Tidal current measurements using VHF radar
and ADCP in the Normand Breton Gulf:
comparison of observations and numerical model
V. Cochin, V. Mariette, A. Coat
P. Broche, G. Mercier, R. Garello, M. Lennon
"Technologie pour la surveillance de l'environnement côtier"
Sea Tech Week,
Brest, France, 19 october 2004
19, October 2004
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Véronique Cochin
PhD ACTIMAR / GET - ENST Bretagne / CNRS (Brest, France)
Vincent Mariette, Alain Coat, Marc Lennon
ACTIMAR (Brest, France)
Pierre Broche,
CNRS/LSEET Université de Toulon et du Var (Toulon, France)
Grégoire Mercier, René Garello
GET – ENST Bretagne / CNRS (Brest, France)
email: [email protected]
url: http://www.actimar.fr
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Validation of VHF COSMER radar sea surface
current measurements in strong tidal areas
VHF COSMER radar deployed during EPEL experiment
(Assessment and forecast of coastal environment, supported by
French Navy Hydrographic and Oceanographic Service, SHOM)
Acknowledgments
SHOM
Fabrice Ardhuin, Bernard Simon, Lucia Pineau-Guillou,
Ronan Le Roy, Michel Le Goff, Annick Pichon, Michel Aidonidis
Conservatoire de l’espace littoral et des rivages lacustres
Conseil Général de l’Ile et Vilaine
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Outline of presentation:
•
Description of COSMER VHF system and sea surface current
measurement principle
•
Sea surface current mapping by VHF radar measurements
•
Validation of COSMER VHF radar measurements:
– versus numerical model simulation
– versus ADCP measurements
•
Data analysis:
– Harmonic analysis
– Eulerian residual currents
– Lagrangian simulation
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
COSMER VHF radar
(Courants Océaniques de Surface MEsurés par Radar):
Developed by LSEET (P. Broche et al., 1987), University of Toulon (France),
Dual monostatic radars, operating at 45 MHz and 47.8 MHz.
Each radar :
• transmits electromagnetic waves which travel along the sea surface by
ground wave propagation and are mainly scattered back from ocean
waves of half the electromagnetic wavelength (Bragg scattering),
moving away or towards the radar site,
•
measures the Doppler spectrum caused by moving ocean waves,
•
computes the radial speed of the surface current carrying the ocean
waves.
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Principle:
2 dominant peaks in the echo Doppler spectrum,
displayed according to the phase velocity of the surface waves
1st order Bragg waves wavelength: λradar / 2
In absence of surface current:
Doppler frequency shift occurs at a known position
in the frequency spectrum,
Bragg frequency, fBragg = (g/πλradar ) 1/2
proportional to radar wavelength (COSMER radar: 0.7Hz)
In presence of underlying surface currents:
phase velocity of surface waves is modified,
additional Doppler shift ∆f
estimation of surface current radial component (Vradial)
Vradial = λradar * ∆f / 2
19, October 2004
(Barrick et al., 1977)
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
COSMER VHF radar in
operational mode
COSMER Operating frequency
45 MHz
47.8 MHz
Radar wavelength
6.66 m
6.27 m
Bragg wavelength
3.33 m
3.13 m
transmit station : 4 antennas
(total length: 12 m)
Pointe du Grouin (Cancale)
receive station: linear phased array,
8 antennas (up to 16)
(total length: 50 m for 16 antennas)
Pointe de la Varde (St Malo)
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
COSMER VHF radar in operational mode
measurement location and radar coverage
during EPEL experiment
•
Radial currents integrated over a
surface
– azimutal resolution:
14 degrees (8 antennas)
– radial resolution:
600m (up to 150m)
•
Range: 30 km
•
Surface currents integrated over
25 cm depth
•
Integration time: 9 minutes
•
Surface current vectors are
estimated combining
two radars measurements
over a 1km x 1km grid
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Sea surface currents mapping by VHF radar measurements
• 28 days continuous time series of
vector surface currents acquired in
February - March 2003
• 30 minutes sample interval
• Quasi real time surface currents
maps, displayed daily on
http://www.shom.fr
Example during spring tides
Friday, 21 March 2003,
From 00:30 am to 9:00 am
Low water: 3:55 am,
High water: 8:34 am
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Validation of COSMER VHF radar measurements
– comparisons versus numerical model simulation
• Statistics over the grid points,
• Hodograph over spring/neap tides cycles,
• Time series over spring/neap tides cycles
– comparisons versus ADCP measurements
• Hodograph over spring/neap tides cycles,
• Time series over spring/neap tides cycles
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Tidal analysis and currents mapping using VHF radar and ADCP measurements in the Normand Breton Gulf
Comparison with numerical
model TELEMAC 2D
TELEMAC 2D:
used by SHOM to forecast horizontal
and vertical tide
adjusted surface currents from
measurements
mesh of 1 km in this area
Blue points
->TELEMAC 2D grid (SHOM atlas)
