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Giant rogue waves revealed by ESA satellites

Jul 22, 2004 14: 31 EST
Giant freakish waves, 25 to 30 meters high, are no mere myth. They come out of nowhere, across the ocean waters, and pose a real danger (and a real huge fear!) for even large ships and oil rigs. So trying to figure out how to spot one of these monsters, as tall as ten-story building, is an important undertaking for seafarers, especially the vulnerable ocean rowers.

Once accepted as a leading cause of large ship sinkings, scientists have been working on finding out more on this phenomenon. Results from the European Space Agency’s (ESA's) ERS satellites helped establish the widespread existence of these 'rogue' waves and are now being used to study their origins.

Severe weather has sunk more than 200 supertankers and container ships exceeding 200 meters in length during the last two decades. Rogue waves are believed to be the major cause in many such cases.

Frightening stories

Mariners who survived similar encounters have had remarkable stories to tell. In February 1995 the cruiser liner Queen Elizabeth II met a 29-metre high rogue wave during a hurricane in the North Atlantic that Captain Ronald Warwick described as "a great wall of water… it looked as if we were going into the White Cliffs of Dover."

And within the week between February and March 2001 two hardened tourist cruisers – the Bremen and the Caledonian Star – had their bridge windows smashed by 30-metre rogue waves in the South Atlantic, the former ship left drifting without navigation or propulsion for a period of two hours.

"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', according to Wolfgang Rosenthal - Senior Scientist with the GKSS Forschungszentrum GmbH research center, in Germany.

Offshore platforms have also been struck: on 1 January 1995 the Draupner oil rig in the North Sea was hit by a wave whose height was measured by an onboard laser device at 26 meters, with the highest waves around it reaching 12 meters.

Objective radar evidence from this and other platforms – radar data from the North Sea's Goma oilfield recorded 466 rogue wave encounters in 12 years - helped convert previously skeptical scientists, whose statistics showed such large deviations from the surrounding sea state should occur only once every 10,000 years.

Giant waves satellite hunters

"Having proved they existed, in higher numbers than anyone expected, the next step is to analyze if they can be forecasted," Rosenthal added. "MaxWave formally concluded at the end of last year although two lines of work are carrying on from it – one is to improve ship design by learning how ships are sunk, and the other is to examine more satellite data with a view to analyzing if forecasting is possible."

A new research project called WaveAtlas will use two years worth of ERS imagettes to create a worldwide atlas of rogue wave events and carry out statistical analyses. ESA's twin spacecraft ERS-1 and 2 – launched in July 1991 and April 1995 respectively – both have a Synthetic Aperture Radar (SAR) as their main instrument. While over the ocean The SAR it works in wave mode, acquiring 10 by 5 km 'imagettes' of the sea surface every 200 km.

These small imagettes are then mathematically transformed into averaged-out breakdowns of wave energy and direction, called ocean-wave spectra. ESA makes these spectra publicly available; they are useful for weather centres to improve the accuracy of their sea forecast models.

"Other features like ice floes, oil slicks and ships are also visible on them, and so there's interest in using them for additional fields of study”, Principal Investigator Susanne Lehner from Miami University said.

Rogue Waves hot areas

So far some patterns have already been found. Rogue waves are often associated with sites where ordinary waves encounter ocean currents and eddies. The strength of the current concentrates the wave energy, forming larger waves – Lehner compares it to an optical lens, concentrating energy in a small area.

This is especially true in the case of the notoriously dangerous Agulhas current off the east coast of South Africa, but rogue wave associations are also found with other currents such as the Gulf Stream in the North Atlantic, interacting with waves coming down from the Labrador Sea.

However the data show rogue waves also occur well away from currents, often occurring in the vicinity of weather fronts and lows. Sustained winds from long-lived storms exceeding 12 hours may enlarge waves moving at an optimum speed in sync with the wind – too quickly and they'd move ahead of the storm and dissipate, too slowly and they would fall behind.

"We know some of the reasons for the rogue waves, but we do not know them all," Rosenthal concluded. The WaveAtlas project is scheduled to continue until the first quarter of 2005. It will surely become a mayor tool for sailors and rowers, or those searching for the ‘Perfect Storm’.

Photo 1: This rare photo of a rogue wave was taken by first mate Philippe Lijour aboard the supertanker Esso Languedoc, during a storm off Durban in South Africa in 1980. The mast seen starboard in the photo stands 25 meters above mean sea level. The wave approached the ship from behind before breaking over the deck, but in this case caused only minor damage. The mean wave height at the time was between 5-10 meters.

Photo 2:Example of an imagette from ERS-2. Imagettes are 10 by 5 km samples of the sea surface acquired by the satellite's SAR instrument as it works in Wave Mode every 200 km. Normally they are then converted into ocean-wave spectra, but raw imagettes are useful in themselves, and have been used to carry out a global census of so-called rogue waves.

Photo 3: Giant wave detected during a global census using three weeks of raw ERS-2 SAR imagette data, carried out by the German Aerospace Center (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. Credits: DLR

Images and facts, courtesy of the European Space Agency

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