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Satellites reveal mariner’s nightmare

More than 200 supertankers and container ships exceeding 200 metres in length have been lost at sea during the past 20 years, many of them apparently to a phenomenon known as the “rogue wave”. The rogue waves are walls of speeding water that can reach 30 metres in height. Recently a European Space Agency (ESA) satellite has provided scientists with imagery that not only proves the waves exist but shows they are far more common, and widespread, than previously suspected.

Until fairly recently, statisticans believed that such waves were anomalies of the sort that would appear only once every 10,000 years and that they existed only in the nightmares of mariners. But objective radar data — including findings from the North Sea’s Goma oilfield that recorded 466 rogue wave encounters in 12 years — have established the waves as real entities on the surface of the sea.

In 2000 the ESA responded to a growing understanding that rogue waves were real by initiating a scientific project called MaxWave to confirm their existence, model how they occur and consider their implications for ship and offshore structure design criteria. In effect, the ESA conducted a worldwide “census” by satellite imagery of the waves on the oceans using Synthetic Aperture Radar (SAR).

Giant wave detected during a global census using three weeks of raw ERS-2 SAR imagette data, carried out by the German Aerospace Centre (DLR). This SAR data set was inverted to individual wave heights and investigated for individual wave height and steepness. The wave shown here has a height of 29.8 m. While studying just three week’s worth of imagery — about 30,000 images — the scientists found more than ten individual giant waves around the globe above 25 metres in height.

The ESA has said a new research project called Wave Atlas will use two years worth of ERS imagettes to create a worldwide atlas of rogue wave events and carry out statistical analyses. The imagettes also include other features like ice floes, oil slicks. Ships are also visible on them, and so there’s interest in using them for additional fields of study. Radar satellites can provide the truly global data sampling needed for statistical analysis of the oceans, because they can see through clouds and darkness, unlike their optical counterparts. In stormy weather, radar images are thus the only relevant information available. The Wave Atlas project is scheduled to continue until the first quarter of 2005.