Last modified on Nov 13, 2024 21:19

Contributors:  G. Taburet (CLS), F. Mertz (CLS), J.-F. Legeais (CLS)

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

Date: 28 May 2024 

Ref: WP2-FDDP-2022-09_C3S2-Lot3_PUGS-of-vDT2021-SeaLevel-products_v1.5

Official reference number service contract: 2022/C3S2_312b_MOi/SC1

Please cite this document as "Taburet et al. (2024) C3S Sea Level vDT2021: Product User Guide and Specifications. Issue 1.5. E.U. Copernicus Climate Change Service. Document ref. WP2-FDDP-2022-09_C3S2-Lot3_PUGS-of-vDT2021-SeaLevel-products_v1.5. 

1. Preamble

1.1. History of modifications

1.2. List of datasets covered by this document

The present document applies to the C3S altimeter sea level Climate Data Record (CDR) and the following temporal extensions (Interim CDR). The current version corresponds to the reprocessed DUACS Delayed-Time vDT2021 products.

1.3. Related documents

1.4. Acronyms

1.5. General definitions

2. Scope of the document

This document is the Product User Guide and Specification (PUGS) document for Version DT2021 sea level products, distributed through the Copernicus Climate Change Service (C3S). This document has been developed within the frame of the 2018/C3S_312b_Lot3_CLS/SC2 contract. It provides the end user with practical information regarding the use of these products.

3. Executive summary

The current version of the C3S sea level product is the DUACS vDT2021 reprocessed delayed-time altimeter sea level products. It benefits from several advances over the previous version. They include:

• 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

This Product User Guide and Specification explains the basic altimetry principles that allow the computation of the altimeter sea level product, and provides a brief description of the associated production system. The details of the input data are provided, including their origin. The technical characteristics of each altimeter mission used in the production system are described, as well as the level 2 altimeter algorithms (geophysical standards and orbit solutions). An empirical correction of the TOPEX-A instrumental drift observed during 1993-1998 is included in the data files. The characteristics of the satellite constellation are described, and the principle of the sea level mapping procedure is provided. Finally, the product characteristics are described (format, nomenclature and data handling variables) and a description of the file content is provided in the Annex.

4. Product description

The sea level products distributed through the Copernicus Climate Change Service (C3S) comprise:

• a time series of daily gridded Sea Surface Height and derived variables obtained by merging measurements from two altimetry satellites. It is generated by the DUACS processing system and includes data from several altimetry missions.
• a time series of monthly gridded Sea Surface Height and derived variables obtained by computing monthly means of daily product.

The sea level products have coverage of the global ocean.
The C3S product mainly focuses on the retrieval of the long-term variability of the ocean, which is only obtained using a stable altimeter constellation and homogeneous corrections and standards in time. One way to address the later constraints is to use a two-satellite constellation throughout the entire altimeter period (see 4.2.2).

The present document refers to the C3S altimeter sea level Climate Data Record (CDR) and the following temporal extensions (Interim CDR). The current version corresponds to the reprocessed DUACS Delayed-Time vDT2021 product.

4.1. Common variables in Altimetry

Altimetry gives access to the Sea Surface Height (SSH) above the reference ellipsoid (see Figure 1) as in Eq (1):

The Mean Sea Surface (MSS_N) in Eq (2) is the temporal mean of the SSH over a period N. It is a mean surface above the reference ellipsoid and it includes the Geoid.

The Sea Level Anomaly (SLA_N) in Eq (3) is the anomaly of the signal around the mean component. It is deduced from the SSH and MSS_N :

The Mean Dynamic Topography (MDT_N) in Eq (4) is the temporal mean of the SSH above the Geoid over a period N.

The Absolute Dynamic Topography (ADT) in Eq (5) is the instantaneous height above the Geoid. The geoid is a gravity equipotential surface that would correspond to the ocean surface if the ocean was at rest (i.e. without any currents and only under the gravity field). When the ocean is influenced by wind, differential heating and precipitation, and other sources of energy, the ocean surface moves away from the geoid. Thus, the departure from the geoid provides information on ocean dynamics.

The ADT is the sum of the SLA_N and MDT_N:

The reference period N considered can be changed as described in Pujol et al (2016).

Figure 1: Different concepts of sea surface height used in altimetry.

4.1.1. Variables in the daily sea level product

The variables disseminated to users as part of the C3S sea level product, are the Sea Level Anomalies (variable name in netcdf: 'sla'), Absolute Dynamic Topography (variable name in netcdf: 'adt'), formal mapping error from sla (variable name in netcdf: 'err_sla'), the geostrophic velocities anomalies (variable names in netcdf: 'ugosa' and 'vgosa'), the formal mapping error on zonal/meridional velocity anomalies (variable names in netcdf: 'err_ugosa' and 'err_vgosa') and the absolute geostrophic velocities (variable names in netcdf: 'ugos' and 'vgos').

