http://www.esa.int/esaCP/SEMOKQL26WD_index_0.html
Ship-sinking monster waves revealed by ESA satellites
21 July 2004
Once dismissed as a nautical myth, freakish ocean waves that rise as tall as
ten-storey apartment blocks have been accepted as a leading cause of large ship
sinkings. Results from 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 metres in length during the last two decades. Rogue waves are
believed to be the major cause in many such cases.
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.
Damage done by a rogue wave
"The incidents occurred less than a thousand kilometres apart from each other,"
said Wolfgang Rosenthal - Senior Scientist with the GKSS Forschungszentrum GmbH
research centre, located in Geesthacht in Germany - who has studied rogue waves
for years. "All the electronics were switched off on the Bremen as they drifted
parallel to the waves, and until they were turned on again the crew were
thinking it could have been their last day alive.
"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'."
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 metres, with the highest waves around it reaching 12 metres.
Giant wave in Bay of Biscay
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 sceptical scientists, whose statistics showed such
large deviations from the surrounding sea state should occur only once every
10000 years.
The fact that rogue waves actually take place relatively frequently had major
safety and economic implications, since current ships and offshore platforms
are built to withstand maximum wave heights of only 15 metres.
In December 2000 the European Union initiated a scientific project called
MaxWave to confirm the widespread occurrence of rogue waves, model how they
occur and consider their implications for ship and offshore structure design
criteria. And as part of MaxWave, data from ESA's ERS radar satellites were
first used to carry out a global rogue wave census.
ERS satellite
"Without aerial coverage from radar sensors we had no chance of finding
anything," added Rosenthal, who headed the three-year MaxWave project. "All we
had to go on was radar data collected from oil platforms. So we were interested
in using ERS from the start."
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.
The SAR works in several different modes; while over the ocean it works in wave
mode, acquiring 10 by 5 km 'imagettes' of the sea surface every 200 km.
Example of an imagette from ERS-2
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.
"The raw imagettes are not made available, but with their resolution of ten
metres we believed they contained a wealth of useful information by
themselves," said Rosenthal. "Ocean wave spectra provide mean sea state data
but imagettes depict the individual wave heights including the extremes we were
interested in.
"ESA provided us with three weeks' worth of data – around 30,000 separate
imagettes – selected around the time that the Bremen and Caledonian Star were
struck. The images were processed and automatically searched for extreme waves
at the German Aerospace Centre (DLR)."
Giant wave detected in ERS-2 imagette data
Despite the relatively brief length of time the data covered, the MaxWave team
identified more than ten individual giant waves around the globe above 25
metres in height.
"Having proved they existed, in higher numbers than anyone expected, the next
step is to analyse 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 analysing
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. The Principal Investigator is Susanne Lehner, Associate
Professor in the Division of Applied Marine Physics at the University of Miami,
who also worked on MaxWave while at DLR, with Rosental a co-investigator on the
project.
"Looking through the imagettes ends up feeling like flying, because you can
follow the sea state along the track of the satellite," Lehner said. "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.
"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."
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.
Giant wave in a wave tank
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.
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| User: "tw" |
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| Title: Re: The reason for many mysterious ship disappearances |
22 Jul 2004 08:29:25 AM |
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"TonyZ2001" <tonyz2001@aol.com> wrote in message
news:20040722064252.19503.00001997@mb-m17.aol.com...
http://www.esa.int/esaCP/SEMOKQL26WD_index_0.html
<snip>
Good lord, a non-abusive, non-hysterical and actually INTERESTING post from
Tony.
Well done lad. The largest journey begins with but a single step.
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| User: "TonyZ2001" |
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| Title: Re: The reason for many mysterious ship disappearances |
23 Jul 2004 05:06:37 AM |
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"tw"
wrote:
"TonyZ2001" <tonyz2001@aol.com> wrote in message
news:20040722064252.19503.00001997@mb-m17.aol.com...
http://www.esa.int/esaCP/SEMOKQL26WD_index_0.html
<snip>
Good lord, a non-abusive, non-hysterical >and actually INTERESTING post from
Tony.
This group is filled with such posts from me over th years, but with people
like Amy and Dani in this group the last few years, it is quite difficult to do
much of it.
They BTW never post anything of value.
Tony
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| User: "jha_amin" |
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| Title: Re: The reason for many mysterious ship disappearances |
23 Jul 2004 03:14:58 PM |
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(TonyZ2001) wrote in message news:<20040723060637.06753.00002124@mb-m13.aol.com>...
"tw"
wrote:
"TonyZ2001" < > wrote in message
news:20040722064252.19503.00001997@mb-m17.aol.com...
http://www.esa.int/esaCP/SEMOKQL26WD_index_0.html
<snip>
Good lord, a non-abusive, non-hysterical >and actually INTERESTING post from
Tony.
This group is filled with such posts from me over th years, but with people
like Amy and Dani in this group the last few years, it is quite difficult to do
much of it.
They BTW never post anything of value.
Tony
Praising yourself does little to enhance the value of your posts.
They are a little like a headache. The only thing of value is the
fact that they soon go away and you appreciate the relief of not being
annoyed by them.
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