This data set contains derived geophysical data products including sea ice freeboard, snow depth, and sea ice thickness measurements in Greenland and the Arctic retrieved from IceBridge Snow Radar, Digital Mapping System (DMS), Continuous Airborne Mapping By Optical Translator (CAMBOT), and Airborne Topographic Mapper (ATM) sensors. The data were collected as part of Operation IceBridge funded campaigns, and are stored in ASCII text files.
We kindly request that you cite the use of this data set in a publication using the following citation. For more information, see our Use and Copyright Web page.
Kurtz, N., M. S. Studinger, J. Harbeck, V. Onana, and S. Farrell. 2012, updated 2014. IceBridge Sea Ice Freeboard, Snow Depth, and Thickness. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://nsidc.org/data/idcsi2.html.
ATM, Snow Radar, DMS, CAMBOT
Greenland and the Arctic
Freeboard: 40 m length scale
Snow Depth: based on synthetic aperture dictated footprint size and averaged to a 40 m length scale in the along-track direction
Thickness: 40 m length scale
31 March 2009 to present
Nathan Kurtz, Michael Studinger, Jeremy Harbeck, Vincent-De-Paul Onana
Cryospheric Sciences Laboratory
NASA Goddard Space Flight Center
Greenbelt, Maryland USA
Earth System Science Interdisciplinary Center (ESSIC)
University of Maryland
College Park, Maryland USA
NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
The production of a sea ice and snow thickness product would not have been possible without the help of many people. We would like to thank the IceBridge Sea Ice Science team and members of the community for support and guidance. We would also like to thank the instrument teams and air crews for long hours in the field and at home collecting and processing the data and the National Snow and Ice Data Center for archiving and publishing the data. This work is funded by NASA's Airborne Science and Cryospheric Sciences Programs.
The data files are in comma delimited ASCII text format. Each data file is paired with an associated XML file. The XML files contain point latitude and point longitude, along with instrument, sensor, and campaign metadata.
Data files are on the FTP site, ftp://n5eil01u.ecs.nsidc.org/SAN2/ICEBRIDGE/IDCSI2.001/ in folders organized by date, for example /2009.03.31/ to /2012.04.10/.
Data files are named according to the following convention and as described in Table 1:
|IDCSI2||Short name for IceBridge Sea Ice Freeboard, Snow Depth, and Thickness|
|YYYY||Four-digit year of data collection|
|MM||Two-digit month of data collection|
|DD||Two-digit day of data collection|
|.txt||Indicates ASCII text file|
Data files range from approximately 8 MB to 60 MB.
The entire data set is approximately 1.3 GB.
Spatial coverage for the IceBridge Sea Ice Freeboard, Snow Depth, and Thickness parameters currently includes Greenland and the Arctic.
Greenland / Arctic:
Southernmost Latitude: 60° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E
Freeboard: adjusted 40 m length scale.
Snow depth: at the 460 m nominal flight altitude the snow radar has a footprint size of 11 m across track dictated by the pulse-limited footprint size, and 14.5 m along-track dictated by the synthetic aperture formed. The data are averaged in the along-track direction to a 40 m length scale.
Thickness: estimates sea ice thickness over a 40 m length scale to provide the highest resolution available from the data.
Referenced to the ITRF-2005 reference frame and projected onto the WGS-84 ellipsoid.
Referenced as a relative distance from the aircraft with latitude and longitude coordinates provided by the GPS system on the aircraft.
Georeferenced as a series of camera locations: latitude, longitude, elevation and altitude above ground (WGS-84) and aircraft orientation roll, pitch, and heading.
Arctic. Polar Stereographic Standard Parallel 70° N, Longitude of the origin (central meridian): 45° W,
Antarctic. Polar Stereographic Standard Parallel 71° S, Longitude of the origin (central meridian): 0°, WGS-84 ellipsoid.
31 March 2009 to the present.
IceBridge campaigns are conducted on an annual repeating basis. Arctic and Greenland campaigns are conducted during March, April, and May; and Antarctic campaigns are conducted during October and November.
The Sea Ice Freeboard, Snow Depth, and Thickness data files contain parameters as described in Table 2.
