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Monthly average polar sea-ice concentration

Metadata also available as - [Outline] - [Parseable text] - [DIF]

Frequently anticipated questions:

What does this data set describe?

Title: Monthly average polar sea-ice concentration
Abstract:
This data set represents the results of calculations carried out on sea-ice-concentration data from the SMMR and SSM/I instruments. The original data were obtained from the National Snow and Ice Data Center (NSIDC). The data set also contains the source code of the programs that made the calculations. The objective was to derive monthly averages for the whole 13.25-year series (1978-1991) and to derive a composite series of monthly averages representing the variation of an average year. The resulting file set contains monthly images for each of the polar regions for each year, yielding 160 files for each pole, and composite monthly averages in which the years are combined, yielding 12 more files. Averaging the images in this way tends to reduce the number of grid cells that lack valid data; the composite averages are designed to suppress interannual variability.
Also included in the data set are programs that can retrieve seasonal ice-concentration profiles at user-specified locations. These nongraphical data retrieval programs are provided in versions for UNIX, extended DOS, and Macintosh computers. Graphical browse utilities are included for the same computing platforms but require more sophisticated display systems.
Supplemental_Information:
The data contained in this data set are derived from the Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/ Imager (SSM/I) data produced by the National Snow and Ice Data Center (NSIDC) at the University of Colorado in cooperation with the U.S. National Aeronautics and Space Administration (NASA) and the U.S. National Oceanic and Atmospheric Administration (NOAA). The basic data come from satellites of the U.S. Air Force Defense Meteorological Satellite Program. NSIDC distributes three collections of sea- ice-concentration grids on CD-ROM: data from the Nimbus-7 SMMR (October 25, 1978 through August 20, 1987) are provided on volume 7 of the SMMR Polar Data series (NASA, 1992); data from the SSM/I are provided on two separate volumes, covering the periods from July 9 of 1987 to December 31 of 1989, and from January 1 of 1990 through December 31 of 1991, respectively. The NASATEAM data from revision 2 of the SSM/I CD-ROM's were used to create the present data set. SMMR images were collected every 2 to 3 days, whereas SSM/I data are provided in daily ice-concentration grids. Apart from a number of small gaps (5 or fewer days) in the record, the only long period for which no data are available is December 3 of 1987 through January 12 of 1988, inclusive.
As ancillary data, the ETOPO5 global gridded elevation and bathymetry data (Edwards, 1989) were interpolated to the resolution of the NSIDC data; the interpolated topographic data are included.
  1. How should this data set be cited?

    Schweitzer, Peter N., 1995, Monthly average polar sea-ice concentration: U.S. Geological Survey Digital Data Series DDS-27, U.S. Geological Survey, Reston, Virginia.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -180.0
    East_Bounding_Coordinate: 180.0
    North_Bounding_Coordinate: 90.0
    South_Bounding_Coordinate: -90.0

  3. What does it look like?

    <http://geochange.er.usgs.gov/pub/sea_ice/Derived/images/gif/north/02n.gif> (GIF)
    Average February Northern Hemisphere sea-ice concentration plotted on a polar stereographic projection of the North Pole
    <http://geochange.er.usgs.gov/pub/sea_ice/Derived/images/gif/south/02s.gif> (GIF)
    Average February Southern Hemisphere sea-ice concentration plotted on a polar stereographic projection of the South Pole

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 25-Oct-1978
    Ending_Date: 31-Dec-1991
    Currentness_Reference:
    SMMR aboard Nimbus-7 satellite became fully operational on 781025; that instrument was replaced but the observations were continued by the SSM/I aboard DMSP F8 satellite.

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: remote-sensing image

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a Raster data set. It contains the following raster data types:

      • Dimensions 304 x 448, type Pixel
      • Dimensions 316 x 332, type Pixel

    2. What coordinate system is used to represent geographic features?

      The map projection used is Polar Stereographic.

      Projection parameters:
      Straight-Vertical_Longitude_from_Pole: 135.0
      Standard_Parallel: 70.0
      False_Easting: 0.0
      False_Northing: 0.0

      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 25000
      Ordinates (y-coordinates) are specified to the nearest 25000
      Planar coordinates are specified in meters

      The ellipsoid used is Hughes.
      The semi-major axis of the ellipsoid used is 6378.273.
      The flattening of the ellipsoid used is 1/298.27941.

  7. How does the data set describe geographic features?

    Ice-concentration grid cell
    Any of the one-byte data elements in the sea-ice- concentration files (Source: self-evident)

    Ice-concentration grid cell value
    The proportion of the ocean area represented by the grid cell that is covered with sea ice. (Source: Calibrations of the SMMR and SSM/I instruments are described in the following three publications: Gloersen, P., Campbell, W.J., Cavalieri, D.J., Comiso, J.C., Parkinson, C.L., and Zwally, H.J., 1993, Arctic and Antarctic sea-ice, 1978-1987: Satellite passive-microwave observations and analysis: NASA Special Publication no. 511, 290 p. National Snow and Ice Data Center, 1994, Nimbus-7 SMMR polar radiances and Arctic and Antarctic sea ice concentrations on CD-ROM user's guide, version 3: Boulder, Colorado, National Snow and Ice Data Center, 23 p. National Snow and Ice Data Center, 1992, DMSP SSM/I brightness temperature and sea ice concentration grids for the polar regions on CD-ROM, User's guide: National Snow and Ice Data Center Special Report 1, [300 p].)

