A study led by UCL (University College London) scientists hasunravelled the physical mechanism behind the poorly understood weatherphenomenon of coastal wind jets - which are thought to have helped theBritish sailing team strike gold at the Olympics.
Reporting in the journal Weather they explain the physics behindcoastal wind jets, which are rivers of fast flowing air that form closeto coasts. The jets are well-known to successful yacht-racingstrategists and may have been used to advantage in races. But, untilnow no one properly understood how these jets form.
Working with colleagues from the University of Reading, PotsdamInstitute for Climate Impact Research, Germany, and the Laboratoire DesEcoulements Geophysiques et Industriels, France, they say that bypredicting when they will form, could give the upper hand in planning arace strategy.
The researchers suggest that their findings may also have significantimplications for the positioning of wind turbine 'farms', which arebeing 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 areoften under-predicted by operational weather forecast models. Improvedunderstanding of them may enable us to optimise wind energy along ourcoasts. We also hope this will lead to better prediction of flooding inhigh winds as they can force strong, localised storm surge in theocean."
Wind develops because of differences in atmospheric pressure.Above the Earth's surface they always blow clockwise around areas ofhigh pressure and anti-clockwise around low pressure regions. This isthe result of the rotation of the Earth, which leads to a force knownas the 'Coriolis force'. However, the direction of the wind changes inrelation to the roughness of the surface it passes over. Overland dragis caused by trees, cliffs and buildings, which reduces wind speed. Inthe northern hemisphere the surface wind is deflected to the left ofits path above the surface and in the southern hemisphere it isdeflected to the right.
Professor Lord Julian Hunt, of the UCL Department of Space andClimate Physics and the NERC Centre for Polar Observation andModelling, explains:
"Coastal meteorology is complex and of great practicalimportance. However, the formation of low-level jets and the associatedvariation of cloudiness are not well understood. When consideringcoastal meteorology we have shown that it is essential to consider theCoriolis force.
"As onshore winds cross the coastline they are slowed by the increaseddrag and elevation of the land. The Coriolis force turns the wind tothe left and depending on whether the coastline is to the right orleft, the winds either converge or diverge. This causes variations incloudiness and precipitation. To maintain a balanced flow, the Coriolisforce must induce a wind jet parallel to the coastline."
The team managed to reproduce the jets in a laboratory experiment andby running a very high-resolution numerical weather model centred overthe Dover Straits region of the English Channel. Although jets causedby differing land and sea temperature have been previously documented,the team demonstrated for the first time that this is not alwaysrequired, suggesting that they are likely to be more widespread aroundour coasts than was previously thought.
Dr Orr added: "The next step is to develop weather models forpersonal computers which can forecast these jets, and thus provide moreaccurate local forecasting of coastal winds. As well as helping ourOlympic sailors to continue winning gold medals, this will be avaluable resource for forecasting of wind energy and flood prevention."
The study was funded by the Natural Environment Research Council.
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