Contributors: F. Mertz (CLS), G. Taburet (CLS), J.-F. Legeais (CLS)
Issued by: CLS / F. Mertz, G. Taburet and J.-F. Legeais
Date:
Ref: WP2-FDDP-2024-11_C3S2-312b-PUGS-of-vDT2024-SeaLevel-products_v1.1
Official reference number service contract: 2022/C3S2_312b_MOi/SC1
1. Preamble
1.1. History of document modifications
1.2. List of datasets covered by this document
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 DT2024 sea level products, distributed through the Copernicus Climate Change Service (C3S). This document has been developed within the frame of the 2022/C3S2_312b_MOi/SC1 contract. It provides the end user with practical information regarding the use of these products.
3. Executive summary
The current version of the Copernicus Climate Change (C3S) sea level product is the Data Unification and Altimeter Combination System (DUACS) version DT2024 reprocessed delayed-time altimeter sea level products. Compared to the former version (vDT2021), new input L2P altimeter standards (named vDT2024) following expert recommendations (Kocha et al., 2023) have been used for measurements of the entire time series (see section 4.2.1).
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 variables provided in the datasets are listed and shown on figures. The processing chain is detailed in [C3S_ATBD]. Here, the upstream specific elements used for Sea level vDT2024 datasets are given. These include the list of missions, the input data including their origin and the corrections applied to the altimeter measurement (geophysical standards and orbit solutions). Moreover, the technical characteristics of each altimeter mission used in the production system are described. Finally, the product characteristics are detailed (format, nomenclature, data handling variables and access details). Additionally, a description of the file content is provided in the Annexes.
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 Data Unification and Altimeter Combination System (DUACS; a processing system) and includes data from several altimetry missions.
- A time series of monthly gridded Sea Surface Height (SSH) and derived variables obtained by computing monthly means of daily product.
The sea level products coverage is 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 section 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 vDT2024 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:
SSH = Orbit - Altimetric Range (Eq.1)
The Mean Sea Surface (MSSN) 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.
MSSN = <SSH>N (Eq.2)
The Sea Level Anomaly (SLAN) in Eq.3 is the anomaly of the signal around the mean component. It is deduced from the SSH and MSSN :
SLAN = SSH - MSSN (Eq.3)
The Mean Dynamic Topography (MDTN) in Eq.4 is the temporal mean of the SSH above the Geoid over a period N.
MDTN = MSSN - Geoid (Eq.4)
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 SLAN and MDTN:
ADT = SLAN + MDTN = SSH - MSSN + MDTN (Eq.5)
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 three-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.
Figure 2: Sea Level Anomaly (sla; Panel A), Absolute Dynamic Topography (adt; Panel B) and formal error (err_sla; Panel C) in m for June 11th 2022.
Figure 3: Geostrophic velocities anomalies with zonal (ugosa; Panel A) and meridian (vgosa; Panel B) components
and associated formal mapping error for zonal (err_gosa; Panel C) and meridian (err_vgosa; Panel D) components in m/s for June 11th 2022.
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:
EKE = 1/2 * (U2 + V2) (Eq.6)
Figure 4: Sea Level Anomaly (sla; Panel A) in m and Eddy Kinetic Energy (eke; Panel B) in cm2/s2 for the month of June 2022.
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
Apart from the processing, additional information is provided in this document and consists in the input data and the different corrections used to compute this particular DT2024 version. This is detailed in section section 4.2.1
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.
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 are named DT-2024 standards. 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 Kocha et al., 2023.
Table 2: Altimeter standards used in the C3S sea level vDT2024 product as described in Kocha et al., 2023. Bold text indicates the standards that have been enhanced compared to previous version (DT2021: Lievin et al., 2020).
