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Issued by: CLS / G. Taburet, F. Mertz and J.-F. Legeais

Date:  

Ref: WP2-FDDP-2022-09_C3S2-Lot3_PUGS-of-vDT2021-SeaLevel-products_v1.45

Official reference number service contract:2021 2022/C3S2_312a312b_Lot3_METNorwayMOi/SC1

Please cite this document as "Taburet et al. (20232024) C3S Sea Level vDT2021: Product User Guide and Specifications. Issue 1.45. E.U. Copernicus Climate Change Service. Document ref. WP2-FDDP-2022-09_C3S2-Lot3_PUGS-of-vDT2021-SeaLevel-products_v1.45. 

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Expand
titleClick here to see the history of modifications


Version

Date

Description of modification

Chapters / Sections

1.0

15/06/2021

Creation


1.1

08/09/2021

Corrections after review


1.2

 

Addition of monthly product


1.3

 

Addition of Sentinel-6A mission


1.4

 

Corrections after review and finalised for publication

all sections

1.5

 

Temporal extension 2023-06-08 to 2023-09-08

4.2.1


List of datasets covered by this document

...

  • New L2P altimeter standards following expert recommendations (Lievin et al., 2020) and (Kocha et al., 2023) for measurements after 2023/06/08
  • Improved editing for L3 product for mapping
  • More precise definition of the error budgets associated with the different altimeter measurements for the Optimal Interpolation process

...

Anchor
table1
table1
Table 1: Source and delay of availability of the different altimeter data used as input to the DUACS system.

Altimeter mission

Type of product

Source

Availability delay

Sentinel-6 MF

NTC

EUMETSAT

~1 month

Sentinel-3A

NTC

EUMETSAT

~1 month

Jason-3

GDR

CNES/EUMETSAT

Reprocessing only

OSTM/Jason-2

GDR

CNES

Reprocessing only

CryoSat-2

GDR

ESA

Reprocessing only

SARAL/AltiKa

GDR

CNES

Reprocessing only

Topex/Poseidon

GDR

CNES

Reprocessing only

Jason-1

GDR

CNES

Reprocessing only

Envisat

GDR

ESA

Reprocessing only

ERS-1

GDR

ESA

Reprocessing only

ERS-2

GDR

ESA

Reprocessing only


The auxiliary products (altimeter standards, geophysical corrections) used in the production are described in Table 2. They are the most up-to-date standards (whose timeliness is compatible with the C3S production planning) and most of them follow the recommendations of the ESA Sea Level CCI project (Quartly et al. 2017; Legeais et al., 2018). More details on the description of these standards can be found in Lievin et al., 2020

...

 and in Kocha et al., 2023 for measurements after 2023/06/08 (in bold in the table).

May 2024 : temporal extension in interim mode (measurements after 2023/06/08)

In May 2024, a temporal extension of the MY DT-2021 series was made available. It differs from previous ones by a change in the altimetric standards and geophysical corrections used in the processing. Indeed, in preparation for an upcoming complete reprocessing of the series, the upstream data are now available in the DT-2024 standards described by (Kocha et al., 2023). These changes mainly consist of:

  • A new L2 product version for Sentinel-3A (BC005) and Sentinel-6A (FC09), including improved retracking and associated Sea State bias correction
  • The use of the new ocean-tide correction FES22
  • The use of the new Mean Sea Surface combining the SIO22; CNES_CLS_22 and DTU21 versions for the geodetic missions and Sentinel-3B

They are described in the Table 2. The use of the new standards contributes to improve the quality of the SLA field and derivates. They induce few regional biases (order of millimetre at regional scales) which are smoothed by the L3/L4 processing for a seamless transition for users and ensure the continuity of the sea level at regional scales.

Anchor
table2
table2
Table 2: Altimeter standards used in the C3S sea level vDT2021 product as described in Lievin et al., 2020. and in bold as described in Kocha et al., 2023

.


Poseidon Topex

Jason-1

OSTM/Jason-2

Jason

...

