Reporting in the journal Weather they explain the physics behind coastal wind jets, which are rivers of fast flowing air that form close to coasts. The jets are well-known to successful yacht-racing strategists and may have been used to advantage in races. But, until now no one properly understood how these jets form.
Working with colleagues from the University of Reading, Potsdam Institute for Climate Impact Research, Germany, and the Laboratoire Des Ecoulements Geophysiques et Industriels, France, they say that by predicting when they will form, could give the upper hand in planning a race strategy.
The researchers suggest that their findings may also have significant implications for the positioning of wind turbine 'farms', which are being installed in coastal waters.
Dr Andrew Orr, of the UCL Department of Space and Climate Physics and the NERC Centre for Polar Observation and Modelling, says:
"The jets can gust up to 40 per cent higher than normal wind speeds, but are sometimes only a few kilometres wide, and consequently are often under-predicted by operational weather forecast models. Improved understanding of them may enable us to optimise wind energy along our coasts. We also hope this will lead to better prediction of flooding in high winds as they can force strong, localised storm surge in the ocean."
Wind develops because of differences in atmospheric pressure. Above the Earth's surface they always blow clockwise around areas of high pressure and anti-clockwise around low pressure regions. This is the result of the rotation of the Earth, which leads to a force known as the 'Coriolis force'. However, the direction of the wind changes in relation to the roughness of the surface it passes over. Overland drag is caused by trees, cliffs and buildings, which reduces wind speed. In the northern hemisphere the surface wind is deflected to the left of its path above the surface and in the southern hemisphere it is deflected to the right.
Professor Lord Julian Hunt, of the UCL Department of Space and Climate Physics and the NERC Centre for Polar Observation and Modelling, explains:
"Coastal meteorology is complex and of great practical importance. However, the formation of low-level jets and the associated variation of cloudiness are not well understood. When considering coastal meteorology we have shown that it is essential to consider the Coriolis force.
"As onshore winds cross the coastline they are slowed by the increased drag and elevation of the land. The Coriolis force turns the wind to the left and depending on whether the coastline is to the right or left, the winds either converge or diverge. This causes variations in cloudiness and precipitation. To maintain a balanced flow, the Coriolis force must induce a wind jet parallel to the coastline."
The team managed to reproduce the jets in a laboratory experiment and by running a very high-resolution numerical weather model centred over the Dover Straits region of the English Channel. Although jets caused by differing land and sea temperature have been previously documented, the team demonstrated for the first time that this is not always required, suggesting that they are likely to be more widespread around our coasts than was previously thought.
Dr Orr added: "The next step is to develop weather models for personal computers which can forecast these jets, and thus provide more accurate local forecasting of coastal winds. As well as helping our Olympic sailors to continue winning gold medals, this will be a valuable resource for forecasting of wind energy and flood prevention."
The study was funded by the Natural Environment Research Council.
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Notes to editors
Journal: Weather 10/10/05
Title: 'Coriolis effects on wind jets and cloudiness along coasts'
Authors: Andrew Orr (1), Julian Hunt (1), Rachel Capon (2), Joel Sommeria (3),Doug Creswell (1), Antony Owinoh (4)
(1) Centre for Polar Observation & Modelling, Dept. of Space & Climate
Physics, University College London, London, UK
(2) Met Office Joint Centre for Mesoscale Meteorology, University of Reading,
Reading, UK
(3) Laboratoire Des Ecoulements Geophysiques et Industriels/Coriolis,
Grenoble, France
(4) Potsdam Institute for Climate Impact Research, Potsdam, Germany
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Weather