Geostrophic current¹ datasets are also disseminated to users as part of the C3S sea level product, and are generated from the SLAs and the ADTs. It is computed using a nine-point stencil width methodology (Arbic et al., 2012) for latitudes outside the 5°S/5°N band. In the equatorial band, the Lagerloef methodology (Lagerloef et al., 1999) is used. A specific variable is also available (variable name in netcdf: 'tpa_correction') and can be added to the SLA to correct for the observed instrumental drift during the lifetime of the TOPEX-A mission (the correction is null after this period). See Section 4.2.1 for more details.

A variable (variable name in netcdf: 'flag_ice') has been added to flag data using OSI SAF CDR sea ice concentration products (OSI-450) until 2016 and ICDR sea ice concentration (OSI-430-b) from 2016 (also distributed in the Climate Data Store, more info in Lavergne et al., 2019). The flag corresponds to the limit of 15% in sea ice concentration.

4.1.2.  Variables in the monthly sea level product

The variables disseminated to users are the Sea Level Anomalies (variable name in netcdf: ‘sla’) and the Eddy Kinetic Energy issued from the geostrophic velocities anomalies (variable name in netcdf: ‘eke’). It is computed using Eq (6):

4.2. Processing

The Delayed-Time DUACS component maintains a consistent and user-friendly altimeter database using state-of-the-art recommendations from the altimetry community.

The processing sequences can be divided into the following main steps (fully described in [C3S_ATBD]):

• Data acquisition
• Input data quality control
• Intercalibrate and unify
• Along-track products generation
• Gridded merged products generation
• Final quality control

4.2.1. Input data and corrections

The altimeter measurements used to compute the C3S sea level product consist of Level-2 products from different missions called Delayed-Time Geophysical Data Records (GDR) or Non Time Critical (NTC) products. Details of the different L2 altimeter products sources and delay of availability are given in Table 1.

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

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.

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

.

4.2.2. Altimetry constellation

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

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

The C3S sea level altimeter product is based on a satellite constellation with a stable number of altimeters in order to ensure the long-term stability of the ocean observation system. The different altimeter satellites included in the product are the reference missions and the complementary missions as well as missions of opportunity, as illustrated in Figure 3 and described below:

• 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.

Figure 3: Satellite constellation in the C3S time series.

4.2.3. Gridded merged product generation

The gridded merged product is based on the along-track altimeter measurements, which have undergone several processing steps, (as described in detail in [C3S_ATBD]). First of all, global and regional inter-mission biases are removed. Then, the along-track measurements are cross-calibrated following Le Traon and Ogor (1998), which allows for the reduction of the long wavelength errors (LWE) and also considers geographically-correlated errors. Along-track high frequency aliased signals are also removed. In addition, the data are filtered (Dufau et al., 2016) with 65km cut-off length low-pass filtering. The along-track measurements are also subsampled for the mapping procedure, keeping one along-track point out of two. All the details are described in section 3.6 of [C3S_ATBD], in Taburet et al. (2019) and Pujol et al (2016). These procedures ensure the long-term stability of the sea level record. An optimal interpolation method is used for the mapping procedure following Ducet et al. (2000) and Le Traon et al. (2003). This ensures mesoscale signal reconstruction. The parameters used for the mapping procedure are a compromise between the characteristics of the physical field to be focused on, and the sampling capabilities associated with the altimeter constellation.

4.2.4. Mean and reference period

The along-track and gridded sea surface heights (sea level anomalies and absolute dynamic topography) are computed with respect to a 20-year reference period (1993-2012). In addition to the reference period, a mean reference convention has been adopted in the DUACS products: the sea level time series has been arbitrarily referenced so that the mean sea level averaged during the year 1993 is set to zero (see Figure 4). This convention explains why the DUACS global mean SLA during the reference period (1993-2012) is different from zero. The obtained value (about 2.5cm without a Glacial Isostatic Adjustment (GIA) correction) is directly related to global sea level rise (see Figure 4, right). The most recently calculated Global Mean Sea Level (1993 to end of 2021) is displayed in Figure 5. 

Figure 4: Left: Averaged map of sea level anomalies during the year 1993. The global mean for the year 1993 is -0.0007m and can be considered as a zero mean. Right: Global mean sea level progression during the period 1993-2012 (without GIA correction) deduced from DUACS L4 gridded products. The dashed line represents the sea level time-series corrected for the TOPEX-A instrumental drift (between 1993-1998). The horizontal line indicates the value of the globally averaged SLA (not corrected for the TOPEX-A drift) during the reference period (1993-2012).

Figure 5: Global mean sea level progression for the period 1993-2021 (without GIA correction) deduced from DUACS L4 gridded products. The dashed line represents the sea level time-series corrected for the TOPEX-A instrumental drift (between 1993-1998).