|thickness||Sea ice thickness||Meters|
|thickness_unc||Sea ice thickness uncertainty||Meters|
|mean_fb||Mean freeboard from the combined ATM and DMS data set||Meters|
|ATM_fb||Mean freeboard from the ATM data set only (may be biased due to the loss of data over thin ice and water)||Meters|
|snow_depth_unc||Snow depth uncertainty||Meters|
|n_atm||Number of ATM measurements used||n/a|
|pcnt_ow||Percentage of open water detected in the DMS imagery over the 40 m area||n/a|
|pcnt_thin_ice||Percentage of grease ice and/or nilas detected in the DMS imagery over the 40 m area||n/a|
|pcnt_grey_ice||Percentage of non-snow-covered grey ice detected in the DMS imagery over the 40 m area||n/a|
|corr_elev||Surface elevation after the removal of geoid, atmospheric pressure, and tidal corrections||Meters|
|elev||Mean ATM elevation||Meters|
|date||Date of measurement in YYYYMMDD format||n/a|
|elapsed||Elapsed time from the start of the day in UTC||Seconds|
|atmos_corr||Atmospheric pressure loading term||Meters|
|ellip_corr||Conversion factor between the WGS-84 and Topex/Poseidon ellipsoids||Meters|
|tidal_corr||Sum of the ocean, load, and earth tides||Meters|
|ocean_tide_corr_part||Ocean tide for the sea surface height||Meters|
|load_tide_corr_part||Load tide for the sea surface height||Meters|
|earth_tide_corr_part||Solid earth tide for the sea surface height||Meters|
|ssh||Local interpolated sea surface height||Meters|
|n_ssh||Number of ATM measurements used to determine the nearest sea surface height estimate||n/a|
|ssh_sd||Standard deviation of ATM elevations used to determine the nearest sea surface height estimate||Meters|
|ssh_diff||Difference between the centroids of the final and initial Gaussian fits to the nearest sea surface height||Meters|
|ssh_elapsed||Elapsed time since the last sea surface height data point was encountered||Seconds|
|ssh_tp_dist||Distance to the nearest sea surface height tie point||Meters|
|surface_roughness||Standard deviation of the ATM elevation points in the 40 m grid||Meters|
|ATM_file_name||Name of the ATM file which the surface elevation measurements were from||n/a|
|Tx||Mean transmit signal strength (40 m resolution) of the ATM data||Relative|
|Rx||Mean received signal strength (40 m resolution) of the ATM data||Relative|
|KT19_surf||Surface temperature from the KT-19 instrument||Celsius|
|KT19_int||Internal temperature of the KT-19 instrument||Celsius|
|low_en_corr||Correction added to the ATM elevation data for low signal strength||Meters|
|sa_int_elev||Height of radar derived snow-air interface relative to the WGS-84 ellipsoid||Meters|
|si_int_elev||Height of radar derived snow-ice interface relative to the WGS-84 ellipsoid||Meters|
|my_ice_flag||Flag for ice type, 0: first year ice, 1: multi-year ice, from 12.5 km resolution AMSR-E data||n/a|
|empty1...empty10||Empty columns which may be used in future versions||n/a|
The sample record shows the header and three records from the 2011 Greenland data file: OIB_20110316_IDCSI2.txt.
Data are available via FTP.
The data files may be opened by any text editor or word processing program that reads ASCII text files.
A MATLAB program is available for reading the ASCII data files and displaying graphical representations of the data.
For details on data quality, see Sea Ice Thickness, Freeboard, and Snow Depth Products from Operation IceBridge Airborne Data (Kurtz et al. 2013).
This data set contains the geophysical data products sea ice thickness, freeboard, and snow depth retrieved from Operation IceBridge Level-1B ATM, Snow Radar, DMS, and CAMBOT data.
For instrument details related to specific campaigns, see Mission, Campaign, and Flight Specific Notes.
IceBridge Sea Ice Freeboard, Snow Depth, and Thickness products are derived from four Operation IceBridge data sets:
Surface temperature data are provided by the KT-19 infrared pyrometer:
Figure 2 describes the retrieval of sea ice thickness, snow depth, and freeboard (Kurtz et al. 2013). In addition to the ATM and Snow Radar instruments, Operation IceBridge DMS and CAMBOT were used to identify features and surface types on the sea ice.
Figure 2. Product Retrieval from Instrument Data
For details on specific seasonal campaigns, see the Appendix in the Operation IceBridge sea ice freeboard, snow depth, and thickness data products manual.
IceBridge Sea Ice Freeboard, Snow Depth, and Thickness is a retrieval of three products obtained from IceBridge data.
Technical summaries are provided below. For further details on derivation techniques, algorithms, processing steps, and error sources, see Sea Ice Thickness, Freeboard, and Snow Depth Products from Operation IceBridge Airborne Data (Kurtz et al. 2013).
Freeboard is retrieved using geolocated aerial photography and a lead discrimination algorithm to maximize the quality and number of laser altimeter data points used to determine the sea surface height. This method is used to deal with loss of data due to specular reflection of the laser pulse away from the receiver when insufficient surface roughness elements are present to cause diffuse scattering. The combination of photography and laser altimetry allows for more accurate retrieval of sea ice freeboard. The primary ATM laser altimeter product is surface elevation referenced to the WGS-84 ellipsoid. The conversion of ATM elevation data into sea ice freeboard is accomplished by subtracting out the instantaneous sea surface height from each elevation measurement (Kurtz et al. 2013).
Retrieval methods for the IceBridge snow radar have been described by Kurtz and Farrell (2011), Kurtz et al. (2013), and Farrell et al. (2012). The Kurtz and Farrell (2011) method is used to retrieve snow depth for the 2009 IceBridge campaign for this product and the Kurtz et al. (2013) method is used for all subsequent campaigns. The retrieval algorithms for the snow radar system detect the snow-air and snow-ice interfaces within the radar waveform and determine the snow depth by multiplying the time separation between the interfaces by the speed of light within the snow pack (Kurtz et al. 2013).