Poseidon Topex

Jason-1

OSTM/Jason-2

Jason

-3

ERS-1

ERS-2

Envisat

Cryosat-2

SARAL AltiKa

Sentinel-3A

Sentinel-6 MF

Orbit

GSFC STD18

POE-E

POE-F

POE-F

Reaper

POE-E

POE-F

POE-F

POE-F

POE



Ionospheric Correction

Filtered dual-frequency altimeter range measurements [Guibbaud et al. 2015]; DORIS on Poseidon

Filtered dual-frequency altimeter range [Guibbaud et al. 2015] (from SSB C-band)

Filtered dual-frequency altimeter from [Guibbaud et al. 2015] & c> 170 from L2 GDRF

Reaper NIC09 model [Scharroo and Smith, 2010]

GIM [Ijima et al., 1999]

Filtered from L2; c>65: GIM [Ijima et al., 1999] corrected for 8mm bias

GIM [Ijima et al., 1999]

Filtered from L2

L2 Filtered dual frequency



Sea State Bias

Non parametric [Tran et al. 2010] ;

BM4 on Poseidon

Non parametric [Tran 2015]

Non parametric [ Tran 2012]

Non parametric from J2 [ Tran 2012] & c>170 from [Tran 2020 report] J3 GDRF

BM3 [Gaspar and Ogor, 1994]

Non parametric [Mertz et al., 2005]

Non parametric [ Tran 2017]

Non parametric [ Tran 2018] Baseline C

Non parametric [ Tran 2018]

Non parametric [ Tran 2012]

Non parametric SSB [Tran 2021] from J3 GDR


Wet Troposphere

GPD+ [Fernandes and Lazaro, 2015]

JMR (GDRE) radiometer

AMR radiometer

AMR radiometer (c>170 from L2 GDRF)

GPD+ [Fernandes and Lazaro, 2015]

MWR radiometer reprocessed

GPD+ [Fernandes and Lazaro, 2015]

Neural Network (5 entries) V4

MWR 3 radiometer

MWR radiometer

Dry Troposphere

ERA5 (1-hour) model based





Dynamical Atmospheric Correction

TUGO

High frequencies forced with analysed ERA5 pressure and wind field + inverse barometer Low frequencies

TUGO HF forced with analysed ERA 5 pressure an d wind field; and after 02/2016 MOG2D HF forced with analysed ECMWF pressure and wind field + inverse barometer Low Frequencies

MOG2D HF forced with analysed ECM WF pressure and wind [Carrere and Lyard, 2003; operational version 3.2.0] + inverse barometer Low Frequencies

TUGO High frequencies forced with analysed ERA5 pressure and wind field + inverse barometer Low frequencies

TUGO High frequencies forced with analysed ERA5 pressure and wind field; and after 02/2016 MOG2D High frequencies forced with analysed ECMWF pressure and wind field + inverse barometer Low frequencies

TUGO HF forced with analysed ERA5 pressure an d wind field; and after 02/2016 MOG2D HF forced with analysed ECMWF pressure and wind field + inverse barometer Low Frequencies

Before 2023/06/07: MOG2D High frequencies forced with analysed ECMWF pressure and wind field [Carrere and Lyard, 2003; operational version 3.2

.0]

.0] + inverse barometer Low frequencies

After 2023/06/08: TUGO High frequencies forced with analysed ECMWF pressure and wind field + inverse barometer

Low frequencies

LF

Ocean Tide

Before 2023/06/07: FES 2014 B  [Carrère et al. 2016]; after 2023/06/08: FES22b [Carrère et al., 2023]

Internal TideZaron 2019 (HRETv8.1 tidal frequencies: M2, K1, S2, O1)
Pole TideDesai et al., 2015 ; Mean Pole Location 2017
Solid TideElastic response to tidal potential [Cartwright and Tayler, 1971; Cartwright and Edden, 1973]
Mean Sea Surface
Composite
Before 2023/06/07: Hybrid (SIO,CNES/CLS15,DTU15) [Sandwell et al.,2017 ; Ole et al.; Pujol et al.,2018]; after 2023/06/08: Hybrid MSS (SIO22; CNES/CLS22, DTU21) [Laloue et al., 2024]
Mean
Dynamic Topography		CNES_CLS18 (Mulet et al, 2021) combined with CMEMS_2020
Glacial Isostatic Adjustment (GIA)The DUACS L4 products are not corrected from GIA effects


Warning
titleWarning:

Between 1993 and 1998, the retrievals of global mean sea level (MSL) have been known to be affected by an instrumental drift in the TOPEX-A measurements, which has been quantified by several studies as discussed in the C3S Product Quality Assessment Report ([C3S_PQAR], section 3.2) and in Legeais et al. (2020). The altimeter sea level community agrees that it is necessary to correct the TOPEX-A record for the instrumental drift to improve the accuracy and the uncertainty of the total sea level record. An empirical correction of this drift based on a global comparison between altimetry and _in situ_ tide gauge measurements (WCRP sea level budget group, 2018) has been proposed in the data files. The correction value included in the dedicated variable can be added to the gridded SLA, to correct for the observed instrumental drift during the lifetime of the TOPEX-A mission (the correction is null after this period). This is a global correction to be added a posteriori (and not before) to the global mean sea level estimate derived from the gridded sea level data. It can be applied at regional or local scales as a best estimate (better than no correction, since the regional variation of the instrumental drift is unknown).

