see how both the fire and weather change in response to each other," Dr Peace said.

"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.

"As we learn and share these findings, we are able to apply our knowledge to future bushfires. Right now, we can use the findings to help fire behaviour analysts and fire meteorologists recognise the conditions that lead to extremely dangerous localised bushfire behaviour.”

Research shows that the drought and heatwave conditions experienced in the lead up to and during all five fires were a key factor in priming the landscape for extreme fire behaviour, but local weather conditions were also important when combined with the very dry vegetation.

Unusual fire activity occurred in the overnight period, when fire intensity and rate of spread is typically expected to decrease. Interactions between strong winds above the ground, topography and the fire plume circulation were key drivers accelerating 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 explained.

“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.”

Pyrocumulonimbus (pyroCb) clouds or fire generated thunderstorms were a feature of the 2019-20 fire season and the number of pyroCb clouds recorded was an Australian record for one season. However, the five fires examined were not all associated with pyroCb's, highlighting that it is not the sole weather phenomenon associated with extreme fire behaviour.

The simulations show that the fire-affected wind near a fire plume can be much stronger than the background winds and that destructive winds can occur, including extreme fire-front winds and fire generated vortices.

“For the bushfires that occurred close to the coast – Yanchep in Western Australia and on Kangaroo Island – the combination of heatwave conditions, the temperature difference between the hot land and the cooler water and local topography led to complex winds that changed the bushfire behaviour,” Dr Peace explained.

Sea breezes, the local environment, and the fire caused erratic, variable winds along active fire lines which at times stretched for several kilometres.

The bushfire simulations undertaken through this research use the Australian Community Climate and Earth System Simulator Fire (ACCESS-Fire) model and are run on the National Computing Infrastructure supercomputer in Canberra. The results show the benefits of enhanced simulation capability and supercomputer power. Due to the level of detail, data and computer power required it is currently not possible to model bushfire behaviour like this when bushfires are burning.

This research was a partnership between the Bushfire and Natural Hazards CRC and the Bureau of Meteorology and was conducted in close collaboration with fire and land management agencies in each state.

The project highlights the complexity of the fire environment and fire management and shows how a coordinated multidisciplinary approach can make effective fire behaviour predictions.

This research was part of the Bushfire and Natural Hazards CRC’s Black Summer research program, funded by the Australian Government and the CRC to investigate key issues from the 2019-20 bushfire season. The research team consisted of Dr Mika Peace, Barry Hanstrum, Dr Jesse Greenslade, Dr Dragana Zovko-Rajak, Dr Abhik Santra, Dr Jeffrey Kepert, Dr Paul Fox-Hughes, Dr Harvey Ye, Tasfia Shermin and Jeffrey Jones from the Bureau of Meteorology.

The research report, Coupled fire-atmosphere simulations of five Black Summer fires using the ACCESS-Fire model, can be accessed at www.bnhcrc.com.au/publications/black-summer-fire-modelling.