Sea ice thickness, hi, is calculated using the corresponding 40 m scale freeboard and snow depth data as input for the hydrostatic balance equation:
|ρw||density of sea water|
|ρi||density of sea ice|
|ρs||density of snow|
ρw and ρi are taken to be 1024 kg m–3 and 915 kg m–3 which are derived from the result of numerous field measurements summarized by Wadhams et al. (1992).
ρs is taken to be 320 kg m–3 following the climatological values compiled by Warren et al. (1999).
Time tags in the 'elapsed' data field in some samples are identical yet have different latitude and longitude, and there are jumps in lat/lon at those points where the time stamps do not change. This timestamp issue appears to be present in each of the files for the 2011 Arctic campaign, apparently arising from the initial 2011 snow radar data set. This should only be an issue with the timestamp, not the location information, so the latitude and longitude fields should be correct. Reprocessed and corrected 2011 snow radar data have been received, and a fully corrected version will be released in the near future with Version 2 of the IDCSI2 data set. Version 2 will cover all data released to date and will include fixes for errors such as this as well as multiple upgrades.
During the processing of this data all averaging was based on the locations of the snow radar footprints. Each lat/lon ˜40 m spot was processed individually. During this process, if ATM data was available for this footprint, the timestamp was recorded from the ATM data within the footprint. If there were no ATM data available for the location, the timestamp from the snow radar was used. The code was initially designed for calculating ATM freeboard and the snow radar / thickness portion was added afterwards. This idiosyncrasy has since been removed and all timestamps are assigned at the beginning, similar to the lat/lon fields and are not dependent upon other data fields.
In the assignment of the timestamp from the snow radar data source, a coding error was made that failed to regularly update the timestamp being assigned to the correct one for the current location being processed. Due to this error, when the final script ran and rearranged every point to ensure that the elapsed time field was monotonically increasing, it also rearranged every other field, including the lat/lon fields. So even though the lat/lon fields appear to have "bad spots," they are actually just out of order in the file.
The lat/lon fields in the original ˜40 m footprint snow radar data set were ingested for the lat/lon positions. Other than being out of order, the values are identical. This means that the user has two options:
The NASA ATM instrument is a scanning airborne laser that measures surface elevation of the ice by timing laser pulses transmitted from the aircraft, reflected from the ground and returning to the aircraft. This laser pulse time-of-flight information is used to derive surface elevation measurements by combining measurement of the scan pointing angle, precise GPS trajectories, and aircraft attitude information.
The University of Kansas CReSIS ultra-wideband snow radar operates over the frequency range from 2 to 8 GHz to map near-surface internal layers in polar firn with fine vertical resolution. The radar also has been used to measure thickness of snow over sea ice. Information about snow thickness is essential to estimate sea ice thickness from ice freeboard measurements performed with satellite radar and laser altimeters.
The NASA Digital Mapping System is an airborne digital camera that acquires high resolution natural color and panchromatic imagery from low and medium altitude research aircraft.
The CAMBOT system is comprised of a Canon Rebel XTi (or alternatively the XSi model) camera and a Mac Mini running custom data acquisition software. The camera is powered with an AC power adapter and connected to the Mac mini via USB. The camera is outfitted with a Canon Zoom Lens EF-S 18-55 mm lens.
Farrell, S. L., N. T. Kurtz, L. Connor, B. Elder, C. Leuschen, T. Markus, D. C. McAdoo, B. Panzer, J. Richter-Menge, and J. Sonntag. 2012. A First Assessment of IceBridge Snow and Ice Thickness Data over Arctic Sea Ice. IEEE Transactions on Geoscience and Remote Sensing, 50(6):2098-2111, doi:10.1109/TGRS.2011.2170843.
Kurtz, N. T., Farrell, S. L., Studinger, M., Galin, N., Harbeck, J. P., Lindsay, R., Onana, V. D., Panzer, B., and Sonntag, J. G. 2013. Sea Ice Thickness, Freeboard, and Snow Depth Products from Operation IceBridge Airborne Data. The Cryosphere, 7:1035-1056, doi:10.5194/tc-7-1035-2013.
Wadhams, P., W. B. Tucker III, W. B. Krabill, R. N. Swift, J. C. Comiso, and N. R. Davis. 1992. Relationship Between Sea Ice Freeboard and Draft in the Arctic Basin, and Implications for Ice Thickness Monitoring. Journal of Geophysical Research, 97(C12):20325-20334.
The acronyms used in this document are listed in Table 3.
|ASCII||American Standard Code for Information Interchange|
|ATM||Airborne Topographic Mapper|
|CAMBOT||Continuous Airborne Mapping By Optical Translator|
|CIRES||Cooperative Institute for Research in Environmental Science|
|DMS||Digital Mapping System|
|FTP||File Transfer Protocol|
|GPS||Global Positioning System|
|ITRF||International Terrestrial Reference Frame|
|NASA||National Aeronautics and Space Administration|
|NSIDC||National Snow and Ice Data Center|
|WGS-84||World Geodetic System 1984|
14 January 2013
01 August 2013
28 October 2013
17 April 2014
17 June 2014