Contributors: Sebastian B. Simonsen (Technical University of Denmark)
Issued by: Technical University of Denmark / Sebastian B. Simonsen
Date:
Ref: C3S2_312a_Lot4.WP2-FDDP-IS-v2_202312_SEC_PUGS-v5_i1.1
Official reference number service contract: 2021/C3S2_312a_Lot4_EODC/SC1
History of modifications
List of datasets covered by this document
Related documents
Acronyms
General definitions
| Term | Definition |
|---|---|
| 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. |
| 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. |
Scope of the document
This document is the Product User Guide and Specification for the version 5.0 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 Greenland, from 1992 to the present. The latest version, Version 5.0, includes updates solely for the Greenland data product, while the production of Antarctic data has been temporarily halted. We refer the reader to Product User Guide and Specification (PUGS) version 4.0 [D4] for a description of the Antarctic SEC, also hosted at the Copernicus Climate Data Store. This document describes the v5.0 Surface Elevation Change product over Greenland in a manner that is understood by the product user with focus on the geophysical data product content, it's structure and format, known limitations and data access.
Executive summary
We document here the description of the Climate Data Records (CDRs) v5.0 for the SEC product, with a guide to their usage and specifications.
In section 1 we describe the products and the 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 2 we give details of public access to the products, licensing, and how to make inquiries about them.
Product Change Log
The same product changes apply to both Antarctic (up to v4) and Greenland products.
Version | Product Changes |
|---|---|
V1 | Initial product, using data from European Remote-sensing Satellite 1 (ERS1), ERS2, Envisat and CryoSat-2. |
V2 | Data from Sentinel-3A added. |
V3 | Data from Sentinel-3B added. |
| V4 | Updated timeseries and revised filtering of raw data. |
| V5 | Data from Sentinel-3 Thematic product is included |
1. Surface elevation change, Greenland
1.1. 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. An example plot from one data grid is shown in Figure 1.
Figure 1: Example accumulated (in meter/year) surface elevation change map produced from v5.0 Surface Elevation Change product from 1992 to 2023
The current version 5.0 of the CDR product builds on the legacy of its preceding versions:
CDRv1. The initial product release builds on the Research and Developement 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.
Similar to its predecessors, the current CDRv5 will be updated monthly as iCDRs, with a 2-month time lag between the acquisition of new measurements and their addition to the product. The product specifications are summarised in Table 1.
Table 1: Greenland SEC product specification summary
| Specification | Description |
|---|---|
| Sensors | ERS1 and ERS2 Radar Altimeter (RA): https://earth.esa.int/eogateway/instruments/ra-ers/description Envisat RA-2: https://earth.esa.int/eogateway/instruments/ra2 Cryosat-2 (Synthetic Aperture Radar Altimeter) SIRAL: https://earth.esa.int/eogateway/instruments/siral/description Sentinel-3A and Sentinel-3B Synthetic Aperture Radar (SAR) Radar Altimeter instrument (SRAL): https://sentinels.copernicus.eu/web/sentinel/technical-guides/sentinel-3-altimetry/instrument/sral |
| Grid | 25km by 25km polar stereographic projection based on 45°W, 70°N on the WGS84 ellipsoid (EPSG: 3413) |
| Time Range | 1992 to 2 months before present |
| Time intervals | 5-year window moving in monthly steps (3-year in the CryoSat-2 and Sentinel-3 era) |
| Validation frequency | Annual |
| Quality flagging | Flags are provided for steep terrain, missing data, and the nearest distance to the original RA-altimeter measurement |
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 [D2].
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 [D1].
1.2. 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
The Copernicus Climate Change Service (C3S) project itself provides key performance indicator targets.
Details of the CDR v5.0 performances against these targets may be found in the related document, the Product Quality Assessment Report [D2].
The requirements summarized for the SEC products, based on the combined GCOS and C3S performance indicator targets are shown in Table 2. 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 [D3].
Table 2: SEC products targets and performance indicators
Statistic | Target | Target source |
|---|---|---|
Stability at pixel-level | 0.1 m/y | GCOS |
Accuracy at basin-level | 0.1 m/y | GCOS |
Accuracy at pixel-level | 0.1 m/y | C3S project |
Surface coverage, aggregated over one year | 65% ERS1, ERS2, Envisat, Sentinel-3A/B | C3S project |
1.3. Data usage information
1.3.1. Product data format and content
The product is provided as a netCDF 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 3.