5. Specifications and target requirements

5.1. Spatial and temporal coverage

The daily time series begins on 01/01/1993. The time series benefits from regular temporal extensions approximately 3 times per year (ICDR production), and the timeliness of the product is of 5 months at the minimum. Such a delay depends on:

• The input data availability (see section 4.2.1)
• The production algorithms (centred temporal windows, [C3S_SQAD])
• The time required for the computation and validation processes.

The time delay can be longer in cases of missing altimeter measurements from a mission, or a longer than usual validation process for instance.

The characteristics of the different missions used in the C3S sea level product are described in Table 3.

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

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.

5.2. Validation and uncertainty estimates

Validation activities are carried out to assess the quality of the product. The validation method is described in the Product Quality Assurance Document [C3S_PQAD] and details of the validation results are provided in the Product Quality Assessment Report [C3S_PQAR].

The description of the altimeter errors and characterization of the uncertainties are available in [C3S_PQAR].

6. Data usage information

6.1. Grid characteristics

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

Table 4: Coverage definition of the cartesian grid.

6.2. Format

The product is stored and delivered to users using the NetCDF (Network Common Data Form) using CF (Climate and Forecast) Metadata convention.

6.3. File nomenclature

6.3.1. Daily product

The nomenclature of the file is the following:

dt_global_twosat_phy_l4_<DateMap>_vDT2021.nc

where:

<DateMap>=the date of the map in the form YYYYMMDD

6.3.2.  Monthly product

The nomenclature of the file is the following:

dt_global_twosat_phy_l4_<MonthMap>_vDT2021-M01.nc

where:

<MonthMap>=the month of the map in the form YYYYM

6.4. Data Handling Variables

6.4.1. Daily sea level product

4 dimensions are defined:

• time
• latitude
• longitude
• nv (useful for grid definition)

The variables are listed in Table 5:

Table 5: Variables of the daily sea level product.

6.4.2. Monthly sea level product

4 dimensions are defined:

• time
• latitude
• longitude
• nv (useful for grid definition)

The variables are listed in Table 6:

Table 6: Variables of the monthly sea level product.

