Anatomy of a firestorm

To a large extent it is a matter of chance whether a potential rain system moving over Victoria accesses a favourable moisture supply. If the tropical moisture source and the middle and high-altitude westerly airflow across southern Australia, known as Rossby Waves, provide a good link then good rains ensue. However, if either the tropical moisture source or the persisting location of the Rossby Waves is unfavourable then seasonal rainfall is low.

Even when there has been low rainfall in the preceding winter and spring it takes a special conjunction of meteorological processes to bring the extremely high temperatures, dry air and strong winds that are characteristic of the mega-fire events. A detailed investigation will be made of the meteorology leading up to and during the recent fires as part of the Royal Commission, but there are general characteristics that are already apparent from the available information. What will need to be evaluated is why these characteristics achieved such intensity.

The analysis of the meteorology of the 2009 fires will be assisted by observing systems and data that were not available during earlier disastrous bushfires. The Bureau of Meteorology's sophisticated satellite surveillance, its network of radar stations and the network of automated meteorological observing stations not only supported the excellent monitoring and early warning services but, with the accumulated climate data, will give new insights into how the extremely hazardous conditions developed.

Daily temperature and humidity over Victoria during summer follows a bi-modal pattern. When the winds are from the southern sector the temperatures are typically mild and humidity is relatively high, having origins from the oceans to the south. When the winds come from the north from the continental interior temperatures are hotter.

Wind direction alone is not sufficient to explain the extremely high temperature and low humidity experienced on Black Saturday, Ash Wednesday or Black Friday. The air spreading across the Victorian countryside had its origins in the middle atmosphere where it is naturally dry. As it descended to the surface the air warmed by compression in the denser air and brought the strong winds of the middle atmosphere with it. To a large extent, the surface temperatures that were reached are a reflection of the altitude from which the air originated.

In the weeks leading up to the major fire event the middle atmosphere airflow across southern Australia had been from the north-west, extending from the Indian Ocean to the Tasman Sea. This in itself was not unusual. It was the heavy monsoonal rains and cyclonic activity that had deluged the north of Australia that gave special character to the situation.

Widespread buoyantly ascending air associated with the clouds and rain of the tropical rain required compensating mass subsidence elsewhere. This was occurring in the north-westerly airflow, drying it out and contributing to the long dry spell over southern Australia through January.

A cooler air mass formed a southern boundary to the dry subsiding air. The boundary extended from north of Perth, crossing the south coast to the east of Esperance and extending to Tasmania. A long high-level cloud line marked the air mass boundary for much of the time in the days leading to the fire event. Radar images suggested light rain at times to the east of Esperance in the cooler air and on the morning of the fires there was rain over and offshore from western Tasmania. On the eastern side of the boundary the air was hot and dry.

On the morning of Black Saturday the air mass boundary acted as a barrier to the air spreading from the tropics and subsiding air was channelled over south-eastern Australia. The dry air from the middle atmosphere was forced downwards in the north-westerly airflow and heated by compression as the density increased near the surface. As the sun heated the ground, the stability of the airflow decreased and the air was able to mix through a great depth bringing wind strength from the middle atmosphere to the surface.

It was the combination of the special dynamics of the airflow and the heating of the surface that brought about the high temperatures, the low humidity and the strong north-westerly winds. On Black Saturday 2009 new temperature records were set over a wide area, including 46.4 degrees in Melbourne with a relative humidity of less than 6 per cent. By comparison, fire-day temperature in 1926 was 39.9 degrees; Black Friday 1939 was 45.6 degrees (the previous record); and Ash Wednesday 1983 was 43 degrees.

Compounding the impact of the hot dry air was the strong winds as air from the middle atmosphere was brought to the surface. The effect of the winds was clearly observable on the Bureau of Meteorology radar as fires developed north of Melbourne. The initially small plumes rapidly intensified and spread south-east to cross the Gippsland coast. Once the initial fires became established in the hot dry airflow the burning debris and embers were carried ahead to spot and start secondary fires, which themselves developed to become sources of new embers.

Horrendous as the fire conditions were during the day, they were compounded late in the afternoon when the air mass boundary moved inland as a cool change. All of a sudden the now extensive north-eastern boundary of the fires became a leading edge as the strong south-westerly winds of the cool change advanced. The air ahead of the change was still dry, temperatures were high, and the firefront was fanned by strong winds. Even after the cool change had passed there were remaining fires fanned by strong winds that posed a continuing danger.