Contributors: Sebastian B. Simonsen and Natalia Havelund (Technical University of Denmark), Thomas Slater and Athul Kaitheri (Northumbria University)

Issued by: Technical University of Denmark / Sebastian B. Simonsen, Northumbria University / Thomas Slater

Date:19 Dec 2024

Ref: C3S2_313d_ENVEO.WP2-DDP-SEC-GIS-AIS-01_202412_PUGS_v1.3

Official reference number service contract: 2024/C3S2_313d_ENVEO/SC1

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History of modifications

List of datasets covered by this document

Acronyms 

General definitions  

Backscatter: The portion of the outgoing radar signal that the target redirects directly back toward the radar antenna.  

Baseline:  A combination of processor versions, auxiliary data and other needed enablers that allows the generation of a coherent set of Earth observation products.

Bias: The tendency of an instrument to preferentially make measurements over a certain type of surface.

Climate Data Record (CDR): A time series of measurements of sufficient length, consistency, and continuity to determine climate variability and change. 

Crossover analysis: A method for deriving elevation change at locations where the orbits of a single or multiple satellites cross.  

Cross-calibration: A method that merges datasets from multiple satellites into one consistent dataset.   

Interim Climate Data Record (ICDR): An updated time series of measurements between the current CDR and the next CDR.

Key performance indicator: A quantifiable measure used to evaluate the success of a product in meeting its performance objectives.

Laser altimeter: An instrument mounted on an aircraft or spacecraft that measures altitude from the ground surface below by timing how long it takes a pulse of laser light to travel to the ground, reflect, and return to the craft. 

Radar Altimetry: Radar altimetry measures the height of a satellite or aircraft above a land or ocean surface by timing how long it takes a pulse of radio waves to travel to the surface below, reflect, and return to the craft.

Stereographic: Stereographic is a planar perspective projection, viewed from the point on the globe opposite the point of tangency. 

Stability: An estimate of the consistency of the measurements over time. 

Surface Elevation Change (SEC): The surface elevation of a point on an ice sheet is the height of the ice sheet surface above a reference geoid (a hypothetical solid figure whose surface corresponds to mean sea level and its imagined extension under land areas). An increase in surface elevation over time at a given location indicates a gain of ice or snow at that location, and conversely, a decrease indicates a loss. The surface elevation change product provides the rate of change given at monthly intervals at each location on a grid covering the ice sheet. The definition of the grid projection includes the geoid used. Given the rates of change, absolute change can be calculated for any time period.  

Tracking: Retrieving the radar echo from a given radar pulse.   

Uncertainty: An estimate of the error in a measurement, due to limitations in the measuring instrument or statistical fluctuations in the quantity being measured. 

Validation: Comparison between two independent datasets to test their agreement. 

Executive summary

This document is the Product User Guide and Specification (PUGS) for the version 5.0 (Antarctica) and 6.0 (Greenland) of the Surface Elevation Change (SEC) products made as part of the Copernicus Ice Sheets and Ice Shelves service. The products contain geographically gridded time series of the rate of change of ice sheet surface elevation in Antarctica and Greenland, from 1992 to the present with a 3 month lag. This document describes the version 6.0 SEC product over Greenland and version 5.0 SEC product over Antarctica in a manner that is understood by the product user with focus on the geophysical data product content, its structure and format, known limitations and data access. 

Each version of a Climate Data Record (CDR) is fully reprocessed with updated data and/or new methods, and each Interim Climate Data Record (ICDR) is added to that CDR, extending its temporal coverage. 

We document here the description of each of the Climate Data Records (CDRs) for the two Polar region SEC products, with a guide to their usage and specifications.

In section 1 (for Antarctica) and section 2 (for Greenland) we describe the products and their inputs, update schedules and validations. We list the target requirements for data quality. We tabulate the contents of the data product files, explain known issues with the dataset, and describe the file-naming conventions used. The data files are produced in netCDF format, and we give examples of their structure and metadata contents. 