7. Appendix A - Specifications of the daily sea level product

netcdf dt_global_twosat_phy_l4_20170515_vDT2021 { dimensions:
time = 1 ; latitude = 720 ;
longitude = 1440 ; nv = 2 ;
variables:
int crs ;
crs:comment = "This is a container variable that describes the grid_mapping used by the data in this file. This variable does not contain any data; only information about the geographic coordinate system." ;
crs:grid_mapping_name = "latitude_longitude" ; crs:inverse_flattening = 298.257 ; crs:semi_major_axis = 6378136.3 ;
float time(time) ;
time:axis = "T" ; time:calendar = "gregorian" ; time:long_name = "Time" ;
time:standard_name = "time" ;
time:units = "days since 1950-01-01 00:00:00" ; float latitude(latitude) ;
latitude:axis = "Y" ; latitude:bounds = "lat_bnds" ; latitude:long_name = "Latitude" ;
latitude:standard_name = "latitude" ; latitude:units = "degrees_north" ; latitude:valid_max = 89.875 ; latitude:valid_min = -89.875 ;
float lat_bnds(latitude, nv) ;
lat_bnds:comment = "latitude values at the north and south bounds of each pixel." ; lat_bnds:units = "degrees_north" ;
float longitude(longitude) ; longitude:axis = "X" ; longitude:bounds = "lon_bnds" ;
longitude:long_name = "Longitude" ; longitude:standard_name = "longitude" ; longitude:units = "degrees_east" ; longitude:valid_max = 359.875 ; longitude:valid_min = 0.125 ;
float lon_bnds(longitude, nv) ;
lon_bnds:comment = "longitude values at the west and east bounds of each pixel." ; lon_bnds:units = "degrees_east" ;
int nv(nv) ;
nv:comment = "Vertex" ;
nv:long_name = "Number of cell vertices" ; nv:units = "1" ;
int sla(time, latitude, longitude) ; sla:_FillValue = -2147483647 ; sla:ancillary_variables = “err_sla” ;
sla:comment = "The sea level anomaly is the sea surface height above mean sea surface; it is referenced to the [1993, 2012] period; see the product user manual for details" ;
sla:coordinates = "longitude latitude" ; sla:grid_mapping = "crs" ; sla:long_name = "Sea level anomaly" ; sla:scale_factor = 0.0001 ;
sla:standard_name = "sea_surface_height_above_sea_level" ; sla:units = "m" ;
int err_sla(time, latitude, longitude) ; err_sla:_FillValue = -2147483647 ;
err_sla:comment = "The formal mapping error represents a purely theoretical mapping error. It mainly traduces errors induced by the constellation sampling capability and consistency with the spatial/temporal scales considered, as described in Le Traon et al (1998) or Ducet et al (2000)" ;
err_sla:coordinates = "longitude latitude" ; err_sla:grid_mapping = "crs" ; err_sla:long_name = "Formal mapping error" ; err_sla:scale_factor = 0.0001 ;
err_sla:standard_name = "sea_surface_height_above_sea_level standard_error"; err_sla:units = "m" ;
int adt(time, latitude, longitude) ; adt:_FillValue = -2147483647 ;
adt:comment = "The absolute dynamic topography is the sea surface height above geoid; the adt is obtained as follows: adt=sla+mdt where mdt is the mean dynamic topography; see the product user manual for details" ;
adt:coordinates = "longitude latitude" ; adt:grid_mapping = "crs" ;
adt:long_name = "Absolute dynamic topography" ; adt:scale_factor = 0.0001 ;
adt:standard_name = "sea_surface_height_above_geoid" ; adt:units = "m" ;
int ugos(time, latitude, longitude) ; ugos:_FillValue = -2147483647 ; ugos:coordinates = "longitude latitude" ; ugos:grid_mapping = "crs" ;
ugos:long_name = "Absolute geostrophic velocity: zonal component" ; ugos:scale_factor = 0.0001 ;
ugos:standard_name = "surface_geostrophic_eastward_sea_water_velocity" ; ugos:units = "m/s" ;
int vgos(time, latitude, longitude) ; vgos:_FillValue = -2147483647 ; vgos:coordinates = "longitude latitude" ; vgos:grid_mapping = "crs" ;
vgos:long_name = "Absolute geostrophic velocity: meridian component" ; vgos:scale_factor = 0.0001 ;
vgos:standard_name = "surface_geostrophic_northward_sea_water_velocity" ; vgos:units = "m/s" ;
int ugosa(time, latitude, longitude) ; ugosa:_FillValue = -2147483647 ;
ugosa: ancillary_variables = “err_ugosa” ;
ugosa:comment = "The geostrophic velocity anomalies are referenced to the [1993, 2012] period" ; ugosa:coordinates = "longitude latitude" ;
ugosa:grid_mapping = "crs" ;
ugosa:long_name = "Geostrophic velocity anomalies: zonal component" ; ugosa:scale_factor = 0.0001 ;
ugosa:standard_name = "surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid" ; ugosa:units = "m/s" ;
int vgosa(time, latitude, longitude) ; vgosa:_FillValue = -2147483647 ;
ugosa: ancillary_variables = “err_vgosa” ;
vgosa:comment = "The geostrophic velocity anomalies are referenced to the [1993, 2012] period" ; vgosa:coordinates = "longitude latitude" ;
vgosa:grid_mapping = "crs" ;
vgosa:long_name = "Geostrophic velocity anomalies: meridian component" ; vgosa:scale_factor = 0.0001 ;
vgosa:standard_name = "surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid" ; vgosa:units = "m/s" ;
int err_ugosa(time, latitude, longitude) ; err_ugosa:_FillValue = -2147483647 ;
err_ugosa:comment = "The formal mapping error represents a purely theoretical mapping error. It mainly traduces errors induced by the constellation sampling capability and consistency with the spatial/temporal scales considered, as described in Le Traon et al (1998) or Ducet et al (2000)" ;
err_ugosa:coordinates = "longitude latitude" ; err_ugosa:grid_mapping = "crs" ;
err_ugosa:long_name = "Formal mapping error on zonal geostrophic velocity anomalies" ; err_ugosa:scale_factor = 0.0001 ;
err_ugosa:standard_name = "surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid standard_error" ;
err_ugosa:units = "m/s" ;
int err_vgosa(time, latitude, longitude) ; err_vgosa:_FillValue = -2147483647 ;
err_vgosa:comment = "The formal mapping error represents a purely theoretical mapping error. It mainly traduces errors induced by the constellation sampling capability and consistency with the spatial/temporal scales considered, as described in Le Traon et al (1998) or Ducet et al (2000)" ;
err_vgosa:coordinates = "longitude latitude" ; err_vgosa:grid_mapping = "crs" ;
err_vgosa:long_name = "Formal mapping error on meridional geostrophic velocity anomalies" ; err_vgosa:scale_factor = 0.0001 ;
err_vgosa:standard_name = "surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid standard_error" ;
err_vgosa:units = "m/s" ; int tpa_correction(time) ;
tpa_correction:_FillValue = -2147483647 ;
tpa_correction:comment = "This 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) on the global mean sea level estimate derived from the gridded sea level map. It can be applied at regional or local scale as a best estimate (better than no correction, since the regional variation of the instrumental drift is unknown). See product manual for more details." ;
tpa_correction:long_name = "TOPEX-A instrumental drift correction derived from altimetry and tide gauges global comparisons from WCRP Sea Level Budget Group, 2018" ;
tpa_correction:scale_factor = 0.0001 ;
tpa_correction:standard_name = "tpa_correction_for_ sea_surface_height_above_sea_level " ; int flag_ice(time, latitude, longitude) ;
flag_ice:_FillValue = -2147483647 ;
flag_ice:comment = "Ice Flag based on CDR OSI SAF products until 2016 (OSI-450), Interim products from 2016 (OSI-430-b) (Lavergne et al.,2019). The flag correspond to the 15% sea ice concentration" ;
flag_ice:coordinates = "longitude latitude" ; flag_ice:grid_mapping = "crs" ;
flag_ice:long_name = " Ice Flag for a 15% criterion of ice concentration" ; flag_ice:scale_factor = 0.0001 ;
flag_ice:standard_name = "OSISAF_sea_ice_concentration_flag" ;