Table 3: Greenland surface elevation change main data variables.
Variable name | Variable description | Type |
|---|---|---|
x | Centre of grid cell on X axis, in m | 32-bit float |
y | Centre of grid cell on Y axis, in m | 32-bit float |
lon | Longitude of grid cell centre, in degrees east | 32-bit float |
lat | Latitude of grid cell centre, in degrees north | 32-bit float |
time | Central time of surface elevation change rate derivation, in hours since 1990.0 | 32-bit float |
dhdt | Surface elevation change rate, in m/year | 32-bit float |
dhdt_uncert | Uncertainty on surface elevation change rate, in m/yr | 32-bit float |
dhdt_ok | Validity flag for surface elevation change rate 0: no data in cell 1: data in cell | Byte |
surface_type | Flag for geographical surface type in cell 0: no ice (may be land or ocean) 1: ice cover > 95% of surface | Byte |
high_slope | Flag for geographical slope class (i.e. low/medium/high) in cell 0: slope<= 2° 1: 2° < slope <= 5° 2: slope > 5° | Byte |
dist | Distance to the nearest observational node, in m | int |
1.3.2. Product known 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.
1.3.3. Product nomenclature
The dataset filename format is:
C3S_GrIS_RA_SEC_ggkm_versx_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
- 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 monthly updates.
1.4. Structure of NetCDF files
The header data for an example netCDF data file is given in Table 4.
Table 4: Example of the NetCDF file structure for the Greenland Ice Sheet Surface elevation change based on the October 2023 version 5.0 dataset.
netcdf file:/Users/naand/space/nfs/g7/PROJ/C3S/SEC/v5/SECicdr/C3S_GrIS_RA_SEC_25km_Vers5_2023-12-05.nc {
dimensions:
x = 65;
t = 378;
y = 123;
variables:
float x(x=65);
:long_name = "Cartesian x-coordinate - easting";
:standard_name = "projection_x_coordinate";
:units = "m";
float y(y=123);
:long_name = "Cartesian y-coordinate - northing";
:standard_name = "projection_y_coordinate";
:units = "m";
float time(t=378);
:long_name = "SEC period central time";
:standard_name = "time";
:units = "hours since 1990-01-01T00:00:00Z";
float start_time(t=378);
:standard_name = "time";
:long_name = "SEC period start time";
:units = "hours since 1990-01-01T00:00:00Z";
float end_time(t=378);
:standard_name = "time";
:long_name = "SEC period end time";
:units = "hours since 1990-01-01T00:00:00Z";
char grid_projection;
:ellipsoid = "WGS84";
:false_easting = 0.0; // double
:false_northing = 0.0; // double
:grid_mapping_name = "polar_stereographic";
:latitude_of_projection_origin = 90.0; // double
:standard_parallel = 70.0; // double
:straight_vertical_longitude_from_pole = -45.0; // double
:EPSG = "3413";
float lat(y=123, x=65);
:_FillValue = 9999.0f; // float
:units = "degrees_north";
:grid_mapping = "grid_projection";
:long_name = "Latitude";
:_ChunkSizes = 123U, 65U; // uint
float lon(y=123, x=65);
:_FillValue = 9999.0f; // float
:units = "degrees_east";
:grid_mapping = "grid_projection";
:long_name = "longitude";
:_ChunkSizes = 123U, 65U; // uint
float dh(y=123, x=65, t=378);
:long_name = "Elevation change";
:grid_mapping = "grid_projection";
:units = "m";
:_ChunkSizes = 62U, 33U, 189U; // uint
float dh_uncert(y=123, x=65, t=378);
:long_name = "Elevation change uncertainty";
:grid_mapping = "grid_projection";
:units = "m";
:_ChunkSizes = 62U, 33U, 189U; // uint
float dhdt(y=123, x=65, t=378);
:long_name = "Rate of elevation change";
:grid_mapping = "grid_projection";
:units = "m/year";
:_ChunkSizes = 62U, 33U, 189U; // uint
float dhdt_uncert(y=123, x=65, t=378);
:long_name = "Rate of elevation change uncertainty";
:units = "m/year";
