Scientists
Research Grants
Scientific Publications
Informatics Research
NSIDC Scientific Expeditions
Scientists blog from Antarctica and provide a glimpse of what it's like to do research in the field. 
What's hot in the news around climate and sea ice and what are scientists talking about now? 
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).
Get Data
DOWNLOADING DATA VIA HTTPS
To learn more about Earthdata Login and to register for an account, please see How to Register with Earthdata Login. Once you have logged in, data can be downloaded via a Web browser, command line, or client. For help with downloading data, please see Options Available for Bulk Downloading Data from HTTPS with Earthdata Login.
IceBridge Portal: Tool to visualize, search, and download IceBridge data.
Earthdata Search: NASA's newest search and order tool for subsetting, reprojecting, and reformatting data.
IceBridge Portal: Tool to visualize, search, and download IceBridge data.
Data Set ID:
RDEFT4
CryoSat-2 Level-4 Sea Ice Elevation, Freeboard, and Thickness, Version 1
Geographic Coverage
| Parameter(s): |
|
|---|---|
| Spatial Coverage: |
|
| Spatial Resolution: |
|
| Temporal Coverage: |
|
| Temporal Resolution: | 30 days |
| Data Format(s): |
|
| 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].Collapse All / Open All
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).
Background color on
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
Background color on
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:
| 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) |
Background color on
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.
Background color on
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.
Background color on
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.
Background color on
Parameter or Variable
Parameter Description
The gridded data file contains fields as described in Table 2. Note:
| 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.
Background color on
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.
Background color on
Quality Assessment
Quality assessment was done through comparison with NASA’s Operation IceBridge data.
Background color on
Data Acquisition and Processing
Background color on
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 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.
Background color on
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
Where:
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.
Background color on
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.
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.
Background color on
References and Related Publications
Contacts and Acknowledgments
Nathan Kurtz and Jeremy Harbeck
NASA/GSFC
Greenbelt , MD 20771
Country: 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
14 December 2017
06 April 2018
No technical references available for this data set.
No FAQs or How Tos available for this data set.
Access complete Knowledge Base
Questions? Please contact:
NSIDC User Services
Phone: 1 303 492-6199
Email: nsidc@nsidc.org
NSIDC User Services
Phone: 1 303 492-6199
Email: nsidc@nsidc.org