// global attributes:
:Conventions = "CF-1.6" ;
:Metadata_Conventions = "Unidata Dataset Discovery v1.0" ;
:cdm_data_type = "Grid" ;
:comment = "Sea Surface Height measured by Altimetry and derived variables" ;
:contact = "http://climate.copernicus.eu/c3s-user-service-desk" ;
:creator_email = "http://climate.copernicus.eu/c3s-user-service-desk" ;
:creator_name = "Copernicus Climate Change Service (C3S)" ;
:creator_url = "http://climate.copernicus.eu" ;
:date_created = "2021-06-01T00:00:00Z" ;
:date_issued = "2021-06-01T00:00:00Z" ;
:date_modified = "2021-06-01T00:00:00Z" ;
:geospatial_lat_max = 89.875 ;
:geospatial_lat_min = -89.875 ;
:geospatial_lat_resolution = 0.25 ;
:geospatial_lat_units = "degrees_north" ;
:geospatial_lon_max = 359.875 ;
:geospatial_lon_min = 0.125 ;
:geospatial_lon_resolution = 0.25 ;
:geospatial_lon_units = "degrees_east" ;
:geospatial_vertical_max = 0. ;
:geospatial_vertical_min = 0. ;
:geospatial_vertical_positive = "down" ;
:geospatial_vertical_resolution = "point" ;
:geospatial_vertical_units = "m" ;
:history = "2021-06-01 00:00:00Z: Creation" ;
:institution = "CLS, CNES" ;
:keywords = "Oceans > Ocean Topography > Sea Surface Height" ;
:keywords_vocabulary = "NetCDF COARDS Climate and Forecast Standard Names" ;
:license = "http://climate.copernicus.eu/c3s-user-service-desk" ;
:platform = "Jason-3, Sentinel-3A," ;
:processing_level = "L4" ;
:product_version = "vDec2021" ;
:project = "Copernicus Climate Change Service (C3S)" ;
:references = "http://climate.copernicus.eu" ;
:software_version = "7.0_DUACS_DT2021_baseline" ;
:source = "Altimetry measurements" ;
:ssalto_duacs_comment = "The reference mission used for the altimeter inter-calibration processing is Topex/Poseidon between 1993-01-01 and 2002-04-23, Jason-1 between 2002-04-24 and 2008-10-18, OSTM/Jason-2
between 2008-10-19 and 2016-06-25, Jason-3 since 2016-06-25." ;
:standard_name_vocabulary = "NetCDF Climate and Forecast (CF) Metadata Convention Standard Name Table
v37" ;
:summary = "SSALTO/DUACS Delayed-Time Level-4 sea surface height and derived variables measured by
multi-satellite altimetry observations over Global Ocean." ;
:time_coverage_duration = "P1D" ;
:time_coverage_end = "2017-05-15T12:00:00Z" ;
:time_coverage_resolution = "P1D" ;
:time_coverage_start = "2017-05-16T12:00:00Z" ;
:title = "DT merged two satellites Global Ocean Gridded SSALTO/DUACS Sea Surface Height L4 product and derived variables" ;
}

