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Exploring Rogue Waves from Observationsin South Indian Ocean
by
Paul C. Liu(NOAA/GLERL, U.S.A.)
and
Keith R. MacHutchon(Liebenberg & Stander International, South Africa)
What is a rogue wave?
Hmax/Hs > 2
How high can this ratio be?
Western Cape Coast Line of South Africa showing FA Platform locationWestern Cape Coast Line of South Africa showing FA Platform location
Atlantic Ocean Indian Ocean
FA Platformat 38S 22E
Wave Measurement
from a Marex Wave Recorder
Significant wave height (Hs)
Maximum wave height (Hmax)
Average zero crossing wave period (Tz)
FA PlatformWind and wave data
Wave data
Wind data
Significantwave height
Maximumwaveheights
Zerocrossingperiods
Hm/HsRatios
Hm/Hs <2
Hm/Hs >2
2< Hm/Hs < 3
3< Hm/Hs < 4
Hm/Hs > 4
Hourly ave1/3 highest
Wind Direction
Hourly maximums
Hourly averages
Hourly averages
FA PLATFORM:Wind & Wave Data Structure
Temporary Record
Time serieswaveelevations
Hourlymaximums
Hourlyaverages
PRIMARY PROCESSEDDATA BY WAVERECORDER
SECONDARY PROCESSED DATA
Wind Speeds
But that’s not the whole story!
What happen when Hmax/Hs
is much larger than 2?
Conventional Rayleigh conditions
44septillionseptillionyearsyears(10(102424))
11quadrillionquadrillionyearsyears(10(101515))
1717millionmillionyearsyears
14.514.5yearsyears
EstimatedEstimatedtimetimeneeded forneeded foroccurrence:occurrence:
1.9x101.9x103131
(19 nonillion)(19 nonillion)5.2x105.2x102121
(5.2 sextillion)(5.2 sextillion)7.9x107.9x101313
(79 trillion)(79 trillion)6.5x106.5x1077(65 million)(65 million)
N = N =
6 6 5 5 4 4 3 3HHmaxmax/H/Hs s ==
Conventional Rayleigh conditions
Comparing the results with some
laboratory experiments by
Chin Wu of University of Wisconsin
Normalize the wave heights withtheir corresponding zero-crossing
wave period, H/gT2
Concluding remarks No. 1
Agulhas Retroflection
Short and mediumperiod waves,with lengths from<200m to 500m,refracted by theAgulhas Current
Agulhas CurrentMedium and long period waves,
with lengths from 200m to 1500m, refracted by the Agulhas Bank
Composite wave fronts arising from the deep sea over a long fetch
Short waves, with lengths <200m, passing over the Agulhas bank
Schematic Diagram showing a possible mechanism for the combination of wave spectra, leading to the presence of Freak Waves, on the Agulhas Bank
Concluding remarks No. 2
Proposing of a new classification:
Typical rogue waves 2.0 < Hmax/Hs < 4.0
Uncommon rogue waves 4.0 < Hmax/Hs
Concluding remarks No. 3
With respect to
typical rogue waves,
Rayleigh distribution needs revision.
With respect to
uncommon rogue waves,
Rayleigh distribution needs overhaul!
Concluding remarks No. 4
We need more, actual, continuous
wave time series measurements
in order to substantially verify or
refute the existence of these
uncommon rogue waves!
Progress in science is sometimes madeby great discoveries. But science alsoadvances when we learn thatsomething we believed to be true isn’t.When solving a jigsaw puzzle, thesolution can sometimes be stymied bythe fact that a wrong piece has beenwedged in a key place.
-- Richard Muller
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