Verification of wave forecast
models
Martin Holt
Jim Gunson
Damian Holmes-Bell
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Verification
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
Models are verified against in-situ observations.

Observations from moored buoys and platforms: wave
height, wind speed, wave period.

Observations from satellites: wave height, wind speed,
wave energy spectrum

Monthly performance statistics are produced and
monitored.

International collaboration to validate ERS-2 and
Envisat missions.
Buoy locations (1995!)
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Global wave model verification exchange
•Started 1995
•Met Office, ECMWF, NCEP, FNMOC, Canada, MeteoFrance
•Each centre co-locates model and moored buoy data in
agreed format and exchanges monthly by ftp.
•Datasets collated and statistics prepared
•Published results: Bidlot, Holmes, Wittmann, Lalbeharry,
Chen 2002, Weather and Forecasting 17
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Global wave model verification exchange
timeseries wave height bias December 1996 to December 2002
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Global wave model verification exchange
timeseries peak period bias December 1996 to December 2002
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Global wave model verification exchange
timeseries wind speed bias December 1996 to December 2002
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Global wave
model
verification
exchange
February 2003
Bias through 5 day forecast
(top) Hs
(middle) windspeed
(lower) peak period
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Global wave
model
verification
exchange
February 2003
SD through 5 day forecast
(top) Hs
(middle) windspeed
(lower) peak period
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Satellite radar altimeter

ERS-2 (and now Envisat)

Co-located wave height & wind speed

1 observation per second =7km

use a 9 second average (for approx 60km
interval) or 20 second average for assimilation

needs careful quality control
– buddy check
– background check
– climatology check
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Satellite
Altimeters
along-track
wave height
ERS-2
23 January 2003
Track crossing central
South Pacific
Some outliers are close to
land
Envisat
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Satellite
Altimeter
ENVISAT RA-2 Ku
Wave height
March 2003 all data
before quality
control.
Red indicates rain
flag set
black crosses show
mean value in bin
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Satellite Synthetic Aperture Radar
• 5km x 5km vignettes taken every 200km ERS-2 (every 100 km
Envisat)
• Values at 12 wavelengths(100m - 1000m)
and 12 directions in half-plane.
• Directional ambiguity
• Nonlinear transformation from wave energy spectrum from SAR
product
Processing
• Software developed at DLR (Susanne Lehner, Johannes SchulzStellenfleth) and the Met Office to perform inversion.
• Azimuthal cutoff  wave model supplies info
• Compare modelled spectra with retrieved spectra
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Comparison of modelled spectra with SAR
example 12 hours data compare Hs
Model
co-locations in
time at 30 minute
intervals
observed
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Comparison of modelled spectra with SAR
example 12 hours data compare Hs for 18
second waves
Demonstrates
need for
improved
observation
retrieval at
longer wave
periods
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Comparison of modelled spectra with SAR
example 12 hours data compare Hs for waves of
10 second period
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Comparison of modelled spectra with ERS-2 SAR
Pacific Swell
example 6.68N
122W
top left: SAR data
top right model wave
energy spectrum
bottom left: retrieved SAR
wave energy spectrum
bottom right comparison
1d spectra (black model
red SAR)
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Comparison of modelled spectra with ERS-2 SAR
Windsea example
48S 30W
top left: SAR data
top right model wave
energy spectrum
bottom left: retrieved SAR
wave energy spectrum
bottom right comparison 1d
spectra (black model red
SAR)
Hs 4.62m model 3.13m SAR
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Comparison of modelled spectra with ERS-2 SAR
Windsea example
66S 91W
top left: SAR data
top right model wave energy
spectrum
bottom left: retrieved SAR
wave energy spectrum
bottom right comparison 1d
spectra (black model red SAR)
Hs 5.85m model 6.75m SAR
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SUMMARY
•For global and regional wave models, validation is
carried out against instrumented moored buoy
observations of wave height, wave period and
windspeed.
•Validation against altimeter data requires careful
quality control when used in near-real time.
•Validation against satellite retrieved wave spectra
needs further development of the retrieval scheme
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SPARE SLIDES FOLLOW
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Observing waves with SAR
Synthetic Aperture Radar
Flown on ERS-2, archived daily
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The SAR observation
• Bragg scattering of microwaves
•Two-scale model of ocean surface:
ripples and long waves
• Doppler effect used to resolve features
Hydrodynamic modulation
Tilt modulation
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Remote Sensing of waves: SAR
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