The supercell that hit Brisbane on November 27 this year caused more than A$500 million worth of damage, produced hail up to 7.5 cm in diameter, and lashed the city with winds of more than 140 km an hour.
In the news, we hear about tornadoes or supercells, and wonder if climate change is beginning to have an impact on these events.
In fact, the evidence suggests that while there has been no increase in severe storm activity in the past, we are likely to see stronger and more frequent storms in the future.
The science of storms
Growing up in Sydney’s western suburbs, I remember the summer thunderstorms appearing in the afternoon to the west, and wondering just why we see these castles in the sky.
Thunderstorms form when moisture and warmth near the Earth’s surface is overlapped by cooler air, causing an “updraft” of rising air. The more warm and moist the air, the stronger the thunderstorm’s updraft.
Thunderstorm clouds are like the bubbles you see in a saucepan of water on the stove, where the heated water rises through cooler water above. Most of the time, a thunderstorm fills the sky for an hour, rises, rains and then disappears as if it was never there.
But in certain situations, these storms can become “severe”, producing hail in excess of 2 cm, wind gusts above 90 km per hour and sometimes tornadoes. To form, severe thunderstorms typically need some degree of changing wind speed and direction at different levels of the atmosphere — known as wind shear.
If you’ve ever looked up at the sky and seen clouds moving in different directions, that is wind shear. Wind shear organises thunderstorms, moving rain away from the updraft, and allowing the storm to grow outside the normal lifetime of a thunderstorm, becoming stronger. The strongest of these organised storms are known as supercells and produce most hail larger than 5 cm, as well as tornadoes.
Severe storms widespread in Australia
Every year, Australia sees many severe thunderstorms, but we only hear about the few that hit populated areas, as someone needs to be present to observe the effects of a storm.
In reality, severe thunderstorms are found over the entire continent, but the intersection between tropical moisture and stronger wind shear means that they are most commonly found over the east coast and interior, stretching from Rockhampton to Melbourne.
To estimate their frequency, we can use a combination of potential updraft strength and wind shear to give an idea of how many days conditions are right for severe thunderstorm development. Using this approach, we can estimate Brisbane gets around 25 favourable days, Sydney 20 and Melbourne 10 days per year.
Has 2014 been particularly stormy?
Overall, the frequency of severe storms in 2014 was about average, or even slightly below. Perhaps we are just forgetting some of those days when the storms weren’t as extreme, or those which missed populated areas.
The Brisbane supercell hailstorm of November 27 has a damage bill rising above A$500 million, but is far from unprecedented in either hail size or damage (in 1985 a similar event caused A$1.7 billion in equivalent damage).
In terms of damaging winds, estimated gusts (around 140 km per hour) were not as strong as the 1985 storm (around 185 km per hour), but similar to the storm that affected the Brisbane suburb The Gap in 2008.
If we just look at days favourable to severe thunderstorms, there is little indication that there is an increasing frequency of severe thunderstorms outside of natural variability since 1979.
Will severe storms become more common?
In a warming climate, results for Australia, the United States and Europe have shown that the the surface air becomes warmer and moisture increases, making updrafts stronger, while the wind shear available to organise storms appear to decrease.
This battle between the elements seems to end with the strength of updrafts winning, and results in more days with stronger severe thunderstorms. Over the east coast, projected increases by the end of the 21st century for Melbourne, Sydney and Brisbane range between 114% and 160% of present levels.
Are we certain though? Several factors remain unexplained in a warming climate.
If the air above the surface warms as well, then it is possible that warming the surface won’t result in as many thunderstorms, but they will be stronger.
If we don’t get as many patterns which pull the conditions favourable to thunderstorms together, then maybe the frequency won’t change or will simply shift the season.
It is important to remember that even as the climate changes, our poor knowledge of past events is insufficient to say with any degree of certainty that a severe thunderstorm is beyond what was possible before.
What this change does mean is an increasing likelihood that we will see severe thunderstorms more often, and the question remains as to whether Australia as a nation is prepared to respond.