Exploring Rogue Waves from Observations
in 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 location
Atlantic Ocean
Indian Ocean
FA Platform
at 38S 22E
Wave Measurement
from a Marex Wave Recorder
Significant wave height (Hs)
Maximum wave height (Hmax)
Average zero crossing wave period (Tz)
Wind Speeds
Hourly maximums
Wind data
Hourly averages
Wind Direction
Temporary Record
Hourly averages
Time series
wave
elevations
Hm/Hs <2
FA Platform
Wind and wave data
Significant
wave height
Hourly ave
1/3 highest
Wave data
Maximum
wave
heights
Hourly
maximums
Zero
crossing
periods
FA PLATFORM:
Wind & Wave Data Structure
Hm/Hs
Ratios
2< Hm/Hs < 3
Hm/Hs >2
3< Hm/Hs < 4
Hm/Hs
Hourly
averages
PRIMARY PROCESSED
DATA BY WAVE
RECORDER
SECONDARY PROCESSED DATA
>4
But that’s not the whole story!
What happen when Hmax/Hs
is much larger than 2?
Conventional Rayleigh conditions
Hmax/Hs =
N=
3
4
5
6.5x107 7.9x1013 5.2x1021
(65 million) (79 trillion)
Estimated 14.5
time
years
needed for
occurrence:
17
million
years
(5.2 sextillion)
1
quadrillion
years
(1015)
6
1.9x1031
(19 nonillion)
4
septillion
years
(1024)
Conventional Rayleigh conditions
Comparing the results with some
laboratory experiments by
Chin Wu of University of Wisconsin
Normalize the wave heights with
their corresponding zero-crossing
wave period, H/gT2
Concluding remarks No. 1
Agulhas
Current
Medium 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 and medium
period waves,
with lengths from
<200m to 500m,
refracted by the
Agulhas Current
Agulhas Retroflection
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 made
by great discoveries. But science also
advances when we learn that
something we believed to be true isn’t.
When solving a jigsaw puzzle, the
solution can sometimes be stymied by
the fact that a wrong piece has been
wedged in a key place.
-- Richard Muller