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8c-typhoon weathermen

# 8c-typhoon weathermen - SCIENCE Weathermen get to grips...

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Unformatted text preview: SCIENCE Weathermen get to grips with typhoons Mick Hamer TYPHOONS have been a major source of embarrassment for meteorologists. Take typhoon Koryn, a tropical storm that started life in the Paciﬁc Ocean on 17 June 1993. It moved west at first, and weather forecasters said it would turn north towards Japan. Instead, Koryn carried straight on, wreaking a trail of destruction across the Philippines before eventually hitting the coast of China and turning south. In future, forecasters should be able to do better, thanks to a team of researchers from Britain and Hong Kong. Modern weather forecasting relies on complicated computer models of the atmosphere. Meteorologists feed in up-to- the-minute measurements of temperature, air pressure, wind direction and the like. The model’s mathematical formulae calcu- late how these factors interact to inﬂuence the weather, and produce a forecast of how a particular weather system will evolve. Forecasting the course of typhoons, however, is extremely difﬁcult. Part of the problem, explains Colin Flood, director of central forecasting at the Britain’s Mete- orological Ofﬁce in Bracknell, Berkshire, is that there are not enough measurements to feed into the models. Many of these meas- urements are made by ships, and ships un— derstandably try to avoid typhoons. But the major problem is one of scale. The Met Office’s global weather model di- vides the Earth’s surface into loo-kilometre squares. The model looks mainly at how the squares affect one another, rather than at variations in temperature, pressure and wind direction within each one. The core of a tropical storm may be no more than 20 kilometres across, so the model cannot represent the turmoil within a typhoon. Meteorologists have tried to work round this problem by creating a sub-model cov- ering the immediate area of a typhoon. This uses a series of bogus observations of the winds and pressures around the centre of an idealised typhoon, in place of real measurements. The sub-model feeds its results into the main computer model and gives a forecast of where the typhoon’s epicentre will be up to ﬁve days ahead. The problem has been that, forecasting 24 hours ahead, these predicn'ons have actually been worse than those produced using an older method of predicting where a typhoon will go. This method, called the Cliper technique, simply predicts where a typhoon will go based on the past experi— ence of similar storms in the same region. The latest and more successful forecast- ing method owes its existence to a chance encounter in China. Two years ago Julian Hunt, chief executive of the Met Office, was in Beijing to give a talk on typhoon forecasting. After the talk Johnnie Chan, a meteorologist based at Hong Kong Poly- technic, came up to Hunt and told him that he could make the Met Office’s forecasts a lot better. “And he was right,” says Hunt. Chan’s idea was to alter the bogus obser- vations around the centre of a typhoon so that they reﬂected the direction and the speed in which the typhoon as a whole is NEW SCIENTIs r}. moving. To represent the movement of typhoon, he realised, it is necessary to all the distribution of winds around the phoon from a circular, symmetrical pa to an asymmetrical pattern. Together ’ Met Ofﬁce scientists Alan Radford Julian Heming, Chan started work on to improve the model. This has led to an enormous impro ment in forecasting the track of typhoo 1993 the average error of forecasting w the centre of a nopical cyclone would b hours later was about 200 kilometres. , the new technique the error is now closer . 100 kilometres. Over longer periods the 3 provement is even more dramatic. At the end of October 1994, the In- passed its ﬁrst test. A typhoon called Wil brewed up in the Pacific, close to wit Koryn had started, and began heading .. wards the Philippines. After a couple of . '2. it turned back on itself, then veered " towards Japan before ﬁnally turning and heading out to sea. This time the Office was able to forecast Wilda’s and turns with impressive accuracy. “—4- .T—mW - Predicting where typhoons will strike lli'TH“ 1mm l'jl‘.” ..._~...\.m ....... A mysterious monthly temperature cycle 9 i AVERAGE temperatures around the world show a regular monthly variation averaging 0-2 °C, a meteorologist in Italy has found. If you have any idea what might cause this monthly cycle, meteor- ologists would like to hear from you, because they are completely bafﬂed by the discovery. The mystery monthly variation has been uncovered by Clive Best of the European Union’s Institute for Systems Engineering and Informatics in Ispra, Italy. He analysed data gathered by the European Centre for Medium Range Weather Forecasting in Reading, covering the ﬁve-year period from 1986 to 1991. These data consist of temperature meas- urements taken at 51 200 places around the world at noon GMT each day. Superimposed on the large tempera- ture changes due to seasonal variations, Best found a regular oscillation which repeats roughly every 30 days (Geophysical Research Letters, vol 21, p 2369). The variation is strongest near the poles, and is bigger in the northern hemisphere than the southern. This, Best behaves, is because the northern hemi- sphere contains more land, which tends to cool and heat up more quickly than the sea. The cycles in the northern- southern hemispheres are precisely: of step with one another. When no regions reach their monthly tempe peak, the southern hemisphere is coldest part of the monthly cycle. The most obvious cause of monthly cycle is the Moon, which the Earth once every month. Chan the tilt of the Moon’s orbit, which over an 18-6-year cycle, have been with rainfall variation in some p a the world. But it is not clear ho»; Moon could exert a large monthly "-2 on temperature. John G ...
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