Microbursts: what causes the deadly, highspeed weather events?
A severe storm on Sydney’s northern beaches on Sunday that left one woman dead and three injured, tore the roof off buildings and felled trees, was a microburst event, according to New South Wales emergency services.
On Monday, NSW State Emergency Service superintendent Stuart Fisher said: “At approximately 3.45pm yesterday, a microburst appeared out of nowhere, situated over the northern beaches, resulting in over 550 requests for assistance and 150,000 phone calls.”
What is a microburst?
A microburst is a small-scale version of a weather event known as a downburst – a powerful current of air that moves downwards to ground level and then spreads outwards in all directions.
According to the Bureau of Meteorology, downbursts typically last five to 15 minutes and have unusually high downward wind speeds. During a downburst, peak winds can exceed 200km/h.
A microburst refers to a downburst concentrated over a small area – less than 4km in diameter. Downbursts can affect wider areas than tornadoes. Those that are larger than 4km in diameter are called macrobursts.
How do they form?
Dr Linden Ashcroft, a climate scientist at the University of Melbourne, said downbursts can occur during severe thunderstorms.
“It happens when we get a lot of updraft – which is when you’ve got a lot of quickly rising air – inside a thunderstorm,” she said.
“This air is rising and rising … and it’s picking up a lot of rainfall and hail. When that updraft weakens for whatever reason, this mass of cold air can’t be held up in the cloud anymore.”
Downbursts form when hail or large raindrops fall through drier air, cooling the surrounding air as they melt or evaporate. This colder air – which is denser than warmer air – falls to the ground.
“As it falls, there’s a bit of evaporation that occurs and that allows it to cool down even more, which makes it fall even faster,” Ashcroft said.
“Then all of a sudden, you get this massive burst or this big collapse of cold air outside the bottom of a thunderstorm.
“You get this huge mass of air smashing into the surface of the Earth and then rushing out in all directions – in 360 degrees.”
According to the BoM, a downburst occurs in a similar process to “pouring a large bucket of water from height on to a path covered in autumn leaves. The leaves are spread forwards and outwards from where the water first hits the ground.”
How common are they?
Microbursts can occur during severe thunderstorms all around the world, Ashcroft said. “They can be dry, but the one we saw in Sydney was wet, so there was a lot of moisture as well that collapsed out with this cold air.”
Certain factors need to be present for them to occur, such as evaporative cooling and strong winds inside the storm cloud.
Determining whether the intensity and frequency of microbursts may change in future is difficult to predict, Ashcroft said. “We do know that severe heavy rainfall events have been identified as becoming more intense [with climate change],” she said.
“We don’t know enough about severe wind events like this to understand how they are changing as a result of climate change. We also have to factor in this year that we are experiencing a La Niña event.”
Victim remembered
A yoga instructor heading home from a neighbourhood Christmas gathering has been identified as the woman killed in Narrabeen on Sunday afternoon.
The storm, which ripped roofs off apartment buildings and took out power for 35,000 homes, struck as 68year-old Susan Cobham was heading home from a party near Narrabeen Surf Life Saving Club.
“Lightning hit a massive Norfolk pine which subsequently fell, crushing her and hitting two other women,” her son Ben Porter posted to her Facebook page. “It was an unlucky freak accident and she has left us far too early and will be sorely missed.”
Cobham, who was educated in Uruguay and previously worked as a scientist for AstraZeneca, was “a terrific person, a real bright spark and will be much missed”, friend Penny Auburn posted.
Former AstraZeneca colleague Karishma Pai said Cobham was “a great colleague and even better a friend to many of us who worked in the lab alongside her”.
Her death has been referred to the coroner, delaying funeral plans. Two women injured in the same tree fall remain in Royal North Shore hospital.
A 71-year-old woman was in a serious but stable condition on Monday afternoon while a 19-year-old woman had been upgraded to stable.