Predicting Rogue Waves
Researchers have used satellite data to create a map showing where massive waves are likely to appear; they hope to use it to save lives.
Scientists from the German Space Agency say they have mapped incidents of extremely large waves, known as rogue waves, using synthetic aperture radar (SAR) satellite data, and will soon publish a massive wave atlas for the first time. Such waves can mysteriously surge 100 feet (or about the height of a 12-story building) and sink massive cargo ships in their wake.
The scientific community has been slow to validate the existence of rogue waves, which are loosely defined as having an individual crest height that is more than twice the average height of surrounding waves. Besides sailors’ eyewitness accounts and discernible ship damage, evidence of abnormally large waves, which are extremely rare, has been limited to only a few rare photos, ocean oil-platform readings, and very occasional buoy data readings.
Difficulties in detecting the phenomenon are posed by the limited areas that traditional wave-height-measuring systems cover. There are a relatively small number of buoys or oil platforms collecting such data, and they are rarely deployed in remote oceans and seas where rogue waves are thought to be more likely to appear. During the very rare circumstances when high waves do surge against buoys or oil platforms, wave-height sensors are often damaged. Buoys that can withstand the impact without being destroyed are often incapable of measuring freak waves that are twice the average crest heights. Measurements of rogue waves, instead, are often indicated as mistaken readings.
Now, with two years of data from European Space Agency (ESA) satellites in tow, German Space Agency scientists say they are able to offer a rogue-wave map by taking advantage of the satellites’ global coverage. The researchers used data from two ESA satellites that orbited the earth 12 times a day and took SAR images every 200 kilometers for two years. SAR is a remote sensing radar system with which images are created by tracking how emitted radio waves bounce off the earth’s surface.
Using data from more than one million images, the German Space Agency scientists then calculated ocean surface heights with equations and models they created. The researchers pinpointed rogue waves up to 30 meters tall in the North Atlantic Ocean near Rockall (an island off the southwest coast of Greenland), in the North Pacific, in the Pacific Ocean southwest of Australia, and near the Cape Horn.
Unlike spectral data emitted by satellites, which enable only average wave sizes to be determined, the German Space Agency research group’s SAR data calculations are more precise, the researchers say. They have been able to determine individual wave heights around the world for the first time, says Susanne Lehner, who was involved in the research.
“Spectral analyses only give an average over an area,” Lehner says. “We derived the surface areas, the wave heights from top to bottom, and crest heights of individual waves.”
By using the German Space Agency’s rogue-wave atlas pinpointing where the monster waves have appeared in the past, real-time weather forecasts could, in theory, help prevent many accidents and deaths on the open ocean by indicating when and where dangerously high rogue waves might occur. Indeed, ship sinkings and ensuing deaths caused by the phenomenon are probably more numerous than officially recorded, Lehner says, given the large number of vessels that simply disappear without a trace every year.
A rogue wave, for example, likely sunk the German München cargo ship in the Atlantic in 1978. Scattered remains of the München’s wreckage indicate the force of the wave that brought it down. The Bremen, a luxury ocean liner, is thought to have encountered a 30-meter wave in the South Atlantic in 2001; it was measured by the height of the impact against the ship’s bridge. The Bremen’s electronic controls and engines were momentarily shut down after the wave hit, which caused the ship to veer sideways against incoming waves. Had the ship’s engines not been started shortly thereafter, the ship’s captains reported, the ship would have almost surely gone down.
The researchers say this map of rogue waves won’t change that much over time, which makes it all the more useful. Now that they’ve located where these huge monster waves have appeared, the researchers know where they are likely to occur again. With their real-time weather data and average-wave-height forecasts, the rogue-wave maps could help save lives. If forecasts showed that, for example, average wave heights were going to be 10 meters or more at a particular coordinate where rogue waves have been known to occur, the appropriate authorities could be alerted.
Some scientists remain cautiously skeptical, however. “Using SAR data to find wind and wave-height measurements is a good idea, and perhaps the only way you can get a global view of these parameters,” says Kristian Dysthe, a mathematician with the University of Bergen, in Norway, who was not involved in the research project. “But uncertainties remain about the [equations] used.”
The ultimate test to determine the accuracy of the German Space Agency’s individual wave maps will depend on buoys and other moorings with sensors that can check and calibrate the results. That’s why William W. Drennan, of the University of Miami, hopes to place buoys in the Southern Hemisphere off the coast of Australia, where, he says, “the waves tend to be the highest.” The moorings will be sturdy enough to withstand the impact of waves up to 30 meters or more, Drennan says.
“I don’t think [the German Space Agency’s] data is incorrect, but you can’t corroborate their measurements of individual waves with data now, because you would have really had to have been there at the right time and place,” Drennan says. “But if you are in the right place long enough, like in the southern ocean, you can do that.”
But where do rogue waves come from? Answering this question is one of the goals that the German Space Agency team shares with researchers at institutes and universities around the world. At present, several theories exist. Crossing seas and waves from different storms, currents and topographies, and what Lehner calls the “nonlinear interaction of different individual waves” are all thought to come into play.
In the meantime, the German Space Agency researchers say they will soon have more ESA satellite data available to add to their wave-atlas map. Images taken by ESA satellites from 2003 to 2007 could be available to complement their wave-measurement database within two years.