ESA satellites home in on rogue waves
Having been dismissed for years by scientists as the stuff of legend, the existence of huge waves of over 30 metres high has now been proven with the help of the Earth remote sensing (ERS) satellites of the European Space Agency (ESA). In 2000, the European Commission launched an initiative called MaxWave, designed to confirm the widespread occurrence of so called rogue waves, model how they occur and consider their implications for the design of ships and offshore structures. That project, funded under the energy, environment and sustainable development section of the Fifth Framework Programme (FP5), was completed at the end of 2003. New initiatives are now being launched to see if it is possible to forecast rogue waves, and to analyse how ships are sunk by such waves in order to improve their design. Over the last 20 years, 'severe weather' has been blamed for sinking more than 200 super tankers and container ships of over 200 metres in length. In the space of one week in 2001, two huge cruise ships - the Bremen and the Caledonian Star - had their bridge windows smashed by 30 metre waves in the South Atlantic. Dr Wolfgang Rosenthal, who coordinated MaxWave, said: 'The incidents occurred less than a thousand kilometres apart from each other [...]. The same phenomenon could have sunk many less lucky vessels: two large ships sink every week on average, but the cause is never studied to the same detail as an air crash. It simply gets put down to 'bad weather'.' Not anymore. Radar data, such as that from the North Sea's Goma oilfield platform, which recorded 466 rogue waves in 12 years, has helped to convince previously sceptical scientists of the existence of these freak mountains of water. In order to validate these observations, the MaxWave project set out to use data from ESA's ERS satellites to carry out the first global rogue wave census. While passing over the world's oceans, the two ERS satellites collect ten by five kilometre radar 'imagettes' of the surface detail, with a resolution of ten metres. 'ESA provided us with three weeks' worth of data - around 30,000 separate imagettes - selected around the time that the Bremen and the Caledonian Star were struck. The images were processed and automatically searched for extreme waves at the German Aerospace Centre,' said Dr Rosenthal. Within that three week timeframe, the analysis of the imagettes revealed more than ten individual giant waves around the globe that were more than 25 metres in height. 'Having proved they existed, in higher number than anyone expected, the next step is to analyse if they can be forecasted,' Dr Rosenthal continued. Therefore, a new research initiative called WaveAtlas is being established to create a worldwide atlas of rogue waves (also using ESA's ERS satellites), and to carry out statistical analyses. The project will be under the coordination of Professor Susanne Lehner from the University of Miami. 'Only 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,' explained Professor Lehner. Some patterns behind the formation of rogue waves have already been observed. The phenomenon often seems to occur at sites where ordinary waves encounter ocean currents and eddies, and it is thought that the strength of the current concentrates the wave energy to form larger waves. This process has been observed in the Agulhas current off the east coast of South Africa, as well as the Gulf Stream in the North Atlantic. However, rogue waves have also been observed well away from such ocean currents, and while scientists believe that weather fronts may also play a part in their creation, many questions still remain to be answered. 'We know some of the reasons for the rogue waves, but we do not know them all,' Dr Rosenthal concluded.