Rise and rotate

Last summer’s huge and widespread Australian bushfires were monumental on just about every scale.

By the time most were out or contained in early March, more than 180,000 square kilometres had been burnt, more than 2700 homes destroyed and at least 34 lives lost. It’s thought that up to a billion wild animals may have perished.

But it’s only recently come to light – in a paper published in Geophysical Research Letters – that the fires’ remarkable residue reached equally monumental heights in size and behaviour.

The intense fires in southern NSW and eastern Victoria in late December and early January spawned at least 18 huge pyrocumulonimbus (pyroCb) clouds.

The smoke plumes from these produced “several previously undocumented phenomena in the stratosphere”, writes the research team of meteorologists and atmospheric scientists at the US Naval Research Laboratory in Washington, DC.

The pyroCb clouds spewed 300,000–900,000 tonnes of smoke into the stratosphere, which is believed to be more than from any previous fire event. The smoke plumes rose to altitudes of more than 16 kilometres.

One plume – about 1000 kilometres wide (roughly the straight-line distance between Canberra and Adelaide) and five kilometres thick – containing very concentrated smoke rose moved eastward from Australia to South America by late January, then reversed course and, moving westward, completely circled the globe over the next few weeks.

During its travels this extraordinary cloud rose to a record altitude of more than 30 kilometres, while generating rotating winds. Some of the smaller smoke plumes exhibited similar rising-and-rotating characteristics.

The researchers believe this is the first example “of smoke causing changes to winds in the stratosphere”, which they claim “opens up a whole new vein of scientific research”.

It’s thought that the big smoke cloud may be one of the largest, if not the largest, wildfire smoke plumes in the stratosphere that’s been visible to satellites.

Researchers monitored the plume with satellites and weather balloons. Its colour – set by dark particles in the smoke – helped it to heat by absorbing sunlight, which was a factor in its tremendous altitude gain.

The 15-metres-per-second winds that swirled around it may have helped keep it from mixing with surrounding cleaner air.

The research team isn’t sure what caused the rotating winds, and other matters still being considered include the smoke plume’s impact on stratospheric chemistry.