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Data Set ID:

CryoSat-2 Level-4 Sea Ice Elevation, Freeboard, and Thickness, Version 1

This data set contains 30-day averaged Arctic sea ice thickness estimates, with freeboard and ice surface roughness, derived from the ESA CryoSat-2 Synthetic Aperture Interferometric Radar Altimeter (SIRAL).

Geographic Coverage

  • Sea Ice > Freeboard
  • Sea Ice > Ice Depth/Thickness
  • Sea Ice > Ice Roughness
Spatial Coverage:
  • N: 90, S: 55, E: 180, W: -180

Spatial Resolution:
  • 25 km x 25 km
Temporal Coverage:
  • 27 August 2010
Temporal Resolution: 30 day
Data Format(s):
  • NetCDF
  • XML
  • PNG
Platform(s) CRYOSAT-2, DMSP 5D-3/F17
Sensor(s): SIRAL, SSMIS
Version: V1
Data Contributor(s): Nathan Kurtz, Jeremy Harbeck
Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Kurtz, N. and J. Harbeck. 2017. CryoSat-2 Level-4 Sea Ice Elevation, Freeboard, and Thickness, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: [Date Accessed].

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Detailed Data Description

This data set contains 25 km, gridded 30-day averaged Arctic sea ice thickness estimates, with freeboard and ice surface roughness.

This data set was developed using a new method to retrieve sea ice freeboard from CryoSat-2 data. The new retrieval procedure is used to estimate Arctic sea ice freeboard and the results are compared to a threshold tracking method and independent freeboard observations from airborne data (Kurtz et al. 2014).


The data files are in NetCDF format (.nc).

Sea ice thickness plots in PNG (.png) format are provided for each data file.

Each data file is paired with an associated XML file (.xml). The XML files contain file, date, location, platform, and instrument metadata.

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File and Directory Structure

Data are available via HTTPS in the following directory:

Within this directory, the folders are organized by date, for example /2010.08.27/.

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File Naming Convention

Data files are named according to the following convention and as described in Table 1:


Table 1. File Naming Convention
Variable Description
RDEFT4 Short name for CryoSat-2 Level-4 Sea Ice Elevation, Freeboard, and Thickness
MM Month
DD Day
.xxx Indicates file type, either NetCDF (.nc) XML (.xml), or PNG (.png)
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File Size

NetCDF data files are approximately 4 MB each. The total NetCDF file volume is approximately 6.6 GB.

PNG files are each about 78 KB. The total PNG file volume is approximately 1.3 GB.

XML files average about 3 KB. The total XML file volume is approximately 0.005 GB.

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The data set is approximately 7.9 GB.

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Spatial Coverage

Spatial coverage for the NASA GSFC CryoSat-2 sea ice product currently is the Arctic Ocean.

Southernmost Latitude: 55° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Spatial Resolution

25 km grid

Projection and Grid Description

Data are gridded to the Polar Stereographic SSM/I Grid and referenced to the WGS-84 ellipsoid, with each grid point center latitude and longitude provided with the data.

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Temporal Coverage

27 August 2010 to present.

As summer data are not reliable for ice thickness, data delivery began on 27 August 2010 and does not include any data from May through August for any subsequent years.

Each data file covers one month. For example, the beginning date is 21 March 2017 and end date is 20 April 2017. 

Temporal Resolution

Temporal resolution includes daily and monthly averages.

Files are 30-day averages, and are available on a daily basis using data starting on 15 September, 2010. Files are only provided that contain a minimum of 10 days of CryoSat-2 data.

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Parameter or Variable

Parameter Description

The gridded data file contains fields as described in Table 2.

Table 2. Data Parameter Description
Parameter Description Units
sea_ice_thickness Sea ice thickness Meters
snow_depth Snow depth Meters
snow_density Snow density Kg/Meters3
lat Latitude of grid point center Degrees
lon Longitude of grid point center Degrees
freeboard Ice freeboard Meters
roughness Ice surface roughness Meters
ice_con Sea ice concentration Percent

Sample Data Record

Figure 1 shows sea ice thickness values from data file as displayed in HDFView.

Figure 1. Sea Ice Thickness Values

The sea ice thickness plot is illustrated in Figure 2.

Figure 2. Sea Ice Thickness Plot RDEFT4_20170420_thickness_plot.png

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Software and Tools

The following external links provide access to software for reading and viewing HDF5 and netCDF data files. Please be sure to review instructions on installing and running the programs.

HDF Explorer: Data visualization program that reads Hierarchical Data Format files (HDF, HDF-EOS and HDF5) and also NetCDF data files.

Panoply NetCDF, HDF and GRIB Data Viewer: Cross-platform application. Plots geo-gridded arrays from NetCDF, HDF and GRIB data sets.

For additional tools, see the HDF-EOS Tools and Information Center.

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Quality Assessment

Quality assessment was done through comparison with NASA’s Operation IceBridge data.

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Data Acquisition and Processing

The primary data source is from ESA's CryoSat-2 Level-1B SAR and SARIn data. 

