Researchers identify unusual Black Saturday weather

A very high-resolution weather forecasting model has provided the first simulations of the small-scale weather patterns in Victoria on 7 February 2009, the day of the Black Saturday bushfires that caused 173 fatalities and destroyed more than 2100 residences, devastating the townships of Marysville and Kinglake.

After fire fighters reported extraordinary behaviour from the Black Saturday bushfires, researchers from the University’s School of Earth Sciences wanted to understand what weather characteristics produce these destructive conditions. 

The study was led by Dr Todd Lane and Chermelle Engel from the School of Earth Sciences and included Professor Michael Reeder from Monash University and Dr Michael Rezny from the ARC Centre of Excellence for Climate System Science.

Dr Lane says the extremely hot, dry and windy conditions created a day of exceptional fire danger that was exacerbated by a variety of localised meteorological phenomena.

“We found that weather events at a horizontal scale of about 10 kilometres introduced variability in the wind, temperature and humidity conditions. These smaller events combined to produce significant variability in fire danger across much of Victoria.”

The analysis used a very high-resolution weather forecasting model, which represented the airflow over the entire state on 400 metre weather grids, which are about ten times smaller than the grids used in operational weather forecasting. Weather conditions observed on the day were used to validate the model.

Ms Engel performed the detailed analysis as part of her position as a research fellow at the University of Melbourne, and the work forms her first paper on this subject. She is currently finishing off her PhD on weather forecast evaluation.

She says the study revealed fine scale structures in the atmosphere most likely to affect fire behaviour: the winds during the day were highly variable, and the variability increased dramatically when the cool change arrived in the late afternoon.

“It was interesting to see the behaviour of the wind as the cool change arrived. The air rose and sank rapidly ahead of and during the change, which may have affected the fire plumes. As the front moved into the mountains, the winds were channeled through the valleys creating complicated patterns of wind direction too, which in some areas caused the cool change to arrive from a different direction than expected.”

Another weather phenomena identified during the day is called a ‘horizontal convective roll’ and is similar to a streamer of wind flowing through the air.

“Similar wind patterns have been observed previously on hot days, but the convective rolls on Black Saturday were unusual as the high temperatures meant they were stronger and spaced wider apart than normal. The rolls create bands of fast and slow wind around 5-10 kilometres apart, causing coherent spatial variations in fire danger. Fire spread is related to wind speed and the rolls might have caused fires to behave differently over a relatively small distance,” Dr Lane says.

Studies like this highlight the complexities of extreme fire weather conditions in Victoria. The models can be used for training and to prepare the community for future weather forecasting technology that has already shown rapid improvements over the past decade.

“With improved computing power in the future, this kind of analysis should be available for operational forecasting on days of fire danger, providing a better guide for public warning systems and fire fighting resources,” Ms Engel says.

The work was funded by the Australian Research Council and is published online in the Quarterly Journal of the Royal Meteorological Society.

www.science.unimelb.edu.au