Magenta points
-> VHF radar interpolation grid (1km x 1km)
31 points on both grid
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Comparison with numerical model TELEMAC 2D
Spring tides and neap tides
Mean difference
3 cm/s
Standard deviation
10 cm/s
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Radar versus TELEMAC 2D
hodograph example
(lat -1.947, lon 48.776)
Time series over a tidal
cycle (neap tide)
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
VHF COSMER radar versus TELEMAC 2D
Observed differences due to techniques of acquisition
Temporal resolution:
Integration time: Radar (9 minutes)
Time interval for data: Radar (30 minutes) / TELEMAC 2D hourly tidal currents
Currents:
Radar (sea surface): 0,25m integration depth
residual current is subtract for tidal currents comparisons
TELEMAC: integration over the water column
estimation of sea surface currents adjusted from measurements (10% mean error)
used of SHOM atlas data
(not simulated data for the same period of time as radar measurements)
Statistics:
Interpolation at tide hour for a total tide cycle (spring tide and neap tide)
fast variation and changing direction of tidal currents during the slack
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Validation of COSMER VHF radar measurements:
comparisons versus ADCP measurements
• Hodograph over spring/neap tides cycles
• Time series over spring/neap tides cycles
Red points
-> In situ instruments (EPEL experiment)
ADCP (AWAC)
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Comparison with
COSMER VHF radar and
ADCP (2m depth)
Hodograph over a tidal cycle
(spring tide)
Time series over a tidal cycle
(spring tide)
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
VHF COSMER radar versus ADCP:
Observed differences due to techniques of acquisition
Spatial resolution:
Radar: Currents integrated over a surface versus subsurface point for ADCP
Temporal resolution:
Integration time: Radar (9 minutes) / ADCP (2 minutes)
Time interval for measurements: Radar (30 minutes) / ADCP (10 minutes)
Currents measured:
Radar (sea surface):
0,25m integration depth
ADCP (2m immersion):
2m integration
measurements at 2m below surface
Blind area
Rapport d’étude n°003/93, SHOM
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Data analysis:
– Harmonic analysis on radar measurements
over 1 month period
– Eulerian residual current
– Lagrangian simulation
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Harmonic analysis :
Extraction of tidal components
M2 tidal ellipses
and tidal ellipses
(1 month period)
Tidal component
M2
S2
Major axis
(mean amplitude)
77 cm/s
45 cm/s
Orientation
(degrees)
93
93
Minor Axis
(mean amplitude)
13 cm/s
8 cm/s
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Eulerian residual circulation:
• Representative of time period measurements
(meteorological conditions)
• Shows wind influence on eulerian residual currents
in strong tidal areas
• Need large measurement period to be integrated
into numerical models
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Residual currents,
Over 1 month measurements
(total tide cycle)
Mean wind direction is from 160 degrees
Mean wind velocity: 5 m/s
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Residual currents over 3 tidal periods measurements
Mean wind direction is from 140 degrees
Mean wind direction is from 70 degrees
Mean wind velocity: 5 m/s
Mean wind velocity: 6 m/s
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Lagrangien Residual Circulation
Simulation using GNOME (General NOAA Oil Modeling Environment)
1 spill, starting at
same position but different time
During neap tide, 1 spill (red line):
48°46’N, 1°54’W,
Start : 13 March 2003, 1:30 am (HW: 1:20 am)
-> 7 hours to leave radar coverage
During spring tide, 1 spill (blue line):
48°46’N, 1°54’W,
Start : 20 March 2003, 8:30 am (HW: 8:35 am)
-> 4 hours to leave radar coverage
1 point per hour
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Lagrangien Residual Circulation
Simulation using GNOME (General NOAA Oil Modeling Environment)
2 spills, starting at
same time but different position,
during neap tide, without wind
48°47’N, 1°55’W (blue line),
Start : 10 March 2003, 11:00 am (HW: 10:50 am)
-> 6 hours to leave radar coverage
48°45’N, 1°53’W (green line),
Start : 10 March 2003, 11:00 am (HW: 10:50 am)
-> more than 24 hours
to leave radar coverage
1 point per hour
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Tidal currents measurements using VHF radar and ADCP in the Normand Breton Gulf: comparison of observations and numerical model
Conclusion
VHF COSMER:
Operational feasibility in mapping coastal ocean surface currents in
strong tidal areas
Validation of measurements, by comparisons with in situ data and
numerical model TELEMAC 2D
Applications for this new concept of measurements:
• Knowledge of the real sea surface currents in the coastal zone, over
a large area
• Tracking of surface pollution drifting
• Real time observations for marine navigation
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