contribution of wide-swath altimetry missions to the ocean...

mercator-ocean.eu/marine.copernicus.eu

Contribution of wide-swath altimetry missions to the

ocean analysis and forecasting system in the global

model

Mounir Benkiran and Mercator Ocean Team

http://www.mercator-ocean.fr

[email protected]

Introduction and Objectives

Introduction:

➢ Satellite altimetry measurements based on nadir-looking radar altimeter, have been a fundamental contribution to the

understanding of the ocean during the past two decades.

➢ Along-track measurements provide a representation of the ocean mesoscale dynamics which is strongly limited by the

spatial and temporal sampling of a given altimeter mission.

➢ The forthcoming altimeter missions based on radar interferometry to obtain wide-swath measurement of sea-surface

elevation could address the shortcomings of the conventional altimetry, extending the capability of existing nadir

altimeters to two-dimensional mapping at much higher resolution (up to wavelengths of 20 km).

➢ In the future surface topography measurement could be based on both a nadir and wide swath altimeter missions. In this

sense, a major challenge will be to combine wide-swath and the conventional along-track altimeters data with

high resolution

Objectives:

➢ Investigate the contribution of wide-swath altimeters into global high resolution OGCMs (1/12°).

➢ Sensitivity of the system to wide-swath altimeters instrumental error.

Objectives:

➢ Investigate the contribution of wide-swath altimeters into global high resolution OGCMs (1/12°).

➢ Sensitivity of the system to wide-swath altimeters instrumental error.

NatRun:

• ORCA12 extended to the south but without the ice shelves, 75

vertical levels

• ERAinterim reanalysis with correction towards the satellite data:

• Wind, Tair, Hum : 3h

• Precipitation, long wave and short wave: 24h

• Atmospheric pressure forcing : 3h

• Bulk formulation IFS

• Absolute winds

• Variable volume (vvl)

Observing System Simulation Experiments(OSSEs) global model (1/12°)

EKE (2015) OSSEs:

• ORCA12 not extended, 50 vertical levels

• ERAinterim reanalysis with correction towards the satellite data:

• Wind, Tair, Hum : 3h

• Precipitation, long wave and short wave: 24h

• Bulk formulation NCAR

• Relative winds

EKE (2015)

Observing System Simulation Experiments(OSSEs) global model (1/12°)

Simulated Wide-swath (7 Kms, cycle: 20,9 days ) data:

NatRun SSH + Error (Ku-band Klystron)

Swath-1 and Swath-2 with 10-day shift in the orbits

Simulated nadir altimeter data (1Hz):

NatRun SSH (along tracks: Jason3, 2 Nadir along Wide-

swath , Sentinel3A, Sentinel3B: SAR mode)

NatRun: OSSEs:

➢ Sentinel3A, Sentinel3B for verification (not assimilated)7-day data (SSH : 3Nadir +2Swath) coverage

Other simulated observations:

the same in all the OSSEs

• SST (multiple sensors; L3S, 10km, daily )

• T&S profiles (CORA4.1)

• Sea Ice Concentration

Assimilation Scheme:

System Assimilation Mercator (SAM)

Observing System Simulation Experiments (OSSEs)

Several OSSEs were designed in order to investigate the impact on the ocean analysis and

forecast in the global model simulating different constellations of satellite altimeters

considering nadir and wide swath altimeter. The OSSEs were performed over a period of one

year (2015) and the results evaluated comparing with the OGCM data which represent the

“truth” ocean in the OSSEs.

OSSE0 : Free Model

OSSE1 : 3 nadir: Jason3, 2Nadir (middle of each Wide-Swath, error : 1 cm)

OSSE2 : 2 Wide-swath, (error : 0.4-1.2 cm)

OSSE3 : 3 Nadir + Wide-Swath, (error : 0.4-1.2 cm)

120 km

1 km

7 km

1 km• Start from the same stateof the ocean

• +Assim SST, ICE, T&S profiles

Impact on the Sea Surface Height

Comparaison with Ssh along S3A and S3B tracks (not assimilated)

~15% Gain

➢ Assim Swath Data Improvement along tracks (S3A & S3B)

➢ Best solution is the combination of Nadirs and Wide-Swaths

Impact on the Sea Surface Height (model grid)

SSH RMS ERROR(NatRun – OSSEs)

+ Assim 2Swath

Impact on the Sea Surface Height (model grid, Gulf Stream)

SSH RMS ERROR

OSSE0: Free Run OSSE1: Assim 3Nadir OSSE3: Assim 3Nadir+2Swath

OSSE0

OSSE1 OSSE3

Which scales are best corrected ?

SSH RMS ERROR as function of spatial scales




250Km



> 500Km

SSH RMS ERROR as function of spatial scales ( L >500Km)

+ Assim 2Swath



200 < L < 500Km

SSH RMS ERROR as function of spatial scales ( 200 < L < 500Km)

+ Assim 2Swath



100 < L < 200Km

SSH RMS ERROR as function of spatial scales ( 100 < L < 200Km)

+ Assim 2Swath



< 100Km

SSH RMS ERROR as function of spatial scales ( L < 100Km)

+ Assim 2Swath

➢ Reduction of SSH error at all scales

SSH (NatRun:Color, OSSEs: Contour; NatRun vs 3-day Forecast)


Impact on the ocean forecast: NatRun vs OSSEs (Agulhas current)

➢ Better forecast of SSH

SUMMARYWide-Swath assimilation:

▪ Improvement along tracks (S3A & S3B, between tracks of the assimilated Nadirs)

▪ Best solution is the combination of Nadirs and Wide-Swaths

▪ Reduction of SSH error at all scales

▪ Improvement prediction and help the system to better solve ocean dynamics up to 50 km spatial scales.

Next Steps :

• Impact on the water column and model dynamic (Current, vertical velocity, Spectrum , Mld…)

• Impact on the dynamic interpolation through model

• Sensitivity of the system to wide-swath altimeters instrumental error.

• Continue improving the data assimilation system ( non correlated errors,….)

http://[email protected]

Thank youfor your attention

contribution of wide-swath altimetry missions to the ocean...

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