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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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