1 verification of wave forecast models martin holt jim gunson damian holmes-bell

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Verification of wave forecast models

Martin HoltJim Gunson

Damian Holmes-Bell

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Verification

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.

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Buoy locations (1995!)

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•Started 1995

•Met Office, ECMWF, NCEP, FNMOC, Canada, Meteo-France

•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

Global wave model verification exchange

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Global wave model verification exchangetimeseries wave height bias December 1996 to December 2002

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Global wave model verification exchangetimeseries peak period bias December 1996 to December 2002

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Global wave model verification exchangetimeseries wind speed bias December 1996 to December 2002

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Global wave model verification exchangeFebruary 2003

Bias through 5 day forecast

(top) Hs

(middle) windspeed

(lower) peak period

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Global wave model verification exchangeFebruary 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 Altimetersalong-track wave height

ERS-2

Envisat

23 January 2003

Track crossing central South Pacific

Some outliers are close to land

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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

• Software developed at DLR (Susanne Lehner, Johannes Schulz-Stellenfleth) and the Met Office to perform inversion.

• Azimuthal cutoff wave model supplies info

• Compare modelled spectra with retrieved spectra

Processing

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Comparison of modelled spectra with SARexample 12 hours data compare Hs

observed

Model

co-locations in time at 30 minute intervals

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Comparison of modelled spectra with SARexample 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 SARexample 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 122Wtop 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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Windsea example 48S 30Wtop left: SAR data




Hs 4.62m model 3.13m SAR

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Windsea example 66S 91Wtop left: SAR data




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

Hydrodynamic modulation

Tilt modulation

• Bragg scattering of microwaves•Two-scale model of ocean surface: ripples and long waves• Doppler effect used to resolve features

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Remote Sensing of waves: SAR

1 verification of wave forecast models martin holt jim gunson damian holmes-bell

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