Cyclone Ita: Cooktown residents take shelter
Residents in the Cooktown region of Far North Queensland are taking shelter ahead of the arrival of category 5 Cyclone Ita, which is expected to bring 300km/h winds and a 1.5-metre storm surge when it makes landfall on Friday evening. Nine News.PT1M58S http://www.canberratimes.com.au/action/externalEmbeddedPlayer?id=d-36h1p 620 349 April 11, 2014
Trees will start to topple, roofs will be sucked up and solar panels might start flying like Frisbees as severe tropical Cyclone Ita makes landfall in far north Queensland this evening, wind experts say.
The Bureau of Meteorology predicts wind speeds for the Category 5 storm to approach 300km/h as it approaches the coast near Cape Flattery, north of Cooktown.
“If we do get 300km/h that is slightly stronger than Cyclone Yasi,” which ripped through north Queensland near Innisfail in 2011, said Grahame Reader, the severe weather manager at the bureau. “Physically, (Ita's) a relatively small storm but it's intense.”
Cyclone Ita now a Category 5 storm. Photo: NOAA, via Reuters
Wind speed measurements are largely drawn from satellite imagery of clouds and radars, with only a few land-based devices, known as anemometers, on hand. (Click here for an amazing global winds graphic.)
James Cook University, though, is testing mobile anemometers mounted on anchored trailers for the first time to get more accurate readings, said John Ginger, research director of the university's Cyclone Testing Station. Estimates now are largely gained after the event when bent-over street signs are examined closely for clues.
Dr Ginger said branches and other debris typically start to break off when wind speeds reach about 100km/h, and shallow-rooted trees begin to topple when sustained gusts reach about 160km/h.
The expected trajectory of Cyclone Ita.
The force of wind rises at four times the speed, so that by 200km/h houses typically start to lose guttering and other material. Houses built to pre-1980s standards also begin to be at risk, while those built since should be able to withstand speeds of 250km/h or more.
John Holmes, director of JDH Consulting and chair of the body setting wind loading standards in Australia, said cyclones lose wind speed quickly once they make landfall.
Cyclone Ita strengthened to become a category 5 storm on Thursday night. Photo: Higgins Storm Chasing
Yasi, for instance, was more like 180km/h in strength when it crossed the coastline – still strong enough to cause significant damage.
“There were some solar panels and things that took off like Frisbees,” Dr Holmes said. “It's not a good place to be wandering around outside in.”
More newly built houses have more steel connections, particularly to prevent roofs blowing off. Airborne debris, for instance, can smash windows or garage doors that weaken the building's protection.
Anemometer - measuring the wind speeds. Photo: Stuart Walmsley
“You tend to get internal pressure in the building and it combines with the roof pressure to lift the roof off completely or parts of the roof,” Dr Holmes said. “They advise you to go to the smallest room because it's the least likely to fail.”
New public cyclone structures – with half the funding coming from the Abu Dhabi emirate – are designed to withstand one-in-10,000-year winds. “If I had an older house, I'd be going to one of these new shelters,” Dr Holmes said.
Storm surge threat
Yasi's damage in 2011. Photo: James Brickwood
The bureau's Mr Reader said Cyclone Ita's risks include not only the winds but also the expected storm surge and then the potential flash flooding from heavy rain.
Kevin Walsh, an associate professor at the School of Earth Sciences at the University of Melbourne, said storm surges – expected to be as much as 1.5 metres with Ita – can be much more damaging than the winds.
“If you are in a coastal location, it's not the winds but it's the rising sea levels and the associated storm wave,” Professor Walsh said. “The density of water is 1000 times the density of air.”
Yasi in 2011. Photo: Rick Rycroft
“If you've got waves crashing on a house, it's a much more effective way of wrecking a house than high winds,” he said.
Rob Sharpe, a meteorologist with Weatherzone, said the storm surge resulting from Ita is likely to add about 1-2 metres to the tide level.
‘‘Unfortunately this may coincide with the usual high tide of 2.4 metres at 7:30pm,’’ Mr Sharpe said.
‘‘It is therefore very likely that tides will exceed the highest astronomical tide for the year. This will then combine with damaging waves, leading to coastal inundation.’’
Cyclone Yasi spurred debate among scientists over the accuracy of wind-speed measures.
Dr Holmes said the bureau relies on examining cloud images drawn from satellites, rather than anemometers. "It's a little bit archaic, going back to the 70s," he said, adding that the measure may lead the bureau to "over-call" the wind speeds.
Mr Reader, from the bureau, said one difficulty is having the devices near enough the eye of the cyclone to pick up the top speed.
Another issue is that the bureau estimates wind speeds at 10 metres above the ground. The mobile anemometers being used for the first time in Queensland are designed to a height of 3.5 metres, in line with a typical roof, said Dr Ginger.
Even if the ground-level speed comes in at less than 200 km/h, "it's still a serious storm event", he said.
The mobile devices, located for Ita at Cooktown, Mossman and Port Douglas, will not provide real-time data for forecasters but should generate accurate estimates of wind speeds within days, Dr Ginger said.
Up until now, researchers have tended to scour storm-hit areas to compare road signs for how much they have been buckled by the winds to get local estimates of wind speeds. (Click here for a technical report on Yasi's damage.)
Cyclone Yasi is the costliest cyclone to hit Australia in the 1980-2013 period, according to Munich Re, a major reinsurer. Overall losses at the time of Yasi in 2011 were $US2.8 billion, with insured losses of just over half that, the company said. There was one recorded fatality.
The next most costly one was Cyclone Larry in 2006, which left a damage bill of $US1.3 billion at the time, with insured losses about a third of that tally. There were no recorded deaths, Munich Re said.
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