However, even if the corrections proposed by the different studies available lead to similar global MSL trends and accelerations (in agreement with climate models), there is not yet a consensus on the best approach to estimate the drift correction at global and regional scales. The recommendation of the Ocean Surface Topography Science Team (OSTST) is to wait for the future release of a reprocessed TOPEX dataset. Therefore, the TOPEX-A correction has been proposed as a separate variable within the C3S sea level data files vDT2021 (and not directly included in the SLA estimate). See the sea level Product Quality Assessment Report [C3S_PQAR] for further details.

...

The complete altimetry satellite constellation used in the C3S sea level product is illustrated in Figure 2.

Anchor
figure2
figure2

Image RemovedImage Added

Figure 2: Overview of the L2P products (input for DUACS system) availability period for each altimetric mission.

...

  • the reference missions are the TOPEX/Poseidon, Jason-1, Jason-2, Jason-3 and Sentinel-6MF, which have been successively introduced into the production system. These missions are essential for the computation of the long- term trend of the MSL since they are used to wedge complementary missions in terms of sea level drift. Sentinel-6MF is the current reference mission used in the system and it has replaced Jason-3 in February 2022.
  • the complementary missions provide additional information for the estimation of mesoscale signal variabilities (>200-300 km) and also increase the observing capacity at high latitudes, which is of great interest for climate. The missions that have successively been included in the C3S product are ERS-1, ERS-2, Envisat, SARAL/Altika and presently Sentinel-3A. Note that the ERS-1 mission was operated in an ice phase (phase D) from 21/12/1993 to 10/04/1994; no ERS-1 altimeter measurements have been used as input to the sea level production system during this period. As no other altimeter data are available, this means that the C3S product is based on TOPEX/Poseidon data only during this 3.5-month period. During the following two successive geodetic phases (phase E, 10/04/1994 – 28/09/1994 and phase F, 28/09/1994 – 21/03/1995), the changes to the ERS-1 mission operations (declared as a new mission: ERS-1 geodetic) have been taken into account in sea level data production.
  • In addition, after the loss of the Envisat mission in April 2012, only the opportunity CryoSat-2 mission has been available. Thus, this opportunity mission was included in the C3S product until SARAL/AltiKa delayed-time measurements become available in March 2013.

Anchor
figure3
figure3


Image Modified


Figure 3: Satellite constellation in the C3S time series.

...

Anchor
table3
table3
Table 3: Characteristics and time availability of the different altimeter data used in input of DUACS system.


Altimeter mission



Cycle  duration  (days)



Latitude range (°N)



Number of  tracks in  the cycle



Inter-track distance at equator (km)



Sun-  synchron ous



Dual-  frequency  Altimeter



Radiometer on board


Temporal period processed by DUACS system for C3S product


Begin date


End date

Topex/Poseidon

10

±66

254

~315

No

Yes

Yes

1992/11/20

2002/04/24

Jason-1

10

±66

254

~315

No

Yes

Yes

2002/04/24

2008/10/19

OSTM/Jason-2

10

±66

254

~315

No

Yes

Yes

2008/10/19

2016/05/26

Jason-3

10

±66

254

~315

No

Yes

Yes

2016/05/26

2022/02/10



ERS-1



35




±81.5




1002



~80




Yes




Yes




Yes



1992/11/20*



1995/05/15

ERS-1 Geodetic

168

-

1994/04/10

1995/03/21

ERS-2

35

±81.5

1002

~80

Yes

Yes

Yes

1995/05/15

2002/05/14

Envisat

35

±81.5

1002

~80


Yes (S-band


2002/05/14

2010/10/18

Envisat-New Orbit

30

±81.5

862

-

Yes

lost after

cycle 65)

Yes

2010/10/26

2012/04/08

Cryosat-2

29 (sub cycle)

±88

840

~98

No

No

No

2012/04/08

2013/03/14

SARAL/AltiKa

35

±81.5

1002

~80

Yes

No

Yes

2013/03/14

2016/03/20

Sentinel-3A

27

±81.5

770

~100

Yes

Yes

Yes

2016/03/20

On-going

Sentinel-6 MF

10

±66

254

~315

No

Yes

Yes

2022/02/10

On-going

The user and service requirements related to the sea level ECV product are described in detail in [C3S_TRD]. The characteristics (spatial and temporal coverage) listed in the above table are in agreement with these target requirements. The [C3S_TRD] document also includes a gap analysis, describing what could be achieved to better answer the user's needs so that the sea level product remains relevant and up-to-date.

