Data Set ID:

RDEFT4

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).

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Parameter(s):	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
Metadata XML:	View Metadata Record

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: https://doi.org/10.5067/96JO0KIFDAS8. [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).

Format

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), which contains additional metadata.

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File and Directory Structure

Data are available via HTTPS in the following directory:

https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/RDEFT4.001/

Within this directory, the folders are named for each year, month, and day of data collection, for example /2010.08.27/. The folder date corresponds to the beginning date of the file it contains. As each file contains data that span one month, the dated directory precedes the actual file name by a month.

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File Naming Convention

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

RDEFT4_20170103.nc

RDEFT4_YYYYMMDD.xxx

Where:

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

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File Size

As of April 2018, the total file volume is 7.1 GB. Since new data are added to this product on a daily basis, the total file volume will be updated accordingly in this document.

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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

Data are delivered on an ongoing, daily basis with an approximately 1.5 month lag between the last file’s end date and the present date.

Temporal Resolution

The files are monthly (i.e., 30-day) averages, and are available on a daily basis starting on 15 September 2010. For example, the RDEFT4_20170420.nc file contains data averaged between 21 March 2017 to 20 April 2017. Only files are 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. Note: Any negative values can be treated as missing data.

**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 RDEFT4_20170420.nc as displayed in HDFView.

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.

HDFView: Visual tool for browsing and editing HDF4 and HDF5 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

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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, and pole tide 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.

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

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

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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 et al. (1999) climatology of snow depth on sea ice with a value of half the climatology applied to first year ice.

Snow density is likewise based on the Warren et al. (1999) climatology.

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.

Sea ice freeboard is then determined by subtracting the gridded sea surface elevation from the gridded sea ice floe elevation and applying a freeboard correction for the lower radar propagation speed in snow, following the method of Kurtz et al. (2014) (Equations 16 and 17). The freeboard correction is only applied where snow depth data are available. It is set to 0 when snow depth data are not available, i.e., outside of the central Arctic region.

Error Sources

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

Equation 1: Errors in Sea Ice Thickness

Where:

Equation 2: Ice Thickness

and h_i is the ice thickness, fb the freeboard of the sea ice, ρ_w, ρ_i, and ρ_s the densities of water, ice, and snow, c the speed of light in vacuum, and c_snow 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

The ESA SIRAL instrument, the primary instrument on board CryoSat-2, 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.

The SIRAL instrument 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 SAR mode is typically operated over sea-ice areas as well as ocean basins and coastal zones, whereas the SARIn mode is usually employed for the steep slopes of ice sheet margins, over small ice caps, and over mountain glacier regions.

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References and Related Publications

Contacts and Acknowledgments

Nathan Kurtz and Jeremy Harbeck
NASA/GSFC
Greenbelt , MD 20771
United States

Acknowledgments:

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

Document Creation Date

17 August 2017

Document Revision Date

07 June 2018

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National Snow and Ice Data Center

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

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