Poseidon Topex | Jason-1 | OSTM/Jason-2 | Jason-3 | ERS-1 | ERS-2 | Envisat | Cryosat-2 | SARAL AltiKa | Sentinel-3A | Sentinel-6 MF | |
Product standard ref | GDR-F | GDR-D | GDR-F | GDR-F |
| BC05 | F09 | ||||
Retracking | MLE3 | MLE4 | MLE4 | MLE4 | OPR | MLE3 (OCE-1) | SAMOSA-3 | MLE4 | SAMOSA | Numerical LRM | |
Orbit | GSFC STD18 | POE-F | POE-F | POE-F | Reaper | POE-F | POE-F | POE-F | POE-F | POE-F | |
Ionospheric Correction | Filtered dual-frequency altimeter range measurements [Ablain and Legeais, 2010] on Topex; DORIS on Poseidon | Filtered dual-frequency altimeter range measurements [Guibbaud et al. 2015] | Filtered dual-frequency altimeter range [Guibbaud et al. 2015] (from SSB C-band) | Filtered dual-frequency altimeter range measurements GDR-F [Nencioli et al., 2022] | 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 dual-frequency range from L2 | Filtered dual-frequency altimeter range [Guibbaud et al. 2015] | |
Sea State Bias | 2D Non parametric [Putnam et al., 2023] on Topex; BM4 on Poseidon | Non parametric [Tran 2015] | Non parametric [ Tran 2012] | 2D Non parametric [Bignolet-Cazalet et al., 2021] | 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 et al 2021] | 2D Non parametric [Bignolet-Cazalet et al., 2021] J3 MLE4 GDR-F |
Wet Troposphere | TMR GDRF radiometer reprocessing | JMR (GDRE) radiometer | AMR radiometer GDR-F | AMR radiometer GDR-F | GPD+ [Fernandes et al., 2015] | MWR radiometer reprocessed | GPD+ [Fernandes and Lazaro, 2016] | Neural Network (5 entries) V4 | GPD+ [Fernandes et al., 2015] | AMR radiometer | |
Dry Troposphere | ERA5 (1-hour) model based | ||||||||||
Dynamical Atmospheric Correction | TUGO High frequencies forced with analysed ERA5 pressure and wind field before S6A era and ECMWF field after+ inverse barometer Low frequencies | ||||||||||
Ocean Tide | FES22b [Carrère et al., 2023] | ||||||||||
Internal Tide | Zaron 2019 (HRETv8.1 tidal frequencies: M2, K1, S2, O1) | ||||||||||
Pole Tide | Desai et al., 2015 ; Mean Pole Location 2017 [Ries and Desai, 2017] | ||||||||||
Solid Tide | Elastic response to tidal potential [Cartwright and Tayler, 1971; Cartwright and Edden, 1973] | ||||||||||
Mean Sea Surface | Hybrid MSS 2023 (SIO22; CNES/CLS22, DTU21) [Laloue et al., 2024] | ||||||||||
Mean Dynamic Topography | MDT_CNES_CLS22 [Jousset et al., 2023] combined with regional CMEMS_2020 Mediterranean and Black Seas [Jousset et al., 2022; Jousset and Mulet, 2020] | ||||||||||
Glacial Isostatic Adjustment (GIA) | The DUACS L4 products are not corrected from GIA effects |
Warning:
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]) 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. For the vDT2024 data, the empirical correction is not yet computed. Consequently, the variable in the files are filled with default value, it will be updated once the correction is available.
4.2.2. Altimetry constellation
The complete altimetry satellite constellation used in the C3S sea level product is illustrated in figure 5.
Figure 5: 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 secondary missions which can be complementary missions or a mission of opportunity as 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 calibrate 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 as illustrated in figure 6. 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 6: Satellite constellation in the C3S time series.
Note that the information about the satellites used to compute each map is given in the global attribute "platform" of each file. The use of such a constant number in the satellite constellation contributes to ensuring the long- term Mean Sea Level (MSL) stability, which is not the case when using all satellites available throughout the altimeter period (see [C3S_PQAR]).
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 [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 7). 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. The most recently calculated Global Mean Sea Level (1993 to end of 2023) is displayed in figure 8.
Figure 7: 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.
Figure 8: Global mean sea level progression for the period 1993-2023 (without GIA correction) deduced from DUACS L4 vDT2024 gridded products.
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 2 times per year (ICDR production), and the timeliness of product availability from measurement acquisition is approximately 5 months at a 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.
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 lost after cycle 65) | Yes | 2002/05/14 | 2010/10/18 | |
Envisat-New Orbit | 30 | ±81.5 | 862 | - | 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.
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.
Area | Latitude coverage | Longitude coverage |
Global Ocean | 90°S/90°N | 0°/360° |
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 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.
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>_vDT2024.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>_vDT2024-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 (see figures in section 4.1.1).
Type | Name | Content | Unit | Scale Factor |
float | time (time) | Time of measurement | days since 1950-01-01 | 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 | This correction is not yet available for the current product version (values filled with 'default_value'). This field will be updated once this correction is produced. 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) | 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 |
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 (see figures in section 4.1.2).
Type | Name | Content | Unit | Scale Factor |
float | time (time) | Time of measurement | days since 1950-01-01 | 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 |
float | climatology_bnds (time,nv) | Useful for grid definition | meters | |
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 | eke (time,latitude,longitude) | Eddy Kinetic Energy | cm2/s2 | 10-4 |
7. Data access information
7.1. CDS Catalogue
The dataset is called "Sea level gridded data from satellite observations for the global ocean from 1993 to present" in the Climate Data Store (CDS). It can be search easily in the CDS Catalogue by entering 'Sea Level' or the full name.
The direct access to information is https://cds.climate.copernicus.eu/datasets/satellite-sea-level-global?tab=overview.
7.2. CDS Download
Data download can be initiatied here: https://cds.climate.copernicus.eu/datasets/satellite-sea-level-global?tab=download
Three versions are provided to users: the current vDT2024 version which is regularly updated, and the former v2021 and v2018 versions which are no longer being extended.
The user can select daily or monthly products, and the desired period (it is possible to to select all years, or days, or months).
A zip file can then be downloaded containing the desired days or months files in NetCDF format.
Examples of the file header contents are shown in Appendix A and Appendix B.
7.3. CDS Documentation
The documentation about Sea Level datasets (ATBD, TR-GAD, PQAR, etc.) for each version can be found in
https://cds.climate.copernicus.eu/datasets/satellite-sea-level-global?tab=documentation.
8. Appendix A - Specifications of the daily sea level product
9. Appendix B - Specifications of the monthly sea level product
10. References
Ablain, M. and Legeais, J. F.: SLOOP Tache 2.4 : Amélioration du filtrage de la correction ionospherique bifréquence, 2010.
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.
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