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

2. Background About 280 SVP type 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 DBCP-18 workshop – 14-15 Oct. 2002 Common name at DBCP : SVP-BW drifters

3. What’s the WOTAN technique ? WOTAN means Wind Observation Through Ambient Noise WOTAN was also the 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. Generic underwater sound spectra according to Nystuen & Selsor (1997) DBCP-18 workshop – 14-15 Oct. 2002 2 kHz8 kHz Frequency (kHz) Spectral level (dB re  Pa²/Hz) 0.1110100 20 40 60 80 100 2 5 12 20 Wind m/s hail Heavy rain Light rain, no wind Light rain, 3 m/s wind snow

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

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

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

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

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

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 : 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 U 10 = a * 10 SL(f)/20 + b where SL(f) is the sound intensity at frequency f expressed in dB relative to 1  Pa 2 /Hz a and b are two empirical coefficients depending on the frequency

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

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

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

15. Linear relationship between wind speed and P o DBCP-18 workshop – 14-15 Oct. 2002 2 kHz6 kHz Best correlation

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

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

18. Algorithm proposed to provide better wind speed estimates Wind speed estimate at 10 meters height is DBCP-18 workshop – 14-15 Oct. 2002 the average of estimates computed at 2, 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

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

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

21. Conclusions (1) Wind speed estimates presently computed from 8 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 and wind 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 DBCP-18 workshop – 14-15 Oct. 2002 The standard deviation of wind speed estimates computed between 1 and 8 kHz can be used to reject dubious values from GTS transmission.

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

24. Acknowledgements UK Met Office for deployment near K3 moored buoy DBCP-18 workshop – 14-15 Oct. 2002 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 … to