:grid_mapping = "grid_projection";
:_ChunkSizes = 62U, 33U, 189U; // uint
float dhdt_stabil(y=123, x=65, t=378);
:grid_mapping = "grid_projection";
:units = "m/year";
:long_name = "Stability of rate of elevation change fit";
:_ChunkSizes = 62U, 33U, 189U; // uint
byte dhdt_ok(y=123, x=65, t=378);
:grid_mapping = "grid_projection";
:flag_values = 0B, 1B; // byte
:flag_meanings = "no_data data_valid";
:long_name = "SEC valid flags";
:_ChunkSizes = 123U, 65U, 378U; // uint
float dist(y=123, x=65, t=378);
:long_name = "Distance to observational node";
:grid_mapping = "grid_projection";
:unit = "m";
:_ChunkSizes = 62U, 33U, 189U; // uint
byte land_mask(y=123, x=65);
:ref = "ESA Glacier CCI Greenland ice cover";
:flag_meanings = "0_LandOcean 1_IceCover";
:long_name = "Land cover";
:grid_mapping = "grid_projection";
:flag_values = 0B, 1B; // byte
:_ChunkSizes = 123U, 65U; // uint
byte high_slope(y=123, x=65);
:flag_meanings = "0Slope_leq_2degrees 1Slope_geq2leq5degrees 2Slope_geq_5degrees";
:grid_mapping = "grid_projection";
:ref = "Slope of the GIMP Greenland DEM";
:long_name = "Slope flag";
:flag_values = 0B, 1B, 2B; // byte
:_ChunkSizes = 123U, 65U; // uint
float area(y=123, x=65);
:long_name = "Grid_area";
:grid_mapping = "grid_projection";
:units = "m^2";
:_ChunkSizes = 123U, 65U; // uint
// global attributes:
:Title = "Surface Elevation change of the Greenland ice sheet from Radar altimetry";
:institution = "Copernicus Climate Change Service, DTU Space - Div. of Geodynamics";
:reference = "Simonsen and Sørensen (2017), Sørensen et al. (2018)";
:contact = "copernicus-support@ecmwf.int";
:file_creation_date = "2023-12-05 12:52:56.546114";
:project = "C3S_312b_Lot4_ice_sheets_and_shelves";
:region = "Greenland";
:missions_used = "ESA Radar altimeters: ERS-1,ERS-2, Envisat, CryoSat-2 and Sentinel-3";
:power_corrections = "ERS-1, ERS-2 and Envisat 5-years, CryoSat-2 3-years";
:grid_projection = "EPSG:3413";
:grid_minx = -739301.6214372054; // double
:grid_miny = -3478140.668199717; // double
:grid_nx = 65L; // long
:grid_ny = 123L; // long
:grid_cell_width_x = "25000 m";
:grid_cell_width_y = "25000.0m";
:Latitude_min = 57.76737214534745; // double
:Latitude_max = 86.04798347855436; // double
:Longitude_min = -104.92422366476225; // double
:Longitude_max = 18.552684627240275; // double
:model_type = "ERS-1, ERS-2 and Envisat combined repeat-track and plan-fitting, with the introduction of CryoSat-2 only plane fit is used";
:cross_cal_method = "Elevation regression, and weighted mean";
:time_coverage_start = "1992-01-01 00:00:00";
:time_coverage_end = "2023-06-01 00:00:00";
:Tracking_id = "04ac2b57-2032-46d2-80ff-e52cf70a4490";
:Internal_ref = "C3S_GrIS_RA_SEC_25km_Vers5_2023-12-05.nc";
:netCDF_version = "NETCDF4";
:product_version = "v5";
:Conventions = "CF-1.7";
:keywords = "EARTH SCIENCE CRYOSPHERE GLACIERS/ICE SHEETS/GLACIER ELEVATION/ICE SHEET ELEVATION";
:license = "C3S general license";
:summary = "Surface elevation change rate derived for Greenland in 25km by 25km grid cells over a 5/3 year window moving monthly cadence.";
}
2. Data access information
2.1. All products
Data will be 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/cdsapp#!/home
Data 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 https://cds.climate.copernicus.eu/cdsapp#!/dataset/satellite-ice-sheet-elevation-change?tab=app.
All requests for information or further data should be channeled through the CDS Knowledge Base at https://confluence.ecmwf.int//display/CKB/
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
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 (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.
