Martin Holt and Jian-Guo Li Met Office, FitzRoy Road, Exeter EX1 3PB, United Kingdom

1. USE OF ALTIMETER AND SAR WAVE DATA AT THE MET OFFICE AND RECENT COMPARISON WITH WAVE MODEL AND BUOYS Martin Holt and Jian-Guo LiMet Office, FitzRoy Road, Exeter EX1 3PB, United Kingdom Email: Jian-Guo.Li@metoffice.gov.uk

2. Application of satellite wave data in Met Office • 1980s: Development of SEASAT satellite data assimilation in wave models. • 1993: Assimilation of ERS-1 SWH and Wind data in operational wave model. • 1996: Switch to ERS-2 data assimilation. • 2001:Turned off DA when quality of ERS-2 wind data became degraded. • 2003: Use of Envisat altimeter RA2 and ASAR data for wave model validation.

3. The Met Office Global Wave Model Met Office Global wave models hindcast twice a day with grid spacing ~ 60km North Atlantic European model ~11km Resolution 13 Frequency 16 direction Test Hires 26 Frequency 24 direction Windsea fitted to a member of the JONSWAP spectral family

4. Quantities used for comparison • 2-D energy spectrum; • 1-D spectrum; • 4-bin SWHs; • SWH.

5. Observations used for comparison ESA Envisat ASAR Level 2 products: ASA_WVW_2P* ESA Envisat RA2 data (SWH): RA2_WWV_2P* Buoy (ID 51028) near Christmas Island (CI 0.02 N 153.87 W) and buoy (ID 42001) in Gulf of Mexico (GM 25.84 N 89.66 W)

6. Along track RA2 SWH __ Land effect

7. Comparison of SWHs ___ Buoy CI ASAR SWH close to CI (<6ºlat/lon) July 04 – Dec 05 Buoy CI SWH July 04- Nov 05 RA2 Wind speed~CI (<6ºlat/lon)July 04- Dec 05 RA2 SWH close to CI (<6 ºlat/lon) July 04- Dec 05 Buoy CI bias, Altimeter good, ASAR large deviation and poor correlation

8. Comparison of SWHs ___ Buoy GM ASAR SWH ~GM(<6deg)July 04- Dec 05 Buoy GM SWH July 04 - Nov 05 RA2 SWH ~GM (<6°) July 04- Dec 05 RA2 Wind speed ~GM July 04- Dec 05 Buoy GM, Altimeter, Wind good, ASAR scarce, large deviation, poor correlation

9. Comparison of model SWHs with global b) ASAR SWHs in March 2005 ASAR/mod mean 2.87/2.86 m ASAR-mod rms 2.27 m Correlation=0.20 • RA2 SWHs in March 2005 Ra2/mod mean 2.73/2.77 m Ra2-mod rms 0.77 m Correlation=0.82 Altimeter global good, land effects; ASAR large deviation, poor correlation

10. ASAR 2-D spectra and Buoy CI 1-D spectra Model ASAR Buoy CI 0.02 N 153.87 W ASAR close to buoy, Model swell larger than buoy, wind sea fine

11. ASAR 2-D spectra and Buoy GM 1-D spectra Model ASAR Buoy GM 25.84 N 89.66 W ASAR land effect, erroneous swell, GM buoy low wind, no swell

12. Validation of NAE wave model with RA2

13. Along track RA2 and NAE model SWH

14. Summary and conclusions • Satellite ocean wave data have been used in Met Office for over 20 years, especially ERS1-2 and Envisat. • Three independent wave observations (RA2, ASAR and Buoys) are compared via a global wave model. • Total SWHs from all observations and the model are in good agreement. • ASAR spectra seem over-estimate long waves (T>15 s). • Land effects appear in both Altimeter and ASAR wave data, more significant in ASAR.

15. Comparison of along track SWHs ASAR swh large fluctuations, land effects Altimeter close to model, 9-pts average Wind speed good agreement

16. Satellite data QC filters • RA2 Altimeter data • 0 < SWHs < 20 m • StdDev < 20.0 m • Rain_flag=0 • Model grid average ~9 RA2 points • ASAR Level 2 data • quality_flag == 0 • confidence_swell == 0 • land_flag == 0 • 1.0 <= image_variance <= 1.6 • 3.0 <= wind_speed <= 19.0 • 2.0 <= signal_to_noise <= 200.0