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Evaluation of SVP-BW drifters thanks to deployments near moored buoys

Evaluation of SVP-BW drifters thanks to deployments near moored buoys. By Pierre Blouch. Presentation : Jean Rolland. DBCP-18 workshop - Martinique 14-15 October 2002. Background. About 280 SVP type drifters reporting wind data have been deployed since 1997 :.

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Evaluation of SVP-BW drifters thanks to deployments near moored buoys

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  1. Evaluation of SVP-BW drifters thanks to deployments near moored buoys By Pierre Blouch Presentation : Jean Rolland DBCP-18 workshop - Martinique 14-15 October 2002

  2. Background • About 280 SVPtype drifters reporting wind data have been deployed since 1997 : - 165 from Metocean (WOCE WSD/BP) - 115 from Pacific Gyre (Minimet) • All of them are using the WOTAN technique to estimate the wind speed • Common name at DBCP : SVP-BW drifters DBCP-18 workshop – 14-15 Oct. 2002

  3. What’s the WOTAN technique ? • WOTAN meansWind Observation Through Ambient Noise • WOTAN was alsothe Sun God in the Germanic Mythology • The WOTAN technique uses the relationship between : - the energy of the submarine ambient noise - and the wind speed blowing above the sea surface DBCP-18 workshop – 14-15 Oct. 2002

  4. hail Heavy rain Light rain, no wind Light rain, 3 m/s wind snow 2 kHz 8 kHz Generic underwater sound spectraaccording to Nystuen & Selsor (1997) 100 20 80 12 Windm/s 5 60 Spectral level (dB re 1mPa²/Hz) 2 40 20 0.1 1 10 100 Frequency (kHz) DBCP-18 workshop – 14-15 Oct. 2002

  5. Aims of the present study • To compare wind observationsprovided by Metocean SVP-BW drifterswith those reported by moored buoys • To check the parameters and the algorithmused by Metocean SVP-BW drifters to provide wind speed data • To adjust these parameters and to improvethe algorithm if necessary and possible DBCP-18 workshop – 14-15 Oct. 2002

  6. Fixed vane Barometer port Hydrophone 10 metres belowthe sea surface Drifter hull What’s a Metocean SVP-BW drifter ? Drogue DBCP-18 workshop – 14-15 Oct. 2002

  7. ~ 17 m/s 8kHz ~ 4 m/s 2 kHz Sound spectrafrom a Metocean SVP-BW drifter DBCP-18 workshop – 14-15 Oct. 2002

  8. 28 days - but drogue lostafter 3 days Trajectory of drifter 32783 DBCP-18 workshop – 14-15 Oct. 2002

  9. On October 3rd, 2002drifter 32792 was closeto moored buoy 44140 78 days 6 days 44 days Trajectories of drifters 32791 & 32792

  10. Comparisons for wind direction DBCP-18 workshop – 14-15 Oct. 2002

  11. Algorithm used to provide wind speed estimates • Wind speed estimate at 10 meters height : U10 = a * 10SL(f)/20 + b where SL(f) is the sound intensity at frequency f expressed in dB relative to 1 mPa2/Hz a and b are two empirical coefficients depending on the frequency • Metocean SVP-BW drifters compute wind speed estimates for 2 and 8 kHz. Coefficients for 2 kHz are presently wrong. DBCP-18 workshop – 14-15 Oct. 2002

  12. Wrong valuesdeclared as good Good valuesdeclared as wrong Comparisons for wind speed DBCP-18 workshop – 14-15 Oct. 2002

  13. 17.5 m/s Relationship between wind speed and sound level at 2 kHz Slope change DBCP-18 workshop – 14-15 Oct. 2002

  14. 17.5 m/s Relationship between wind speed and sound level at 4 kHz Slope changefor all frequencies DBCP-18 workshop – 14-15 Oct. 2002

  15. Bestcorrelation 2 kHz 6 kHz Linear relationship betweenwind speed and Po DBCP-18 workshop – 14-15 Oct. 2002

  16. 4kHz Wind speed vs frequency and Po(adjustment) DBCP-18 workshop – 14-15 Oct. 2002

  17. 6 kHz Bestcorrelation 2 kHz Sound spectra (adjustment) DBCP-18 workshop – 14-15 Oct. 2002

  18. Algorithm proposed to provide better wind speed estimates • Wind speed estimate at 10 meters height is the average of estimates computed at2, 3.15, 4, 5 and 6 kHz • Wind speed estimate is flagged if the standard deviation of estimates computed between 1 and 8 kHz is higher than 2.5 m/s DBCP-18 workshop – 14-15 Oct. 2002

  19. Proposed wind speed computation Standard deviation greater than 2.5 m/s DBCP-18 workshop – 14-15 Oct. 2002

  20. Present wind speed computation (reminder) DBCP-18 workshop – 14-15 Oct. 2002

  21. Conclusions (1) • Wind speed estimates presently computed from8 kHz are inaccurate. This frequency isn’t the most suitable for the searched goal. • Calibration values presently used for 2 kHz are incorrect • The best correlation between sound energy andwind speed is obtained between 2 and 6 kHz DBCP-18 workshop – 14-15 Oct. 2002

  22. Conclusions (2) • A slope change occurs at about 17.5 m/s in the relationship between sound energy and wind speed. This could be due to breaking waves. • The quality of wind speed estimates is significantly improved by using an average between 2 and 6 kHz • The standard deviation of wind speed estimates computed between1 and 8 kHz can be used to reject dubious values from GTS transmission. DBCP-18 workshop – 14-15 Oct. 2002

  23. Conclusions (3) • Calibration values found here are valid for present Metocean WOTAN drifters only • Software modifications will be implementedon our next Metocean SVP-BW drifters • Evaluationshould continue, mainly on strong winds (i.e. higher than 20 m/s) thanks to drifters providing sound spectra DBCP-18 workshop – 14-15 Oct. 2002

  24. Acknowledgements to • UK Met Office for deployment near K3 moored buoy • Environment Canada for the two other deployments • Metocean for the software modifications inside the drifters • Any body who would help us to continue this evaluation in rough seas by providing sound spectra from some drifters deployed near moored buoys • … DBCP-18 workshop – 14-15 Oct. 2002

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