In section 3 we give details of public access to the products, licencing, and how to make enquiries about them. 

The products are hosted on the Copernicus Climate Data Store.

Product Change Log

The same product changes apply to both Antarctic and Greenland products until version 4.0. Production of Antarctic SEC product was halted during version 5.0 of Greenland SEC. Hence the current release is version 5.0 of Antarctic SEC and version 6.0 of Greenland SEC product.

Surface elevation change, Antarctica

Product Description

The product contains surface elevation change (SEC) rates and their uncertainties from the Antarctic ice sheet, ice shelves, ice rises, and islands on a regular geographic grid at regular (monthly) time intervals. The change rate is calculated over a 5-year period.

The current version 5.0 of the Antarctica CDR product builds on the legacy of its preceding versions: 

CDR version 1.0    The initial product release builds on the R&D efforts of the Antarctic ice sheet Climate Change Initiative (CCI) project’s surface elevation measurements from the European Remote-sensing Satellite (ERS)1, ERS2, Envisat and CryoSat-2.  

CDR version 2.0    Adds the use of observations from Sentinel-3A.  

CDR version 3.0    The time series have been reprocessed, with the upgraded baselines for Envisat and CryoSat-2 data and with the inclusion of data from Sentinel-3B.  

CDR version 4.0    The time series have been reprocessed, with the upgraded data filtering.  

CDR version 5.0   The time series have been reprocessed, with the full mission processing of Baseline E for CryoSat-2 and land ice thematic product for Sentinel-3. 

The product is updated monthly as an Interim Climate Data Record: ICDR. Due to the time taken for processing the raw data and its release, there is a time lag of 2 months between the acquisition of new measurements and their addition to the SEC product (ICDR or CDR). CDRs are produced and released when there is a considerable improvement or update in the existing SEC production methodology. This is followed by designing a validation strategy and carrying out the validation. Whereas, ICDRs are timely updates over the previously submitted CDR and act as an interim record until the release of the next CDR. These ICDRs are produced and submitted at 3-month intervals. The product specifications are summarized in Table 1. 

Table 1: Antarctic SEC product specification summary

Figure 1 below illustrates the timeline of missions or mission phases that were used for producing the Antarctic SEC dataset.

Figure 1: Timeline of the Radar Satellite Missions for producing the Antarctic SEC dataset.

Validation is performed with respect to observation campaigns by the Airborne Topographic Mapper, a scanning laser altimeter flown on board aircraft by Operation IceBridge (Studinger 2014), mission webpage (https://nsidc.org/data/icebridge). Each campaign lasts for one Antarctic Spring season (October to December), and so validation is performed annually. The same Operation IceBridge validation has been performed for each version of the product, but a second validation against ICESat-2's satellite-mounted laser altimeter has been performed since v4.0 of the CDR, mission webpage at https://nsidc.org/data/icesat-2. ICESat-2 was launched in 2018 and can only be used to validate recent data. Details of methodology may be found in the related document, the Product Quality Assessment Report. 

Details of the product file format and contents are below. Details of the algorithmic steps used to create the product can be found in the related document, the  Algorithm Theoretical Basis Document. 

Target requirements

Targets are set by two separate bodies. The Global Climate Observing System (GCOS) maintains definitions of ECVs and their requirements (https://gcos.wmo.int/en/essential-climate-variables/ice-sheets-ice-shelves/ecv-requirements) and the Copernicus Climate Change Service (C3S) project itself provides key performance indicator targets. 

Details of the CDR version 5.0 performances against these targets may be found in the related document, the Product Quality Assessment Report. 

The requirements summarised for the SEC products, based on the combined GCOS and C3S performance indicator targets are shown in Table 2. These apply to both Antarctic and Greenland products. More information about the target requirements and the gaps within the current product characteristics can be found in the Target Requirement and Gap Analysis Document. 

