When the RRS Discovery set out to sea, the crew was expecting stormy weather. Meteorologists had predicted a violent storm, and the scientists -- a team from Britain's National Oceanography Center -- wanted to observe it from up close. What they ended up experiencing went far beyond anything they could have imagined -- and could have cost them their lives.
Near the island of Rockall, 250 kilometers (155 miles) west of Scotland, enormous waves came racing toward the vessel. When they checked their measuring instruments later, the scientists discovered that the tallest of these monster waves had hit nearly 30 meters (98 feet) at wind force 9. And it didn't come alone. "We were shaken up these waves for 12 hours," said Naomi Holliday, the leader of the expedition. Entire sets of giant waves hammered the ship. After the adrenaline levels of the scientists had fallen somewhat, astonishment spread among the crew. The standard computer programs had predicted stormy weather for February 8, 2000, but not such a tempest. Even more astonishing, the giant waves had not appeared individually, but in a group. Previously waves of such size were assumed to only appeared alone.
What Holliday characterized as a "dangerous situation" has turned out to be a spate of luck. The Discovery's crew witnessed the largest waves ever measured by a scientific instrument on the open sea, according to an article the scientists have only now published in the journal Geophysical Research Letters.
Biggest waves ever measured
With a height of up to 29.1 meters (95 feet) from trough to crest, the single waves are the highest ever measured. In terms of so-called significant wave height, they established a new record, according to the scientists: 18.5 meters (61 feet). Significant wave height is the median height of a wave's upper third. It corresponds roughly to the sea swell that experienced sailors can estimate with the naked eye.
More important than the record, however, is how the waves were born. "They were not caused by very strong winds," Holliday told SPIEGEL ONLINE. The strongest phase of the storm had already been over for a day when the largest water masses hit the RRS Discovery.
The scientists think a so-called resonance effect was responsible for the monstrous waves: waves and wind travelled across the Atlantic at practically the same speed. The storm was able to pump energy into the waves efficiently for a long time, building them up to giant size. According to the article published by Holliday and her team, the rapid increase in wave height at the beginning of the event supports this hypothesis.
Trouble for sailors and shipbuilders?
The new data may spell trouble for sailors and shipbuilders, the British scientists believe. Their research results suggest that giant waves may be much more common than previously believed. "Of course we can't make general claims about all the world's seas on the basis of the specific event we observed," Holliday said. "But computer simulation can do this for us."
According to Holliday, plugging the new data into the standard formulas shows that existing computer simulations are slightly off the mark -- at least as far as the formation of giant waves is concerned. "The waves we observed were not predicted by the computer simulation," Holliday explained. That has implications for the construction of ships and oil rigs. "The safety standards are partly based on the computer simulations."
Why was the difference between simulation and reality not noticed earlier? Because of the relative scarcity of measuring buoys and ships collecting scientific data, according to Holliday: "Direct wave height measurements are extremely rare." Cargo ships tend to avoid powerful storms, and oil rigs are so few and far between they hardly ever encounter giant waves.
For this reason alone, the measurements taken by the British research expedition are "spectacular," confirmed Wolfgang Rosenthal, a marine weather expert at a Geesthacht research institute associated with Germany's GKKS ship-building society. Waves of the sort observed by Holliday's team had already been analyzed theoretically, but the only practical knowledge about them came from vague reports. The new measurements confirm the theories that have been developed. "Nothing like this has ever been documented before," Rosenthal said.
Not freak waves
The significant wave height of 18.5 meters (61 feet) is particularly interesting, according to Rosenthal. "The giant 29 meter (95 feet) waves fit well with this statistically," Rosenthal said. He explains that the giant waves observed at Rockall are not the same as the notorious "freak waves" that appear out of nowhere during relatively mild weather, destroying even large vessels. Only those waves are considered freak waves whose overall height is at least twice their significant wave height. When the significant wave height is in the region of 18.5 meters (61 feet), giant waves roughly 30 meters (98 feet) tall become possible -- as they did near Rockall in 2000, and as Holliday and her colleagues were able to find out for themselves.
But Rosenthal doubts that the new data will have a significant effect on security standards in shipbuilding. "A single case doesn't render the existing computer simulations obsolete," he said. Nonetheless, questions about the accuracy of computer simulations have been raised for some time with regard to sea swells under extreme weather conditions. Rosenthal explained that this is partly a result of the weak measurements obtained by means of satellite-based radar. "The stronger the wind gets, the weaker and harder to measure the radar signal reflected by the waves," he said.
Holliday -- whose team includes an expert for computer simulations of sea swells -- is convinced her measurements will contribute to an improvement in computer models. "The existing models strongly underestimate maximal wave heights," she said. "The people in charge of simulations are going to have to find out what they're doing wrong."