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

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

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

Generic underwater sound spectraaccording to Nystuen & Selsor (1997)

DBCP-18 workshop – 14-15 Oct. 2002

2 kHz 8 kHz

Frequency (kHz)

Sp

ec

tra

l le

ve

l (d

B

r e

Pa

²/H

z)

0.1 1 10 100

20

40

60

80

100

2

5

12

20

Windm/s

hail

Heavy rain

Light rain, no wind

Light rain, 3 m/s wind

snow

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 improve the algorithm if necessary and possible

DBCP-18 workshop – 14-15 Oct. 2002

What’s a Metocean SVP-BW drifter ?

DBCP-18 workshop – 14-15 Oct. 2002

Fixed vane

Drifter hull

Barometer port

Drogue

Hydrophone

10 metre

s below

the sea su

rface

SVP-BW drifter 32792 (WMO 44602)sound spectra on April 23th, 2002

35

40

45

50

55

60

65

70

75

80

85

0.1 1.0 10.0 100.0Frequency (kHz)

So

un

d l

evel

(d

B)

23/04/02 09:00

23/04/02 08:00

23/04/02 07:00

23/04/02 05:00

23/04/02 04:00

23/04/02 02:00

Sound spectrafrom a Metocean SVP-BW drifter

DBCP-18 workshop – 14-15 Oct. 2002

2 kHz

8 kHz

~ 4 m/s

~ 17 m/s

Trajectory of drifter 32783

DBCP-18 workshop – 14-15 Oct. 2002

28 days - but drogue lostafter 3 days

Trajectories of drifters 32791

& 327926 days

44 days

78 days

On October 3rd, 2002drifter 32792 was closeto moored buoy 44140

Comparisons for wind direction

DBCP-18 workshop – 14-15 Oct. 2002

SVP-BW drifter 32792 (WMO 44602)Comparisons with moored buoy WMO 44141

0

45

90

135

180

225

270

315

360

22/04/02 23/04/02 24/04/02 25/04/02 26/04/02 27/04/02 28/04/02

Win

d d

irec

tio

n i

n d

egre

es

SVP-BW drifter 32792

Moored buoy 44141

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

U10 = a * 10SL(f)/20 + b

where SL(f) is the sound intensity at frequency fexpressed in dB relative to 1 Pa2/Hz

a and b are two empirical coefficients depending onthe frequency

Comparisons for wind speed

DBCP-18 workshop – 14-15 Oct. 2002

SVP-BW drifter 32792 (WMO 44602)Wind speed (8 kHz) according to Weather Class (Nystuen & Selsor 1997)

Comparisons with moored buoy WMO 44141

0

5

10

15

20

25

30

35

22/04/02 23/04/02 24/04/02 25/04/02 26/04/02 27/04/02 28/04/02

m/s

Moored buoy 44141

SVP-BW drifter 32792 (8 kHz) with :

WC=1 - Wind only, low sea states (2-10 m/s)

WC=2 - Wind only, high sea states (over 10 m/s)

WC=5 - Shipping or other contaminating acoustic noise present

Wrong valuesdeclared as good

Good valuesdeclared as wrong

Relationship between wind speed and sound level at 2 kHz

DBCP-18 workshop – 14-15 Oct. 2002

Wind speed observed on moored buoysversus sound level measured at 2 kHz by the drifters

0

5

10

15

20

25

0 1000 2000 3000 4000 5000 6000 7000

Po = 10^(SL/20)

Win

d s

pee

d (

m/s

)

32783-62108 (drogue lost after 3 days)

32791-44137 (6 days of comparisons)

32792-44141 (44 days of comparisons)

32792-44138 (78 days of comparisons)

17.5 m/s Slope change

Wind speed observed on moored buoysversus sound level measured at 4 kHz by the drifters

0

5

10

15

20

25

0 500 1000 1500 2000 2500 3000 3500 4000

Po = 10^(SL/20)

Win

d s

pee

d (

m/s

)

32783-62108 (drogue lost after 3 days)

32791-44137 (6 days of comparisons)

32792-44141 (44 days of comparisons)

32792-44138 (78 days of comparisons)

Relationship between wind speed and sound level at 4 kHz

DBCP-18 workshop – 14-15 Oct. 2002

17.5 m/s Slope changefor all frequencies

Linear relationship betweenwind speed and Po

DBCP-18 workshop – 14-15 Oct. 2002

Coefficients of determination (r2) between wind speed observations on moored buoys

and 10^(SL/20) measured by drifters

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 1 10 100Frequency in kHz

Co

effi

cien

t o

f d

eter

min

atio

n

32783-62108 (drogue lost after 3 days)

32791-44137 (6 days of comparisons)

32792-44141 (44 days of comparisons)

32792-44138 (78 days of comparisons)

2 kHz 6 kHz

Bestcorrelation

Wind speed vs frequency and Po

(adjustment)

DBCP-18 workshop – 14-15 Oct. 2002

4kHz

Shapes of sound spectra if they were generated by wind only

40

45

50

55

60

65

70

75

80

85

90

0 1 10 100Frequency in kHz

So

un

d le

ve

l in

dB

35 m/s

26 m/s

21 m/s

11 m/s

17 m/s

7 m/s

4.5 m/s

3 m/s

2 m/s

Sound spectra (adjustment)

DBCP-18 workshop – 14-15 Oct. 2002

2 kHz

6 kHz

Bestcorrelation

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

Proposed wind speed computation

DBCP-18 workshop – 14-15 Oct. 2002

SVP-BW drifter 32792 (WMO 44602)Wind speed (average between 2 and 6 kHz)

Comparisons with moored buoy WMO 44141

0

5

10

15

20

25

30

35

22/04/02 23/04/02 24/04/02 25/04/02 26/04/02 27/04/02 28/04/02

m/s

Moored buoy

Drifter (accepted)

Drifter (flagged)

Standard deviation greater than 2.5 m/s

Present wind speed computation (reminder)

DBCP-18 workshop – 14-15 Oct. 2002

SVP-BW drifter 32792 (WMO 44602)Wind speed (8 kHz) according to Weather Class (Nystuen & Selsor 1997)

Comparisons with moored buoy WMO 44141

0

5

10

15

20

25

30

35

22/04/02 23/04/02 24/04/02 25/04/02 26/04/02 27/04/02 28/04/02

m/s

Moored buoy 44141

SVP-BW drifter 32792 (8 kHz) with :

WC=1 - Wind only, low sea states (2-10 m/s)

WC=2 - Wind only, high sea states (over 10 m/s)

WC=5 - Shipping or other contaminating acoustic noise present

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 andwind speed is obtained between 2 and 6 kHz

DBCP-18 workshop – 14-15 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 – 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.

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

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

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