8. Appendix B - Specifications of the monthly sea level product

netcdf dt_global_twosat_phy_l4_202112_vDT2021-M01 {
dimensions:
        time = 1 ;
        latitude = 720 ;
        longitude = 1440 ;
        nv = 2 ;
variables:
        int crs ;
                crs:comment = "This is a container variable that describes the grid_mapping used by the data in this file. This variable does not contain any data; only information about the geographic coordinate system." ;
                crs:grid_mapping_name = "latitude_longitude" ;
                crs:inverse_flattening = 298.257 ;
                crs:semi_major_axis = 6378136.3 ;
        float time(time) ;
                time:axis = "T" ;
                time:bounds = "climatology_bnds" ;
                time:calendar = "gregorian" ;
                time:long_name = "Time" ;
                time:standard_name = "time" ;
                time:units = "days since 1950-01-01 00:00:00" ;
        float climatology_bnds(time, nv) ;
        float latitude(latitude) ;
                latitude:axis = "Y" ;
                latitude:bounds = "lat_bnds" ;
                latitude:long_name = "Latitude" ;
                latitude:standard_name = "latitude" ;
                latitude:units = "degrees_north" ;
                latitude:valid_max = 89.875 ;
                latitude:valid_min = -89.875 ;
        float lat_bnds(latitude, nv) ;
                lat_bnds:comment = "latitude values at the north and south bounds of each pixel." ;
                lat_bnds:units = "degrees_north" ;
        float longitude(longitude) ;
                longitude:axis = "X" ;
                longitude:bounds = "lon_bnds" ;
                longitude:long_name = "Longitude" ;
                longitude:standard_name = "longitude" ;
                longitude:units = "degrees_east" ;
                longitude:valid_max = 359.875 ;
                longitude:valid_min = 0.125 ;
        float lon_bnds(longitude, nv) ;
                lon_bnds:comment = "longitude values at the west and east bounds of each pixel." ;
                lon_bnds:units = "degrees_east" ;
        int nv(nv) ;
                nv:comment = "Vertex." ;
                nv:long_name = "Number of cell vertices" ;
                nv:units = "1" ;
        int sla(time, latitude, longitude) ;
                sla:_FillValue = -2147483648 ;
                sla:cell_methods = "time: mean within years" ;
                sla:coordinates = "longitude latitude" ;
                sla:grid_mapping = "crs" ;
                sla:long_name = "Averaged Sea Level Anomalies 2021/12" ;
                sla:scale_factor = 0.0001 ;
                sla:standard_name = "sea_surface_height_above_sea_level" ;
                sla:units = "m" ;
        int eke(time, latitude, longitude) ;
                eke:_FillValue = -2147483648 ;
                eke:cell_methods = "time: mean within years" ;
                eke:coordinates = "longitude latitude" ;
                eke:grid_mapping = "crs" ;
                eke:long_name = "Averaged Eddy Kinetic Energy 2021/12" ;
                eke:scale_factor = 0.0001 ;
                eke:standard_name = "specific_kinetic_energy_of_sea_water" ;
                eke:units = "cm2/s2" ;

// global attributes:
                :Conventions = "CF-1.6" ;
                :Metadata_Conventions = "Unidata Dataset Discovery v1.0" ;
                :cdm_data_type = "Grid" ;
                :comment = "Monthly Mean of Sea Level Anomalies referenced to the [1993, 2012] period" ;
                :contact = "http://climate.copernicus.eu/c3s-user-service-desk" ;
                :creator_email = "http://climate.copernicus.eu/c3s-user-service-desk" ;
                :creator_name = "Copernicus Climate Change Service (C3S)" ;
                :creator_url = "http://climate.copernicus.eu" ;
                :date_created = "2022-06-30T19:08:21Z: Creation" ;
                :date_issued = "2022-06-30T19:08:21Z: Creation" ;
                :date_modified = "2022-06-30T19:08:21Z: Creation" ;
                :geospatial_lat_max = 89.875 ;
                :geospatial_lat_min = -89.875 ;
                :geospatial_lat_resolution = 0.25 ;
                :geospatial_lat_units = "degrees_north" ;
                :geospatial_lon_max = 359.875 ;
                :geospatial_lon_min = 0.125 ;
                :geospatial_lon_resolution = 0.25 ;
                :geospatial_lon_units = "degrees_east" ;
                :geospatial_vertical_max = "0.0" ;
                :geospatial_vertical_min = "0.0" ;
                :geospatial_vertical_positive = "down" ;
                :geospatial_vertical_resolution = "point" ;
                :geospatial_vertical_units = "m" ;
                :history = "2022-06-30T19:08:21Z: Creation" ;
                :institution = "CNES, CLS" ;
                :keywords = "Oceans > Ocean Topography > Sea Surface Height" ;
                :keywords_vocabulary = "NetCDF COARDS Climate and Forecast Standard Names" ;
                :license = "http://climate.copernicus.eu/c3s-user-service-desk" ;
                :processing_level = "L4" ;
                :product_version = "vDec2021" ;
                :project = "Copernicus Climate Change Service (C3S)" ;
                :references = "http://climate.copernicus.eu" ;
                :software_version = "7.0_DUACS_DT2021_baseline" ;
                :source = "Altimetry measurements" ;
                :ssalto_duacs_comment = "The reference mission used for the altimeter inter-calibration processing is Topex/Poseidon between 1993-01-01 and 2002-04-23, Jason-1 between 2002-04-24 and 2008-10-18, OSTM/Jason-2 between 2008-10-19 and 2016-06-25, Jason-3 since 2016-06-25." ;
                :standard_name_vocabulary = "NetCDF Climate and Forecast (CF) Metadata Convention Standard Name Table v37" ;
                :summary = "Delayed Time Level-4 monthly means of Sea Level Anomalies and derived Eddy Kinetic Energy from two-satellite altimetry observations over the Global Ocean." ;
                :time_coverage_duration = "P1M" ;
                :time_coverage_end = "2021-12-31T00:00:00Z" ;
                :time_coverage_resolution = "P1M" ;
                :time_coverage_start = "2021-12-01T00:00:00Z" ;
                :title = "DT merged two-satellite Global Ocean L4 Monthly Means of Sea Level Anomalies and Derived Eddy Kinetic Energy" ;
}

9. References

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., and 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.

