Diapositiva 1

1
Performance of a Wide Swath Altimeter to control
a model of North Sea dynamics
Baptiste MOURRE ICM Barcelona (Spain) Pierre
DE MEY Matthieu LE HENAFF Yves MENARD Christian
LE PROVOST Florent LYARD LEGOS Toulouse
(France) Pierre-Yves LE TRAON IFREMER Brest
(France)
Joint OS SWH meeting in support of Wide-Swath
Altimetry Measurements Washington D.C. October
30th, 2006
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Objective
Perform Observing-Systems Simulation
Experiments (OSSEs) to estimate the contribution
of a Wide Swath Altimeter to monitor North Sea
dynamics.
Particular case of Coastal ocean/continental
shelf region.
Integrated approach optimal combination of the
observations with a numerical ocean model thanks
to data assimilation techniques.
3
Model and study area

• Modelling area the European shelf focus on
  the North Sea

MOG2D model

• (2D Gravity Wave Model)
• Barotropic
• Finite-element
• Nonlinear
• 7 31 Dec 1998

Model and data assimilation approach

• Simulated processes ocean response to
  meteorological forcing (wind and pressure). No
  tides included.
• high-frequency gravity waves
  (Kelvin-type,
  characteristic scales 1d / 100 km / 50 cm)

4
Assimilation of observations the ensemble Kalman
filter
(Evensen 2003)
Proper representation of model error covariances
tricky issue when approaching coastal areas !
Use of ensemble methods model error statistics
are empirically computed from an ensemble of
possible states of the ocean. Ensemble
variances approximate model error variances.
Model and data assimilation approach
5
Sea level ensemble variances
Saptio-temporal scales of model error
100 km
Mean variances over the study period (cm2).
6
Contribution of a Wide Swath Altimeter

• 200-km swath
• 15-km resolution
• Jason 10-day orbit

Wide Swath Altimeter performance

• Observation error
• White noise, from 3,9 to 5,3 cm rms, depending on
  the distance to nadir.
• (Uncorrelated observation error first)

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Instantaneous reduction of sea level ensemble
variances
Jason
One example track on 31 December 1998, 0am.
5
0
Wide Swath Altimeter performance
Sea level ensemble variances after analysis (cm2)
ANALYSIS
5
0
Sea level ensemble variances before analysis
(cm2) 12/31/1998 0am
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Local reduction of sea level ensemble variances
Sea level ensemble variances (cm2)
Jason
Reduction by the assimilation
- 29.7
Wide Swath Altimeter performance
Wide Swath Atlimeter
- 52.6
Days (December 1998)
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Local reduction of sea level ensemble variances
Sea level ensemble variances (cm2)
Jason
- 24.3
Wide Swath Altimeter performance
Wide Swath Atlimeter
- 39.7
Days (December 1998)
10
Global space-time reduction of ensemble variances
Jason
Jason T/P
Wide Swath (10-day orbit)
Wide Swath Altimeter performance
Sea level ensemble variance reduction ()
11
Global space-time reduction of ensemble variances
Jason
Jason T/P
Wide Swath (10-day orbit)
Wide Swath Altimeter performance
Sea level ensemble variance reduction () Zonal
velocity ensemble variance reduction ()
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Global space-time reduction of ensemble variances
Jason
Jason T/P
Wide Swath (10-day orbit)
Wide Swath Altimeter performance
Nadir (3-day o.)
Wide Swath (3d)
Nadir (17-day o.)
Wide Swath (17d)
Sea level ensemble variance reduction () Zonal
velocity ensemble variance reduction ()
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Global space-time reduction of ensemble variances
Jason
Jason T/P
Wide Swath (10-day orbit)
Wide Swath Altimeter performance
2 Jason
3 Jason
Temporally interleaved
Sea level ensemble variance reduction () Zonal
velocity ensemble variance reduction ()
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Summary of Wide Swath performance (with
uncorrelated obs. error)

• Significant contribution compared to
  conventional altimeters.
• Performance score 70 for sea level
  correction
• 130 for velocity correction
• 1 Wide Swath Altimeter 2 nadir alt. for sea
  level control
• 3 nadir alt. for velocity control
• Here (particular context of high-frequency
  oceanic processes), temporal resolution is still
  lacking to control the main part of model error.
• Interesting complementarity with tide gauges.

Wide Swath Altimeter performance
15
Impact of the along-track correlated roll error

• New study with simplified measurement model

Wide Swath Altimeter performance
1 nadir 1 slope measurement
White noise 3,9 cm rms
White noise 4 cm rms
along-track correlated roll error f ?
0.02 Hz ? L ? 350 km amplitude 11.6
cm (Enjolras, 2006)
Observation error
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Global space-time reduction of ensemble variances
Nadir alone (no roll error)
Nadir slope (no roll error)
Wide Swath Altimeter performance
Sea level ensemble variances reduction () Zonal
velocity ensemble variances reduction ()
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Global space-time reduction of ensemble variances
Nadir alone (no roll error)
Nadir slope (no roll error)
Wide Swath Altimeter performance
Nadir slope (roll error - in obs.
analysis)
Sea level ensemble variances reduction () Zonal
velocity ensemble variances reduction ()
18
Global space-time reduction of ensemble variances
Nadir alone (no roll error)
Nadir slope (no roll error)
Wide Swath Altimeter performance
Nadir slope (roll error - in obs.
analysis)
Nadir slope (roll error -
correlations ignored in the analysis)
Sea level ensemble variances reduction () Zonal
velocity ensemble variances reduction ()
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Conclusions

• Without roll error
• 1 Wide Swath Altimeter 2 nadir alt. for sea
  level control
• 3 nadir alt. for velocity control
• With roll error
• - Slight degradation of the performance, but
  contribution still valuable (if correlations
  represented in the analysis !).
• - True even if the roll frequency is not
  precisely known (not shown here).

20
Thank you
Results in Mourre et al., Ocean Dynamics,
2006 Le Hénaff et al., in preparation, 2006