Monday, 9 July 2007

Twisted sisters have struck before

AN exposed region of New Zealand has been walloped by a spate of tornadoes – and it’s not the first time.
That’s why, at this stage, weather experts don’t believe the July 4 and 5 events that hit Taranaki, on the west coast of the North Island, can be linked to global warming.
Between 1951 and 1970, the region was struck by 28 twisters, says weather expert Dr Richard Turner, of the National Institute of Water and Atmospheric Research (Niwa) in Wellington.
The worst year in that period was 1956, when five whipped through Taranaki.
From 1971-90 there were a dozen twisters, which is another name for the speedily spinning elongated triangles of wind produced by thunderstorms.
We are now in another high-occurrence period, with 19 tornadoes recorded since 1991, including the region’s strongest twister and now the largest number in one hit.
According to storm experts online, a series of twisters from one thunderstorm is called “a family” of tornadoes.
About seven struck Taranaki at 5.30pm on July 5, with at least two of the family – we can called them twisted sisters – storming through homes at Oakura (see cars moved about in above picture, courtesy of Taranaki Daily News, photographer, Mark Dwyer).
There was also a lone wind the day before, on July 4, which whirled its way through central New Plymouth.
All of the tornadoes were spawned by thunderstorms rumbling over the Tasman Sea.
But there is no clear relationship between the tornadoes and a weather pattern, like El Nino, says Dr Turner, a mesoscale research meteorologist.
For now, he’s ruling out any links to global warming.
That’s backed up by National Climate Centre leader Dr David Wratt, also of Niwa.
“The conditions that lead to tornadoes are not built into climate change models,” Dr Wratt says.
But there have been significant changes in temperatures, which have been going up.
“There is more heavy rain in the west and less in the east, and that could be to do with climate change,” Dr Wratt says.
“There is some evidence there have been strengthening winds to the south of New Zealand.”
Dr Turner says the other place prone to tornadoes is Greymouth, which is on the West Coast of New Zealand’s South Island. With the Southern Alps behind, it has similar conditions to Taranaki. But it’s the Central Plateau, not Mt Taranaki, which affects the weather’s big picture.
Unlike the United States, where the prairies or flat expanses of land are the most prone to twisters, New Zealand’s tornadoes are born offshore and then head towards terra firma.
“They form out to sea because there’s a good supply of moisture for the thunderstorm,” Dr Turner says.
Thunderstorms have columns of swiftly rising air, called updrafts. Sometimes these change direction, causing them to spin. When the outstretched spinning air is contracted, it rotates even faster.
“Like a skater spinning with arms outstretched and then when they bring their arms in, they speed up,” he explains. “The updraft can rotate at a rapid rate. Then the downdrafts come and they have the spin as well.”
This updraft, which is in the middle of the storm, sucks up moisture from the sea.
“And we get heavy rain out of it, or hail, which is a sign that spinning air is coming down closer to the surface.”
It’s from angry storms like these that tornadoes take shape, or get squeezed out of.
“Often times you can get thunderstorms rapidly rotating, but no tornadoes form. I don’t know if anybody has any really good answer as to why.”
Dr Turner says nobody has forecast a tornado in New Zealand. “You can kind of predict the environment that tornadoes can occur reasonably well. The problem is, nine times out of 10 a tornado won’t develop.”
In the US, there are “tornado watches”, which tell people that thunderstorms with the power of producing twisters are coming. Once the whirling winds touch down, the watch turns into a “warning.
“One problem with Taranaki, they tend to come off the sea, which means you can’t get away.”
Also, they happen too fast for a warning to be effective.
The worst tornado to hit Taranaki occurred on August 15, 2004, near Waitara. Most members of a farming family were asleep when a twister hit their home, which was lifted off its foundations and demolished. Two people died in the natural disaster.
That event was an F3 on the Fujita Scale, which goes from 0-5, with an F6 being “off the scale”.
Last week’s tornadoes were rated between F0 and F2.
New Zealand’s most disastrous tornado struck Frankton in Hamilton on August 25, 1948. It killed three people, injured 80 and damaged 163 homes, but was only rated as an F2. “It does not have to be an F5 to cause major damage,” Dr Turner says.

No comments: