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Data Set ID:
RDGBV4

Level-4 9ka Greenland Ice Sheet Balance Velocity, Version 1

This data set contains calculated balance velocity of the Greenland Ice Sheet during the last three quarters of the Holocene epoch (9ka).

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Geographic Coverage

Parameter(s):
  • Snow/Ice > Ice Velocity
Spatial Coverage:
  • N: 81.51, S: 58.91, E: 6.62, W: -88.33

Spatial Resolution:
  • 5 km x 5 km
Temporal Coverage:
  • 23 June 1993 to 26 April 2013
Temporal Resolution: Not specified
Data Format(s):
  • NetCDF
  • XML
Platform(s) DC-8, P-3B
Sensor(s): ACORDS, ICORDS, MCRDS, MCoRDS
Version: V1
Data Contributor(s): Joseph MacGregor, Mark Fahnestock, Ginny Catania, John Paden, Prasad Gogineni, Mathieu Morlighem, William Colgan

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.

MacGregor, J. A., M. Fahnestock, G. Catania, J. Paden, P. Gogineni, M. Morlighem, and W. Colgan. 2016. Level-4 9ka Greenland Ice Sheet Balance Velocity, 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/FZHPYHXJ4JGV. [Date Accessed].

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Detailed Data Description

Format

The data files are HDF5-compliant NetCDF (.nc) format.

The data file is paired with an associated XML file. XML files contain point latitudes and longitudes, and file and campaign metadata.

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File and Directory Structure

Data are available in the https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/RDGBV4.001/ directory.

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File Naming Convention

File names:

RDGBV4_Greenland_1993_2013_01_balance_velocity_9ka.nc
RDGBV4_Greenland_1993_2013_01_balance_velocity_9ka.nc.xml

RDGBV4_Location_YYYY_yyyy_0X_balance_velocity_9ka.nc

Where:

Variable Description
Table 1. File Naming Convention
RDGBV4 Short name for Level-4 9­ka Greenland Ice Sheet Balance Velocity data
Location Location, e.g. Greenland
YYYY_yyyy Temporal coverage of data collection from YYYY to yyyy, e.g. 1993_2013
0X Data product version number, e.g. 01
balance_velocity Ice sheet balance velocity
9ka Temporal covearge of the Level-4 data product, 9000 to 0 years before the present
xxx Indicates file type, e.g. NetCDF (.nc), XML (.xml)
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File Size

The RDGBV4_Greenland_1993_2013_01_balance_velocity_9ka.nc data file is approximately 44 MB.

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Volume

The entire data set is approximately 44 MB.

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Spatial Coverage

Spatial coverage for the Level-4 9­ka Greenland Ice Sheet Balance Velocity data includes Greenland.

Greenland:
Southernmost Latitude 58.91° N
Northernmost Latitude: 81.51° N
Westernmost Longitude: -88.33° W
Easternmost Longitude: 6.62° E

Spatial Resolution

5 km grid on EPSG:3413

Projection and Grid Description

Table 2. EPSG:3413 NSIDC Sea Ice Polar Stereographic North
Projection: Polar Stereographic
Latitude of the origin 90°
Longitude of the origin (central meridian) -45°
Standard parallel 70°
Scaling factor 1
False eastings 0
False northings 0
Ellipsoid WGS84
Datum WGS84
Units meters
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Temporal Coverage

 These model results are derived from radar data collected over the Greenland Ice Sheet between 1993–2013. They make use of internal reflections that were dated to less than 9 thousand years ago.

Temporal Resolution

The key model results constitute 9-thousand year averages only.

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Parameter or Variable

The key variables presented in this data set are the boundary conditions for the 9-thousand-year mean ice flux in the interior of the Greenland Ice Sheet, and then the resulting balance-velocity calculation for that period. Differences with readily available modern estimates of related variables (accumulation rate, surface speed) are provided. In the case of the boundary conditions, the variables were originally mapped onto a 1-km grid and then filtered to a 5-km grid. Lo/hi grids represent the 95% confidence bounds for those variables, and are based on separate gridding of the confidence bounds derived for along-track values.

Parameter Description

Parameter Description Units
Table 3. netCDF File Description
D Characteristic number D qualifying the suitability of the local-layer approximation for a 9-ka old particle. dimensionless
D1_mask Mask where (D < 1) = 1, (D > 1) = 0 and (not on the Greenland Ice Sheet) = NaN dimensionless
accumulation_rate 9-ka mean ice-equivalent accumulation rate (std/lo/hi) m/a ice equivalent
accumulation_rate_difference Difference between RACMO2 modern estimate of ice-equivalent accumulation rate and 9-ka mean (absolute/relative) m/a ice equivalent
characteristic_length_accumulation_rate Characteristic length of variability in accumulation rate along the 9-ka particle path km
characteristic_length_thickness Characteristic length of variability in ice thickness along the 9-ka particle path km
deceleration_rate 1-D modeled deceleration rate due to LGP-Holocene viscosity contrast m/a
depth_isochrone Depth of 9-ka isochrone (std/uncert) m
length_particle_path Horizontal length of particle path traveled in 9 ka km
shape_factor Shape factor for portion of ice column between 0-9 ka (std/lo/hi) dimensionless
shear_layer_thickness Dansgaard–Johnsen basal shear layer thickness (std/lo/hi) m
speed_balance 9-ka mean surface speed (std/lo/hi/ref) m/a
speed_difference Difference between modern surface speed and 9-ka mean (std/lo/hi) m/a
vertical_strain_rate Mean vertical strain rate for ice column between 0-9 ka (std/lo/hi) 1/a
x Projected x-dimension grid centered on Greenland km
y Projected y-dimension grid centered on Greenland km

Sample Data Record

Figure 1. Holocene-averaged and modern surface speed across the Greenland Ice Sheet. (MacGregor et al. 2016)

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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.

Quality Assessment

The fundamental quality assessment step for this dataset came earlier, when originally tracing and dating the radar internal reflections used to model the 9-ka-average balance ice flux. See Radiostratigraphy and Age Structure of the Greenland Ice Sheet (RRRAG4) for additional information. Following modeling of along-track depths, accumulation rates and strain rates, some manual removing of clearly anomalous model values was performed. Such values tended to be along the edge of the model domain, toward the coast, where fewer reflections could be mapped or where their dating was more uncertain.

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Data Acquisition and Processing

Theory of Measurements

See Radiostratigraphy and Age Structure of the Greenland Ice Sheet (RRRAG4) and MacGregor et al. 2016 and its Supplementary Materials for more information.

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Data Acquisition Methods
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Derivation Techniques and Algorithms

Figure 2 illustrates the technique for recovering the age-bounded balance velocity.

Figure 2. Schematic illustrating conventional (full-thickness) and Holocene-averaged balance-flux methods. The Holocene-averaged balance velocity is calculated by balancing the input and output ice fluxes, averaged over the period between α age and the present, through a horizontally square column of ice bounded vertically by the subaerial ice surface and the depth of the isochrone of α age. Both balance methods assume implicitly that the ice sheet is in steady state during the period represented by the portion of the ice column considered.  (Schematic is not to scale.)

Processing Steps

First, the boundary conditions (depth to 9-ka isochrone, accumulation and vertical strain rate) were determined using a 1-D ice-flow model along the radar tracks (along-track) at 1-km intervals. These results were then gridded (ordinary kriging) onto a 1-km grid. Finally, they were filtered to a 5-km grid using a 2-D Gaussian with half-width depending on the local ice thickness.

Error Sources

The greatest direct source of uncertainty in the results is the original mapping and dating of the radar internal reflections. Second to that are the assumptions associated with the balance-velocity model, particularly the steady state assumption and the determination of the region where one-dimensional ice-flow models are acceptable. See Radiostratigraphy and Age Structure of the Greenland Ice Sheet (RRRAG4) and MacGregor et al. 2016 and its Supplementary Materials for more information.

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Sensor or Instrument Description
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References and Related Publications

Contacts and Acknowledgments

Joseph A. MacGregor
Research Physical Scientist
NASA Goddard Space Flight Center
Cryospheric Sciences Laboratory (Code 615)
Greenbelt, MD 20771

Technical Contact

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
e-mail: nsidc@nsidc.org

Acknowledgments: 

This work was supported by NSF (grants ARC 1107753, ARC 1108058, and ANT 0424589) and NASA (grants NNX12AB71G and NNX15AD55G). We thank the organizations (Center for Remote Sensing of Ice Sheets and Operation IceBridge) and innumerable individuals who aided in the collection and processing of radar data used in this study. Most of the early radar data used in this study were collected as part of NASA’s Program for Arctic Regional Climate Assessment.

Document Information

DOCUMENT CREATION DATE

03 October 2016

DOCUMENT REVISION DATE

No technical references available for this data set.
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Email: nsidc@nsidc.org