...

The product is delivered in a Cartesian grid with the coverage definition detailed in the table below:

Anchor
Table 4
Table 4
Table 4: Coverage definition of the cartesian grid.

Area

Latitude coverage

Longitude coverage

Global Ocean

90°S/90°N

0°/360°


Note

Note that the latitudinal coverage of the maps depends on the ice coverage and nominally reaches 82° of latitude (except for data from CryoSat-2) because of the orbital inclination of the satellites. When no measurement is available (at higher latitudes or over the continents), the grid is filled with the default '_FillValue'.


Note

Note that the values taken into account to generate a map are ocean values. The mapping process (see section 4.2.3) computes some slight extrapolation to the coasts. This avoids the production of gaps in the data that can occur near the coast, and it also allows for a more precise computation of velocities.

...

The variables are listed in Table 5:

Anchor
Table5
Table5
Table 5: Variables of the daily sea level product.

Type

Name

Content

Unit

Scale Factor

float

time(time)

Time of measurement

days since 1950-01-01
00:00:00 UTC

none

float

latitude(latitude)

Latitude of measurement

degrees_north

none

float

longitude(longitude)

Longitude of measurement

degrees_east

none

float

lat_bnds (latitude,nv)

latitude values at the north and south bounds of each pixel.

degrees_north

none

float

lon_bnds(longitude,nv)

longitude values at the west and east bounds of each pixel.

degrees_east

none

int

nv(nv)

Useful for grid definition

none

none

int

crs

Describes the grid_mapping used by the data in this file. This variable does not contain any data; only information about the geographic coordinates system.

none

none

int

sla(time,latitude,longitude)

Sea level Anomaly

meters

10-4

int

err_sla(time,latitude,longitude)

Formal mapping error

meters

10-4

int

ugosa(time,latitude,longitude)

Geostrophic velocity anomalies: eastward zonal component

m/s

10-4

int

vgosa(time,latitude,longitude)

Geostrophic velocity anomalies: northward meridian component

m/s

10-4

int

err_ugosa(time,latitude,longitude)

Formal mapping error on zonal velocity anomalies

m/s

10-4

int

err_vgosa(time,latitude,longitude)

Formal mapping error on meridional velocity anomalies

m/s

10-4

int

adt(time,latitude,longitude)

Absolute dynamic topography

meters

10-4

int

ugos(time,latitude,longitude)

Absolute geostrophic velocity: eastward zonal component

m/s

10-4

int

vgos(time,latitude,longitude)

Absolute geostrophic velocity: northward meridian component

m/s

10-4

int

tpa_correction

TOPEX-A instrumental drift correction derived from altimetry and tide gauges global comparisons

m

10-4

int

flag_ice

Ice Flag based on CDR OSI-SAF products until 2016 (OSI-450), Interim products from 2016 (OSI- 430-b)

-

10-4

Monthly sea level product

4 dimensions are defined:

...

Anchor
Table6
Table6
Table 6: Variables of the monthly sea level product.

Type

Name

Content

Unit

Scale Factor

float

time(time)

Time of measurement

days since 1950-01-01
00:00:00 UTC

none

float

latitude(latitude)

Latitude of measurement

degrees_north

none

float

longitude(longitude)

Longitude of measurement

degrees_east

none

float

lat_bnds (latitude,nv)

latitude values at the north and south bounds of each pixel.

degrees_north

none

float

lon_bnds(longitude,nv)

longitude values at the west and east bounds of each pixel.

degrees_east

none

floatclimatology_bnds(time,nv)Useful for grid definitionmeters

int

nv(nv)

Useful for grid definition

none

none

int

crs

Describes the grid_mapping used by the data in this file. This variable does not contain any data; only information about the geographic coordinates system.

none

none

int

sla(time,latitude,longitude)

Sea level Anomaly

meters

10-4

inteke(time,latitude,longitude)Eddy Kinetic Energycm2/s210-4

Appendix A - Specifications of the daily sea level product

...

Arbic, B. K., Scott, R. B., Chelton, D. B., Richman, J.G., and Shriver, J. F.: Effects on stencil width on surface ocean geostrophic velocity and vorticity estimation from gridded satellite altimeter data, J. Geophys. Res., 117, C03029, doi:10.1029/2011JC007367, 2012.

