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Why are cyclone paths so difficult to predict?

Kate Doyle, Tuesday March 19, 2019 - 08:17 EDT
ABC image
Oma formed from the remnants of the system which flooded north Queensland two weeks earlier. - ABC

Some cyclones seem to hurtle straight into the coast while others drunkenly stagger about, stalling, changing direction and even going back upon themselves.

The uncertainty can cause dismay to those potentially in a cyclone's path, who wait with baited breath as it decides where and if it will make landfall.

It is obvious that cyclones are destructive storms that are difficult to predict, but why do they move around so bizarrely and why are they difficult to forecast?



Why are cyclones so hard to predict?

BOM meteorologist Jackson Browne said some cyclones were well-behaved while others were not.

"Research has shown that in the Australian region, cyclones exhibit more erratic paths than cyclones in other cyclone basins," he said.

Mr Browne said small changes in the initial environment surrounding a cyclone could make large differences in its eventual path, making it harder to predict its movement because it is difficult to work out exactly what the conditions are at sea.

"Compared to land areas the oceans don't have a lot of observations," he said.

"This data is valuable to numerical computer models and makes it harder for them to accurately predict paths.

"Accuracy of model predictions decrease the longer the forecast time, so the apparent spread in where a cyclone might travel to is also large."


What controls the path of a cyclone?

Mr Browne said a tropical cyclone is like a cork in a stream as it is pushed and pulled along by the winds that surround it.

Fledgling cyclones are controlled by lower to mid-levels of the atmosphere, but mature cyclones, by virtue of being deeper and stronger systems, tend to be steered by winds over a greater depth, according to Mr Browne.



He said because cyclones are rotating storms on a rotating planet, they have an inherent and very small motion towards the poles and to the west, but they can also be influenced by low and high pressure systems as they move through.

"U-turns, wobbles, loops and near-stationary dwelling are all possible," Mr Browne said.

"Slow-moving tropical cyclones tend to exhibit these erratic behaviours, while faster-moving cyclones are under a stronger steering pattern and move around closer to a straight line.

"Very rarely, tropical cyclones can interact with other tropical cyclones and become engaged in a spiral or orbit around one another, known as the Fujiwhara Effect.

"Usually the larger circulation will win out and absorb the smaller one."



Recent example

Cyclone Oma was a classic example of an erratic path which left even the experts unsure of where it would go next.

The system first grabbed headlines when it brought torrential rain to north Queensland as part of the monsoon trough in February 2019.

It settled over central Queensland for more than a week and caused devastating floods before finally moving east and into the Coral Sea.

A few weeks later it intensified into a tropical cyclone near New Caledonia and started moving south-east towards the Queensland coast.

Initially the BOM said it would likely move south-east over New Zealand, but a few days later the chances of it crossing low on the Queensland coast near the population centres of Brisbane, the Gold and Sunshine coasts increased.

Eventually the southern Queensland coast was issued with a cyclone watch and there was coastal flooding and high winds, but Oma weakened and swung north without crossing the coast.



Earlier in the same season, Cyclone Owen barrelled across Cape York peninsula from east to west before doing a U-turn in the Gulf of Carpentaria and crossing the coast from west to east.

There was speculation and modelling that suggested the system would travel all the way down the Queensland coast, however this didn't eventuate.


- ABC

© ABC 2019

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