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

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

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 – Oct. 2002

Generic underwater sound spectra according to Nystuen & Selsor (1997) DBCP-18 workshop – Oct kHz8 kHz Frequency (kHz) Spectral level (dB re  Pa²/Hz) Wind m/s hail Heavy rain Light rain, no wind Light rain, 3 m/s wind snow

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 – Oct. 2002

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

Sound spectra from a Metocean SVP-BW drifter DBCP-18 workshop – Oct kHz 8 kHz ~ 4 m/s ~ 17 m/s

Trajectory of drifter DBCP-18 workshop – Oct days - but drogue lost after 3 days

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

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

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 – Oct 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

Comparisons for wind speed DBCP-18 workshop – Oct Wrong values declared as good Good values declared as wrong

Relationship between wind speed and sound level at 2 kHz DBCP-18 workshop – Oct m/sSlope change

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

Linear relationship between wind speed and P o DBCP-18 workshop – Oct kHz6 kHz Best correlation

Wind speed vs frequency and P o (adjustment) DBCP-18 workshop – Oct kHz

Sound spectra (adjustment) DBCP-18 workshop – Oct kHz 6 kHz Best correlation

Algorithm proposed to provide better wind speed estimates Wind speed estimate at 10 meters height is DBCP-18 workshop – Oct 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

Proposed wind speed computation DBCP-18 workshop – Oct Standard deviation greater than 2.5 m/s

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

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 – Oct. 2002

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 – Oct The standard deviation of wind speed estimates computed between 1 and 8 kHz can be used to reject dubious values from GTS transmission.

Conclusions (3) Calibration values found here are valid for present Metocean WOTAN drifters only DBCP-18 workshop – Oct 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

Acknowledgements UK Met Office for deployment near K3 moored buoy DBCP-18 workshop – Oct 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