Carrère, L., andFand F. Lyard,Modeling the barotropic response of the global ocean to atmospheric wind and pressure forcing - comparisons with observations, Geophys. Res. Lett.,30, 1275, doi: 10.1029/2002GL016473, 2003.

...

Carrere, L., Faugère, Y., and Ablain, M.: Major improvement of altimetry sea level estimations using pressure-derived corrections based on ERA-Interim atmospheric reanalysis, Ocean Sci., 12, 825–842, doi:10.5194/os-12, 825–842, doi:10.5194/os-12-825- 2016, 2016-825- 2016, 2016.

Carrère L., Lyard F., Cancet, M., Allain, D. J. Fauchet, E., Dabat, M.-L., Tchilibou, M., Ferrari, R., and Faugère, Y.: The new FES2022 Tidal atlas. Presented at the 2023 SWOT Science Team meeting (Toulouse). Available online (last access: 28 May 2024): https://doi.org/10.24400/527896/a03-2022.3287, 2023.

Cartwright, D. E. and Tayler, R. J.: New computations of the tide generating potential, Geophys. J. R. Astr. Soc., 23, 45–74, 1971.

Cartwright, D. E. and Edden, A. C.: Corrected tables of tidal harmonics, Geophys. J.  R. Astr. Soc., 33, 253–264, 1973.

Desai S., J. Wahr, B. Beckley, Revisiting the pole tide for and from satellite altimetry, J. of Geodesy, Vol 89, issue 12, pp 1233-1243, 2015, DOI: 10.1007/s00190-015-0848-7

...

Guibbaud, M., A. Ollivier and M. Ablain, A new approach for dual-frequency ionospheric correction filtering, ENVISAT Altimetry Quality Working Group  (QWG), 2015 available in the Section 8.5 of the 2012 Envisat annual activity report:report: https://www.aviso.altimetry.fr/fileadmin/documents/calval/validation_report/ EN/annual_report_en_2012.pdf (last access: 15 June 2021)

Jousset S., Mulet S., New Mean Dynamic Topography of the Black Sea and Mediterranean Sea from altimetry, gravity and in-situ data. Presentation OSTST 2020,  https://wwwmeetings.aviso.altimetry.fr/fileadmin/documents/calval/validation_report/EN/annual_report_en_2012user_upload/tx_ausyclsseminar/ files/OSTST2020_JOUSSET_MULET_MDT.pdf (last access: 15 June 2021)Jousset S., Mulet S., New Mean Dynamic Topography of the Black Sea and Mediterranean Sea from altimetry, gravity and in-situ data. Presentation OSTST 2020,  https://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/ files/OSTST2020_JOUSSET_MULET_MDT.pdf (last access: 15 June 2021),  2020.,  2020.

Kocha, C., Lievin, M., Pageot, Y., Rubin, C., Philipps, S., Dibarboure, G., Denis, I., Guinle, T., and Nogueira Loddo, C.: 30 years of sea level anomaly reprocessed to improve climate and mesoscale satellite data record. OSTST meeting., , https://doi.org/10.24400/527896/a03-2023.3804, 2023.

Laloue A., P. Veillard, P. Schaeffer, M.-I. Pujol, O. Andersen, D. Sandwell, A. Delepoulle, G. Dibarboure, Y. Faugere, Merging recent Mean Sea Surface into a 2023 Hybrid model (from Scripps, DTU, CLS and CNES), submitted to Journal of Geophysical Research, 2024

Lavergne, T., Sørensen, A. M., Kern, S., Tonboe, R., Notz, D., Aaboe, S., Bell, L., Dybkjær, G., Eastwood, S., Gabarro, C., Heygster, G., Killie, M. A., Brandt Kreiner, M., Lavelle, J., Saldo, R., Sandven, S., and Pedersen, L. T.: Version 2 of the EUMETSAT OSI SAF and ESA CCI sea-ice concentration climate data records, The Cryosphere, 13, 49–78, https://doi.org/10.5194/tc-13-49-2019 , 2019.

...

Zaron, E. D.: Baroclinic Tidal Sea Level from Exact-Repeat Mission Altimetry, Journal of Physical Oceanography, 49, 193–210, https://doi.org/10.1175/JPO-D-18-0127.1, 2019.

 CopernCopernicus Climate Change Service

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This document has been produced in the context of the Copernicus Climate Change Service (C3S).

The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation Agreement agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.

The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.

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