Table 2: SEC products targets and performance indicators 

Example visualization of key variables

An example plot from one data epoch is shown in Figure 2 below. The figure represents the average rate of surface elevation change in Antarctica during 1992-2024.

Figure 2: Example data from Antarctic surface elevation change product WP2-FDDP-SEC-CDR-AntIS averaged over the period 1992 to 2024

Data usage information

Data format and file naming 

The product is provided as a netCDF file (CF-1.7 compliant) containing stacked grids of the surface elevation change rate and associated uncertainty, and validity flags. The grids cover the Antarctic ice sheets, shelves, rises and islands at 25km x 25km resolution in a polar stereographic projection with centre longitude 0E and true scale latitude 71S, i.e. EPSG 3031 (https://epsg.io/3031). There is one grid per month, each cell containing the rate of surface elevation change derived from a 5-year period centred on that grid's timestamp. This timestamp starts from November 1994 and goes up to March 2022 which are respectively the midpoints of the first (May 1992 to April 1997) and last (October 2019 to September 2024) 5-year windows where observations are available to estimate SEC. The change rate and its uncertainty are given in m/year. Missing data is indicated by a floating-point NaN (not a number) value. Single-layer flag grids are provided for surface ice-cover type and slope range on the same projection as the data. The main variables are listed in Table 3 below. 

Table 3: Antarctic surface elevation change main data variables. 

The dataset filename format is 

C3S_AIS_RA_SEC_ggkm_versx_YYYYMM-yyyymm_yyyy-mm-dd.nc, where: 

• C3S is the overall project 

• AIS indicates the Antarctic ice sheet region 

• RA indicates that the data source is radar altimetry 

• SEC is the ECV addressed, i.e. surface elevation change 

• gg is the grid resolution in km 

• versx is the version number of the processing code 
• YYYYMM-yyyymm is the start and end of the dataset (start: YYYYMM, end: yyyymm)

• yyyy-mm-dd is the creation date of the dataset 

The latest file created should always be used, as the files are accumulative – each one contains all previous data as well as its monthly updates. 

Quality flags and data masks 

Quality flags are used to denote whether valid data exists in a grid cell or not. Flags are also used to distinguish surface types and geographical slope classes. 

File contents 

The header data for an example netCDF data file is given in Table 4. 

 Table 4: Sample of the structure of the provided NetCDF file for the Antarctic surface elevation change

Examples of known climate applications and best practices 

SEC product finds several applications across scientific research, environmental monitoring, and policy-making. It is used in sea level rise studies to make estimates of ice sheet melt contributions. The product is also used as a long-term record input to validate and calibrate models simulating ice sheet and glacier dynamics. Additionally, the SEC product is also used for mass balance studies as in Shepherd et al, 2012.

Known Issues and Limitations

Please note: the gridded data and its uncertainties are well-defined at pixel level. No smoothing has been applied across the geographic grid. If the pixels are to be combined to drainage basin level then the user should be sure to consider the varying terrain and ice dynamics within the basin - a simple mean value will not be representative. 

Surface elevation change, Greenland

Product Description

The product contains surface elevation change rates and their associated uncertainties for the Greenland ice sheet and is provided on a regular grid at a monthly temporal resolution. The basis for the elevation change estimate for the older satellites (ERS-1, ERS-2, and Envisat) is a running 5-year mean, whereas for the ongoing satellite missions (Cryosat-2, Sentinel-3 A and B) the elevation change estimate is based on the monthly evaluation of a 3-year baseline due to better mission performances in the newer satellites. An example plot from one data grid is shown in Figure 3.

The current version 6.0 of the CDR product builds on the legacy of its preceding versions:

CDRv1.    The initial product release builds on the Research and Development efforts of the Greenland ice sheet Climate Change Initiative (CCI) project’s surface elevation measurements from ERS1, ERS2, Envisat, and CryoSat-2. 

CDRv2.    Adds the use of observations from Sentinel-3A. 