Carrère, L, F. Lyard, M. Cancet, A. Guillot, N. Picot, 2015: FES2014: a new tidal model on the global ocean with enhanced accuracy in shallow seas and in the Arctic region, OSTST2015: http://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/fi les/29Red1100-2_ppt_OSTST2014_FES2014_LC.pdf (last access: 15 June 2021)

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- 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

Ducet, N., Le Traon, P.-Y., and Reverdun, G.: Global high resolution mapping of ocean circulation from TOPEX/Poseidon and ERS-1 and -2, J. Geophys. Res., 105, 19477–19498, 2000.

Dufau, C., Orstynowicz, M., Dibarboure, G., Morrow, R., and Le Traon, P.-Y.: Mesoscale Resolution Capability of altimetry: present & future, J. Geophys. Res, 121, 4910–4927, doi:10.1002/2015JC010904, 2016

ESRIN, “Detailed Processing Model of the Sentinel-3 SRAL SAR altimeter ocean waveform retracker, Development of SAR Altimetry Mode Studies and Applications of Ocean, Coastal Zones and Inland Water (SAMOSA project)”, ESRIN Contract No. 20698/07/I-LG, SAMOSA3 WP2300, 10 September 2015, version 2.5.0.

Fernandes J., C. Lázaro, M. Ablain, N. Pires, Improved wet path delays for all ESA and reference altimetric missions, Remote Sensing of Environment, Vol 169, p 50-74, 2015, https://doi.org/10.1016/j.rse.2015.07.023

Gaspar, P., and F. Ogor, Estimation and analysis of the Sea State Bias of the new ERS-1 and ERS-2 altimetric data (OPR version 6). Report of task 2 of IFREMER Contract n° 96/2.246 002/C, 1996.

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: 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://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/ files/OSTST2020_JOUSSET_MULET_MDT.pdf (last access: 15 June 2021),  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.

Legeais J.-F., W. Llovel, A. Melet, and B. Meyssignac: Evidence of the TOPEX-A Altimeter Instrumental Anomaly and Acceleration of the Global Mean Sea Level. In: Copernicus Marine Environment Monitoring Service Ocean State Report, Issue 4, J Oper Oceanogr, under review.

Legeais, J.-F., Ablain, M., Zawadzki, L., Zuo, H., Johannessen, J. A., Scharffenberg, M. G., Fenoglio-Marc, L., Fernandes, M. J., Andersen, O. B., Rudenko, S., Cipollini, P., Quartly, G. D., Passaro, M., Cazenave, A., and Benveniste, J.: An improved and homogeneous altimeter sea level record from the ESA Climate Change Initiative, Earth Syst. Sci. Data, 10, 281-301, https://doi.org/10.5194/essd-10-281-2018 , 2018.

Le Traon, P.-Y. and F. Ogor: ERS-1/2 orbit improvement using TOPEX/POSEIDON: the 2 cm challenge. J. Geophys. Res., 103, 8045-8057, 1998.

Le Traon, P.-Y, Faugere, Y., Hernamdez, F., Dorandeu, J., Mertz, F., and Ablain, M.: Can We Merge GEOSAT Follow-On with TOPEX/Poseidon and ERS-2 for an Improved Description of the Ocean Circulation?, J. Atmos. Ocean. Technol., 20, 889–895,2003.

Lievin M., Kocha C., Courcol B., Philipps S., Denis I., Guinle T., Nogueira C., Loddo, Dibarboure G., Picot N., Bignalet-Cazalet F.: REPROCESSING of SEA LEVEL L2P products for 28 years of altimetry missions, Presentation OSTST2020,  https://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/ files/OSTST2020_Reprocessing_L2P_2020.pdf (last access: 15 June 2021),  2020.

Mertz F., F. Mercier, S. Labroue, N. Tran, J. Dorandeu, 2005: ERS-2 OPR data quality assessment; Long-term monitoring – particular investigation. CLS.DOS.NT-06.001

Mulet, S., Rio, M.-H., Etienne, H., Artana, C., Cancet, M., Dibarboure, G., Feng, H., Husson, R., Picot, N., Provost, C., and Strub, P. T.: The new CNES-CLS18  global mean dynamic topography, Ocean Sci., 17, 789–808, https://doi.org/10.5194/os-17-789-2021z , 2021.

