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JUST A WEATHER REPORT
Subject: MONSTER WAVES
Vessel Measures Record Ocean Swells
By Markus Becker
A British research team has observed some of the biggest sea
swells ever measured. A whole series of giant waves hammered
into their ship that were so big, according to computer models
used to set safety standards for ships and oil rigs, they shouldn't
even exist.
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.
Click here to launch the image gallery of waves (12 Photos).
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."
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