CDRv3.    The time series have been reprocessed, with the upgraded baselines for Envisat and CryoSat-2 data and with the inclusion of data from Sentinel-3B. 

CDRv4.    The time series have been reprocessed, with the upgraded data filtering. 

CDRv5.    The time series have been reprocessed, with the Thematic data product from Sentinel-3.

CDRv6.    The time series have been reprocessed, and a new version of ArcticDEM has been included

Similar to its predecessors,  the current CDRv6 will be updated every third month as iCDRs, with a 3-month time lag between the acquisition of new measurements and their addition to the product. The product specifications are summarised in Table 5, the specific time periods for each sensor can be seen in Figure 1.

Table 5: Greenland SEC product specification summary

Validation is performed with respect to airborne measurements collected with the Airborne Topographic Mapper, a scanning laser altimeter, during Operation IceBridge campaigns (Studinger 2014), mission webpage at https://nsidc.org/data/icebridge. Validation is performed annually since Arctic campaigns take place once a year during the spring season (March-May). Additionally, independent data from the National Aeronautics and Space Administration (NASA) Ice, Cloud and Elevation Satellite (ICESat)-2 mission is used for validation after the termination of Operation IceBridge campaigns in 2019. 

Details of the product file format and contents are provided in the following section. Details of the methodology may be found in the related document, the Algorithm Theoretical Basis Document.

Target requirements

Targets are set by two separate bodies. The Global Climate Observing System (GCOS) maintains definitions of Essential Climate Variables (ECVs) and their requirements at https://gcos.wmo.int/en/essential-climate-variables/ice-sheets-ice-shelves/ecv-requirements  

and the Copernicus Climate Change Service (C3S) project itself provides key performance indicator targets.

Details of the CDR v6.0 performances against these targets may be found in the related document, the Product Quality Assessment Report.

The requirements summarized for the SEC products, based on the combined GCOS and C3S performance indicator targets are shown in Table 6. More information about the target requirements and the gaps within the current product characteristics can be found in the Target Requirement and Gap Analysis Document.

Table 6: SEC products targets and performance indicators

Example visualization of key variables

An example plot from one data epoch is shown in the Figure 3  below, illustrates the accumulated SEC from 1992-2025 [m]. 

Figure 3: Example accumulated (in meters) surface elevation change map produced from version 6.0 Surface Elevation Change product from 1992 to 2025

Data usage information 

Data format and file naming 

The dataset filename format is:
C3S_GrIS_RA_SEC_ggkm_versx_YYYYMM-yyyymm_yyyy-mm-dd.nc, where:

• C3S is the overall project
• GrIS indicates the Greenland ice sheet region
• RA indicates that the data source is radar altimetry
• SEC is the ECV addressed, i.e. surface elevation change
• gg the grid size in km
• versx is the version number of the processing code
• YYYYMM-yyyymm is the start and end of the dataset (start: YYYYMM, end: yyyymm)
• yyyy-mm-dd is the creation date of the dataset

The latest file created should always be used, as the files are cumulative – each contains all previous data and its three monthly updates.

The product is provided as a netCDF (CF 1.6 compliant) file containing stacked grids of the surface elevation change rate and associated uncertainty, and validity flags. Given in a north polar stereographic projection, with center longitude 45W and latitude 70N, the grids cover the Greenland ice sheet at 25 km x 25 km. Solutions for the Greenland surface elevation change are given at a temporal resolution of one month, which combined with the 25 km grid gives a data array spanning the dimension of (65x123x364)¹. The change rate and its uncertainty are given in m/year. Missing data is indicated by a floating-point NaN (not a number) value. Single-layer flag grids are provided for surface type and high slope. All single-layer flags are gridded using the same north-polar stereographic projection as the data. The main variables are listed in Table 7.

The data period for this dataset is determined by the "time" variable. Users should always reference the SEC data (dh/dt) together with the "time" variable for correct interpretation.