Ole Baltazar Andersen, G. Piccioni, L. Stenseng and P Knudsen. The DTU15 MSS (Mean Sea Surface) and DTU15LAT (Lowest Astronomical Tide) reference surface.https://ftp.space.dtu.dk/pub/DTU15/DOCUMENTS/MSS/DTU15MSS+L AT.pdf (last access 2021/06/15)

Pujol, M.-I., Faugère, Y., Taburet, G., Dupuy, S., Pelloquin, C., Ablain, M., and Picot, N.: DUACS DT2014: the new multi-mission altimeter data set reprocessed over 20 years, Ocean Sci., 12, 1067-1090, doi:10.5194/os-12- 1067-2016, 2016 http://www.ocean-sci.net/12/1067/2016/os-12-1067- 2016.pdf

Pujol, M.‐I., Schaeffer, P., Faugère, Y., Raynal, M., Dibarboure, G., & Picot, N. ( 2018). Gauging the improvement of recent mean sea surface models: A new approach for identifying and quantifying their errors. Journal of Geophysical Research: Oceans, 123, 5889– 5911. https://doi.org/10.1029/2017JC013503

Quartly, G. D., Legeais, J.-F., Ablain, M., Zawadzki, L., Fernandes, M. J., Rudenko, S., Carrère, L., García, P. N., Cipollini, P., Andersen, O. B., Poisson, J.-C., Mbajon Njiche, S., Cazenave, A., and Benveniste, J.: A new phase in the production of quality-controlled sea level data, Earth Syst. Sci. Data, 9, 557– 572, https://doi.org/10.5194/essd-9-557-2017 , 2017.

Sandwell D., Schaeffer P., Dibarboure G., Picot N., (2017). High Resolution Mean Sea Surface for SWOT. https://spark.adobe.com/page/MkjujdFYVbHsZ/ & http://doi.org/10.24400/527896/a01-2021.004 (last access 2021/06/15)

Scharroo R., W H. F. Smith, A global positioning system–based climatology for the total electron content in the ionosphere, J. Geoph. Res., Vol. 115, Issue A10, 2010, https://doi.org/10.1029/2009JA014719

Schaeffer P., I. Pujol, Y. Faugere, A. Guillot, N. Picot, The CNES CLS 2015 Global Mean Sea surface. Presentation OSTST 2016, http://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/fi les/GEO_03_Pres_OSTST2016_MSS_CNES_CLS2015_V1_16h55.pdf (last access 2021/06/15)

Taburet, G., Sanchez-Roman, A., Ballarotta, M., Pujol, M.-I., Legeais, J.-F., Fournier, F., Faugere, Y., and Dibarboure, G.: DUACS DT-2018: 25 years of reprocessed sea level altimeter products, Ocean Sci., 15, 1207–1224, 2019, https://doi.org/10.5194/os-15-1207-2019

Tran, N., Labroue, S., Philipps, S., Bronner, E., and Picot, N.: Overview and Update of the Sea State Bias Corrections for the Jason-2, Jason-1 and TOPEX Missions, Mar. Geod., 33, 348–362, 2010.

Tran N., Philipps, S., Poisson, J.-C., Urien, S., Bronner, E., and Picot, N.: Impact of GDR_D standards on SSB corrections, Presentation OSTST2012 in Venice, http://www.aviso.altimetry.fr/fileadmin/documents/OSTST/2012/oral/02_frid ay_28/01_instr_processing_I/01_IP1_Tran.pdf (last access: 2021/06/15), 2012.

Tran N, D. Vandemark, H. Feng, A Guillot and N Picot: “Updated wind speed and sea state bias models for Ka-band altimetry”, poster in 2014 SARAL/Altika workshop, 2014.  https://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/ files/Poster_PEACHI_ssb_tran2014.pdf (last access 2021/06/15)

Tran N.: “Envisat ESL Phase-F: Sea State Bias”, Technical report CLS-DOS-NT- 15-031, 1rev0-07/05/2015.

Tran N., D. Vandemark, E. D. Zaron, P. Thibaut, G. Dibarboure, et N. Picot, « Assessing the effects of sea-state related errors on the precision of high-rate Jason-3 altimeter sea level data », Adv. Space Res., vol. 68, no 2, p. 963‑977, juill. 2021, doi: 10.1016/j.asr.2019.11.034.

WCRP Global Sea Level Budget Group: Global Sea Level Budget 1993-Present, Earth Syst. Sci. Data Discuss., https://doi.org/10.5194/essd-2018-53 , 2018.

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.

 Copernicus Climate Change Service

10. Related articles

• Page:
  Global sea level change indicators from 1950 to 2050 derived from reanalysis and high resolution CMIP6 climate projections: Product User Guide
• Page:
  Global sea level change time series from 1950 to 2050 derived from reanalysis and high resolution CMIP6 climate projections: Product User Guide
• Page:
  Sea Level version DT2021: Product User Guide and Specification (PUGS)
• Page:
  Sea Level (SL)
• Page:
  Sea level change