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

This presentation discusses the evaluation of SVP-BW drifters and their wind data reporting capabilities near moored buoys. The study aims to compare wind observations from the drifters to those reported by buoys, adjust parameters and algorithms used by the drifters, and improve wind speed data. The findings suggest that improvements can be made to the drifters' wind speed estimates by using an average between 2 and 6 kHz frequencies. The standard deviation of wind speed estimates between 1 and 8 kHz can be used to reject unreliable values. Further evaluations are recommended, particularly in strong winds.

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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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