23 October 2015

 
Image: Destroyed By Burning Forestby Toa55 freedigitalimages.net.

 

As we approach summer and thoughts turn to hot, dry conditions, we take a look in this edition of ARChway at important research at an ARC Centre of Excellence that is helping us to understand how dangerous fire conditions form.

The Variability Research Programme at the ARC Centre of Excellence for Climate Systems Science has investigated the weather in Victoria responsible for some of the most destructive fires in Australia’s history.

One of the most surprising findings of this research is that atmospheric conditions far from southeastern Australia strongly influences whether or not Victoria experiences extreme fire weather. Surprisingly, two of the most important ingredients are tropical cyclones in the eastern Indian Ocean and low-pressure systems in the Atlantic.

Low-pressure systems in the southern Atlantic that form in lee of the Andes impinge on the jetsteam, producing waves. These waves in the upper atmosphere, known as Rossby waves, cause the jetstream to meander. Sometimes, as they propagate eastward from South America, the Rossby waves amplify and break, much like an ocean wave breaking close to a beach. During summer, the conditions in the upper atmosphere permit these Rossby waves to propagate towards Australia, before breaking above Victoria.

In December 2013, then ARC Centre of Excellence PhD student Tess Parker (now Research Fellow at the University of Oxford) wrote the paper, The influence of tropical cyclones on heatwaves in southeastern Australia, published in Geophysical Research Letters. This paper showed how breaking Rossby waves in the upper atmosphere were responsible for heatwaves over Victoria. The paper also showed that tropical cyclones in the eastern Indian Ocean intensified heatwaves. The reason for this is that the outflow from tropical cyclones amplified the Rossby waves and promoted breaking. This process intensified and slowed the associated high-pressure systems creating strong blocking weather patterns, producing a succession of extremely hot days.

Heatwaves generally come to an abrupt end as dry, hot northwesterlies are replaced by cool, moister southwesterlies. These transitions are commonly called cold fronts, and most catastrophic bushfires have been associated with extremely strong cold fronts. Moreover, almost all bushfire fatalities in Victoria have occurred with the passage of cold fronts.

Recently, research led by Professor Michael Reeder has shown how extremely strong cold fronts are also related to Rossby waves generated in the south Atlantic. In his paper, Rossby waves, extreme fronts and wildfires in southeastern Australia, Professor Reeder also explained how these events are generally predictable because mathematical models used in weather forecasting simulate such fronts rather well.

Researchers at the ARC Centre of Excellence for Climate Systems Science have also been investigating how the weather at very fine scales affects the behaviour of bushfires. To better understand the processes, Professor Reeder and his colleagues used an extremely high-resolution weather prediction model to simulate the condition during Black Saturday. The results are found in The meteorology of Black Saturday, published by the Royal Meteorological Society[1]

The work revealed yet another surprise: an undular bore, both detected in the observations and simulated by the model, was found to have been responsible for a dramatic and unexpected night-time reinvigoration of the Beechworth fire. 

An undular bore is a very large pressure jump followed by a series of waves, the leading edge of which is like a very large, very slow vortex turned over on its side. The importance of this discovery is that bushfires generally become less active at night and that an undular bore could have such an effect had not been considered before.

In piecing together how these and other fire weather phenomena were produced, researchers at the ARC Centre of Excellence for Climate Systems Science continue to improve our understanding and prediction of the weather that generates dangerous fire conditions.  

This research is continuing and aims to help Australians better prepare and respond to extreme fire events.

For more information about this research programme, please contact the ARC Centre of Excellence for Climate Systems Science.

This article was kindly submitted by the ARC Centre of Excellence for Climate Systems Science for publishing in ARChway.




[1] An animation of a similar simulation showing how the Kilmore East bushfires on Black Saturday were affected by the passage of an extreme cold front is available for viewing on YouTube (produced by Dr Jussi Toivannen).