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Preamble

History of modifications

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title	Click 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	06 Oct 2022	Addition of monthly product
1.3	05 Jan 2023	Addition of Sentinel-6A mission
1.4	27 Mar 2023	Corrections after review and finalised for publication	all sections

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.

Acronyms

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title	Click here to see the acronyms list

Acronym	Definition
ADT	Absolute Dynamic Topography
AMR	Advanced Microwave Radiometer
BM3	Electromagnetic Bias 3 parameters
BM4	Electromagnetic Bias 4 parameters
C3S	Copernicus Climate Change Service
CCI	ESA's Climate Change Initiative
CDR	Climate Data Record
CLS	Collecte Localisation Satellite
CMEMS	Copernicus Marine and Environment Service
CMS	Copernicus Marine Service (ex CMEMS)
CNES	Centre National d'Etudes Spatiales
DORIS	Doppler Orbitography and Radiopositionning Integrated by Satellite
DTU	Technical University of Danemark
DUACS	Data Unification and Altimeter Combination System
ECMWF	European Centre for Medium-Range Weather Forecasts
EKE	Eddy Kinetic Energy
ERA5	ECMWF Reanalysis v5
ERS-1	European Remote-Sensing-1
ERS-2	European Remote-Sensing-2
ESA	European Space Agency
EUMETSAT	European Organisation for the Exploitation of Meteorological Satellites
FES	Finite Element Solution
GDR	Geophysical Data Record
GIA	Glacial Isostatic Adjustment
GIM	Global Ionosphere Maps
GNSS	Global Navigation Satellite Systems
GPD+	GNSS derived Path Delay Plus
GSFC	Goddard Space Flight Center
ICDR	Interim Climate Data Record
IGDR	Interim Geophysical Data Record(s)
JMR	Jason-1 Microwave Radiometer
L2	Level 2
L2P	Level-2 Plus
L3	Level 3
L4	Level 4
LWE	Long Wavelength Errors
MDT	Mean Dynamic Topography
MOG2D	2 Dimensions Gravity Waves model
MSL	Mean Sea Level
MSS	Mean Sea Surface
MWR	MicroWave Radiometer
NIC	New Ionosphere Climatology
NTC	Non Time Critical
POE	Precise Orbit Ephermeris
PUGS	Product User Guide and Specification
SARAL	Satellite with ARgos and ALtika
SLA	Sea Level Anomaly
SIO	Scripps Institution of Oceanography
SSB	Sea State Bias
SSH	Sea Surface Height
TUGO	Toulouse Unstructured Grid Ocean model
WCRP	World Climate Research Programme

General definitions

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title	Click here to see the general definitions

	General definitions
SSH	is the Sea Surface Height above the reference ellipsoid measured by altimeters.
MSS	is the Mean Sea Surface above the reference ellipsoid and is calculated thanks to the SSH.
MDT	is the Mean Dynamic Topography, i.e. the Mean SSH above the geoid. It is estimated from a combination of MSS and a geoid model.
SLA	is the Sea Level Anomaly. It is given by the difference between SSH and MSS.
ADT	is the Absolute Dynamic Topography above the geoid. It is given by the sum of SLA and MDT.
L4	(Level 4) gridded products are spatially complete global maps combining cross-calibrated altimeter measurements from several missions using an optimal interpolation. These variables are fully described in section 4.1.

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.

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).
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.

Product description

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

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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.

Common variables in Altimetry

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

LaTeX Formatting
Eq (1) $SSH = Orbit - Altimetric Range$

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

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

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.

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¹The geostrophic velocities are directly derived from the first derivative of the altimeter heights and correspond to the balance between the forces of pressure and those related to the Earth's rotation (the ageostrophic part of the velocities linked to acceleration is not included).

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):

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LaTeX Formatting
Eq (6) $ EKE= (UU+VV)*1/2 $

Processing

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

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

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.

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

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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.

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Table 2: Altimeter standards used in the C3S sea level vDT2021 product.

	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	MOG2D High frequencies forced with analysed ECMWF pressure and wind field [Carrere and Lyard, 2003; operational version 3.2.0] + inverse barometer Low frequencies
Ocean Tide	FES 2014 B [Carrère et al. 2016]
Internal Tide	Zaron 2019 (HRETv8.1 tidal frequencies: M2, K1, S2, O1)
Pole Tide	Desai et al., 2015 ; Mean Pole Location 2017
Solid Tide	Elastic response to tidal potential [Cartwright and Tayler, 1971; Cartwright and Edden, 1973]
Mean Sea Surface	Composite (SIO,CNES/CLS15,DTU15) [Sandwell et al.,2017 ; Ole et al.; Pujol et al.,2018]
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

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Warning

title	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], 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.

Altimetry constellation

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

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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.

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Note
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 section 3.1 of [C3S_PQAR]).

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.

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.

Note

Note that the proposed correction of the TOPEX-A instrumental drift has been chosen so that the correction is null after the end of the lifetime of the TOPEX mission in 1999 (dashed line in Figure 4 and in Figure 5). With this approach, the corrected Global MSL does not equal to zero in 1993. This approach is the preferred approach to ensure the continuity of the initial and corrected GMSL after 1999 (e.g. for ocean modellers).

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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).

Specifications and target requirements

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:

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

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

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^* ERS-1: No ERS-1 data between 23 December 23,1993 and April 10, 1994 (ERS-1 phase D - 2nd ice phase).

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].

Data usage information

Grid characteristics

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

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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.

Format

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

File nomenclature

Daily product

The nomenclature of the file is the following:

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

Monthly product

The nomenclature of the file is the following:

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

Data Handling Variables

Daily sea level product

4 dimensions are defined:

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The variables are listed in Table 5:

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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:

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The variables are listed in Table 6:

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	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
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

Appendix A - Specifications of the daily sea level product

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title	Click here to see specifications of the daily sea level product

Code Block

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" ;
}

Appendix B - Specifications of the monthly sea level product

Expand

title	Click here to see specifications of the monthly sea level product

Code Block

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" ;
}

Anchor
references
references
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.

...

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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 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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Acronyms

General definitions

Scope of the document

Executive summary

Product description

Common variables in Altimetry

Variables in the daily sea level product

Variables in the monthly sea level product

Processing

Input data and corrections

Altimetry constellation

Gridded merged product generation

Mean and reference period

Specifications and target requirements

Spatial and temporal coverage

Validation and uncertainty estimates

Data usage information

Grid characteristics

Format

File nomenclature

Daily product

Monthly product

Data Handling Variables

Daily sea level product

Monthly sea level product

Appendix A - Specifications of the daily sea level product

Appendix B - Specifications of the monthly sea level product

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References

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Space shortcuts

Page tree

Page History

Versions Compared

Old Version 1

New Version Current

Key

Preamble

History of modifications

List of datasets covered by this document

Related documents

Acronyms

General definitions

Scope of the document

Executive summary

Product description

Common variables in Altimetry

Variables in the daily sea level product

Variables in the monthly sea level product

Processing

Input data and corrections

Altimetry constellation

Gridded merged product generation

Mean and reference period

Specifications and target requirements

Spatial and temporal coverage

Validation and uncertainty estimates

Data usage information

Grid characteristics

Format

File nomenclature

Daily product

Monthly product

Data Handling Variables

Daily sea level product

Monthly sea level product

Appendix A - Specifications of the daily sea level product

Appendix B - Specifications of the monthly sea level product

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Related articles

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References