The "start_time" and "end_time" fields are processing outputs that indicate the background time period used to produce the SEC data. To avoid misinterpretation, users should rely solely on the "time" variable when analyzing the dataset.

Table 7: Greenland surface elevation change main data variables.

Quality flags and data masks 

Quality flags are provided to show the slope in the grid cell and the geographical surface type on which the data are located. Also, a distance flag can be used to see how far away the raw data points used to derive the dhdt are from the grid center. 

File contents 

The header data for an example netCDF data file is given in Table 8.

Table 8: Example of the NetCDF file structure for the Greenland Ice Sheet Surface elevation change based on the December 2024 version 6.0 dataset.

Examples of known climate applications and best practices

The SEC product has multiple applications in scientific research, environmental monitoring, and policy development. It is utilized in studies of sea-level rise to estimate contributions from ice sheet melting, and, it serves as a long-term data source to validate and calibrate models that simulate ice sheet and glacier dynamics. Additionally, it has been used in deriving Greenland mass balance from 1992-2020 in the study by Simonsen et al 2021. 

Known Issues and Limitations

Please note: The gridded data and its uncertainties are well-defined at the pixel level (25 km x 25 km). If they are to be combined to drainage basin level the user should be sure to consider the varying terrain and ice dynamics within the basin - a simple mean value will not be representative. The performed kriging procedure has the capability of extrapolating data over undesired distances and the distance-flag should be consulted before any averaging of elevation change is performed.

The user should note that the applied satellite data originate from a nadir-looking altimeter, hence, no direct observations of SEC are available between satellite tracks. To compensate for this, we use ordinary kriging to interpolate data. However, the final solution may still exhibit some "stripiness due to slight differences in observational periods." Nevertheless, by employing ordinary kriging, we incorporate the distance to the nearest observational point into the error estimate, accounting for the stripiness in the final SEC estimate. This approach enhances users' ability to contextualize and utilize SEC data in their specific applications.

Data access information

All products

Data are made available through the Copernicus Climate Data Store (CDS), which is the sole data distributor. Registration (free) is required to access the CDS and its toolbox software suite. The CDS is a web-based service, with its homepage at: https://cds.climate.copernicus.eu/

Data (DOI: 10.24381/cds.056d0df7) can be downloaded from the website and used under the License to Use Copernicus Products (included on the download page). The data is provided in NetCDF (Network Common Data Form) files which are a user-friendly way for scientists and researchers to store and share complex scientific data, like climate models and Earth observations, in digital containers. These files make it easier to analyze and understand things like weather patterns and ocean behaviour, facilitating collaboration and advancements in various fields of study. Data may also be viewed online.

All requests for information or further data should be channeled through the CDS Knowledge Base. This will be handled by the Copernicus User Support Service team (level 1) and will be forwarded to the Copernicus User Support Specialists (level 2) if the request is regarding specific technical aspects of their products. Inquiries forwarded to the Copernicus User Support Specialist team will be acknowledged within 3 working days and a notification sent to the user. In case of specific scientific issues, the inquiries will be channeled to the ECV and data specialist of the C3S2_313d_Lot4 project and should be resolved within 3 working weeks. The user forum will also enable discussions among users and product developers.

References

Studinger, M. (2014). IceBridge ATM L4 Surface Elevation Rate of Change, Version 299 1, Antarctica subset. N. S. a. I. D. C. D. A. A. Center. Boulder, Colorado, USA. DOI: 10.5067/BCW6CI3TXOCY

Shepherd, A. et al, (2012): A reconciled estimate of ice-sheet mass balance. Science, 338 (6111), 1183-1189. DOI:10.1126/science.1228102

 Simonsen, S. B., Barletta, V. R., Sørensen, L. S., & Colgan, W. (2021). Greenland Ice Sheet mass balance (1992‐2020) from calibrated radar altimetry. Geophysical Research Letters, 48(3), Article e2020GL091216. https://doi.org/10.1029/2020GL091216

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