Ice type is determined from the OSI-403-b: Sea Ice Type Maps on 10 km Polar Stereographic Grid product. 

Sea ice concentration is determined from the NSIDC Near-Real-Time DMSP SSMIS Daily Polar Gridded Sea Ice Concentrations product.

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Data Acquisition Methods

Primary data sources are ESA’s CryoSat-2 Level-1B SAR and SARIn data.

Geophysical corrections for the wet and dry tropospheric delay time, ionospheric delay, oscillator drift, inverse barometer effect, ocean equilibrium tide, long period ocean tide, load tide, solid earth tide, pole tide, and radar propagation speed in snow have been applied from the ESA CryoSat-2 L1B data products.

Gridded CryoSat-2 data is generated from the processed along-track waveform product CryoSat-2 Level-1B Waveforms, Sea Ice Elevation, and Surface Roughness (RDWES1B) (publication pending).

Sea ice concentration is from the near real time DMSP SSMI/S daily polar gridded data set.

Sea ice type is from the OSI-403-b ice type data set.

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Derivation Techniques and Algorithms

Sea ice elevation is determined from CryoSat-2 using a physical model to determine the best fit to each CryoSat-2 waveform. The fitted waveform is used to determine the retracking correction for surface elevation retrieval as well as the surface roughness within the footprint. Sea ice freeboard is then determined by subtracting the gridded sea surface elevation from the gridded sea ice floe elevation and applying the radar propagation speed correction where snow depth data is available.

Snow depth is constructed from a modified Warren climatology of snow depth on sea ice with a value of half the climatology applied to first year ice.

Sea ice thickness is retrieved assuming hydrostatic balance and nominal densities of snow, ice, and water. Retrievals are only done when the sea ice concentration is at least 70%.

Sea ice concentration is from the near real time DMSP SSMI_S daily polar gridded data set with the pole hole set to a constant value of 100%.

Ice surface roughness is derived using the physical model to fit the CryoSat-2 waveform. The surface roughness is the standard devation of the ice surface elevation with an assumed Gaussian height distribution.

Processing Steps

Sea ice freeboard is determined from CryoSat-2 using a physical model to determine the best fit to each CryoSat-2 waveform.

The fitted waveform is used to determine the retracking correction and also allows determination of the surface roughness within the footprint.

For sea ice floes, the dominant backscattering layer is taken to be from the sea ice surface and thus sea ice freeboard is here defined as the height of the ice layer above the local sea surface.

The DTU10 Ocean wide Mean Sea Surface (DTU10 MSS) is subtracted from each elevation measurement and the elevations from leads and sea ice floes are placed onto a 25 km polar stereographic grid.

Sea ice freeboard is then determined by subtracting the gridded sea surface elevation from the gridded sea ice floe elevation and applying the radar propagation speed correction where snow depth data are available.

Error Sources

Errors in the retrieval of sea ice thickness can be written as follows:

Equation 1: Errors in Sea Ice Thickness


Equation 2: Ice Thickness

and hi is the ice thickness, fb the freeboard of the sea ice, ρw, ρi, and ρs the densities of water, ice, and snow, c the speed of light in vacuum, and csnow the speed of light in snow (which is a function of snow density).

Error sources are thus as follows:

  • Freeboard error from the associated surface elevation retrieval error, an estimate through comparison with IceBridge, is 0.065 m for a 25 km grid cell.
  • Snow depth error is estimated to be between 0.04 to 0.06 m from the interannual variability of snow depth as reported in Table 1 of Warren et al. 1999.
  • Density errors for snow, ice, and water: density errors for water are negligible and estimated from previous studies to be 100 kg/m3 for snow and 10 kg/m3 for ice.
  • Interpolation errors occur due to filling of gaps where no observations are available. These vary depending on the distance used to interpolate between points.
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Sensor or Instrument Description

CryoSat-2 is a radar altimeter which operates at a center frequency of 13.575 GHz and has a receive bandwidth of 320 MHz. The SAR processing of CryoSat-2 utilizes an unfocused aperture synthesis technique which utilizes Doppler beam formation to reduce the footprint size in comparison with a beam-limited altimeter. The effective footprint size after postprocessing is pulse-limited at 1650 m in the across-track direction and pulse-Doppler-limited to be 380 m in the along-track direction. The power-detected echoes contain 128 range bins in SAR mode and 512 range bins in SARIn mode (Kurtz et al. 2014).

The ESA SIRAL instrument is a radar altimeter which measures the surface elevation through knowledge of the spacecraft position and the time delay between the emission of the radar pulse and subsequent reflection from the surface.

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References and Related Publications

Contacts and Acknowledgments

Nathan Kurtz and Jeremy Harbeck
Greenbelt , MD 20771
Country: United States 


We acknowledge support from NASA’s Cryospheric Sciences program from grant NNH12ZDA001N. We would also like to thank the European Space Agency for processing and providing CryoSat-2 data.

Document Information


17 August 2017


14 December 2017

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