    ValueDefinition
    -99Missing data
    -88Land area

    Range of values
    Minimum:0
    Maximum:100
    Units:percent sea-ice cover
    Resolution:1

    ETOPO5 grid cell
    Any of the two-byte data elements in the files etopo5.n and etopo5.s (Source: self-evident)

    ETOPO5 grid cell value
    Height of the land surface or depth of the sea floor, relative to mean sea level. (Source: NOAA/NGDC. 1992. Integrated Global Elevation and Bathymetry. Digital Data. NOAA/NGDC/WDC-A, Boulder, Colorado. Digital raster data on a 5-minute geographic (lat/long) 2160x4320 grid. In: Global Ecosystems Database Version 1.0: Disc A. Boulder, CO: NOAA National Geophysical Data Center. 1 independent single-attribute spatial layer on CD-ROM. 18.6 MB. [First published in 1989.])

    Range of values
    Minimum:-10783
    Maximum:8000
    Units:meters
    Resolution:1

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

  3. To whom should users address questions about the data?

    Peter N. Schweitzer
    U.S. Geological Survey
    Mail Stop 954 National Center
    U.S. Geological Survey
    12201 Sunrise Valley Drive
    Reston, VA 20192
    USA

    (703) 648-6533 (voice)
    (703) 648-6252 (FAX)
    pschweitzer@usgs.gov

Why was the data set created?

The purpose of this data set is to provide paleoclimate researchers with a tool for estimating the average seasonal variation in sea-ice concentration in the modern polar oceans and for estimating the modern monthly sea-ice concentration at any given polar oceanic location. It is expected that these data will be compared with paleoclimate data derived from geological proxy measures such as faunal census analyses and stable-isotope analyses. The results can then be used to constrain general circulation models of climate change.

How was the data set created?

  1. From what previous works were the data drawn?

    SMMR CD-ROM from NSIDC (source 1 of 4)
    NASA, 1992, Radiances on a North Polar Stereographic Grid 4/1/ 87 - 8/20/87 and Arctic and Antarctic Sea Ice Concentrations 10/25/78 - 8/20/87 volume 7 of Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) Polar Data: Goddard Space Flight Center SMMR Polar Data volume 7.

    Other_Citation_Details:
    The volume also contains brightness temperature data from 870401 through 870820. The CD-ROM is labelled USA_NASA_971_SMR_0007. The data are distributed by NSIDC Distributed Active Archive Center, Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309-0049.
    Type_of_Source_Media: CD-ROM
    Source_Contribution:
    Monthly average sea-ice-concentration files in the present data set for the period 781025 through 870820 were derived from these Nimbus-7 SMMR ice-concentration data.

    DMSP/SSMI ICE Grids volume 1 (source 2 of 4)
    NSIDC, 1992, DMSP SSM/I ice concentration grids for the polar regions: DMSP SSM/I brightness temperature and sea-ice concentration grids for the polar regions on CD-ROM volume 1, revision 2.

    Other_Citation_Details:
    The data are distributed by NSIDC Distributed Active Archive Center, Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309-0049.
    Type_of_Source_Media: CD-ROM
    Source_Contribution:
    Monthly average sea-ice-concentration files in the present data set for the period 19870709 through 19891231 were derived from these DMSP F8 SSM/I ice-concentration data.

    DMSP/SSMI ICE Grids volume 2 (source 3 of 4)
    NSIDC, 1993, DMSP SSM/I ice concentration grids for the polar regions: DMSP SSM/I brightness temperature and sea-ice concentration grids for the polar regions on CD-ROM volume 2, revision 2..

    Other_Citation_Details:
    The data are distributed by NSIDC Distributed Active Archive Center, Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309-0049.
    Type_of_Source_Media: CD-ROM
    Source_Contribution:
    Monthly average sea-ice-concentration files in the present data set for the period 19900101 through 19911231 were derived from these DMSP F8 SSM/I ice-concentration data.

    ETOPO5 (source 4 of 4)
    Edwards, Margaret H., 1989, Global gridded elevation and bathymetry (ETOPO5): Digital raster data on a 5-minute geographic (lat/long) 2160x4320 (centroid-registered) grid: U.S. National Oceanic and Atmospheric Administration, National Geophysical Data Center, Boulder, Colorado.

    Type_of_Source_Media: CD-ROM
    Source_Contribution:
    Elevation and bathymetry used in the graphical data retrieval programs of DDS-27 were derived from ETOPO5.

  2. How were the data generated, processed, and modified?

    Date: 1994 (process 1 of 6)
    Calculate monthly averages and composite monthly averages.
    The Bourne shell script do_avgs describes the manner in which the program average was used to calculate the monthly and composite monthly average sea-ice concentration files. The program average is given a list of sea-ice-concentration data files in HDF format. For each grid cell in the images, it calculates the arithmetic average of the corresponding cell in the input files and writes the average image into a new file. This program is run once for each month of each year represented in the original data to yield monthly averages. Then the composite monthly averages are calculated using the monthly averages for each year.

    Data sources used in this process:

    • DMSP/SSMI Ice Grids volume 1
    • DMSP/SSMI Ice Grids volume 2

    Date: 1994 (process 2 of 6)
    Create GIF and PICT images of monthly and composite monthly averages.
    The Bourne shell script do_imgs converts the monthly average and composite monthly average files into GIF and PICT images. This script uses two programs from the freely available PbmPlus toolkit developed by Jeff Poskanzer.

    Date: 1994 (process 3 of 6)
    Regrid ETOPO5 elevation and bathymetry data to fit sea-ice concentration grid.
    The ETOPO5 global gridded elevation and bathymetry data set describes world topography on a grid whose cells measure 5 minutes of latitude by 5 minutes of longitude. Since the ice-concentration data are given in a polar stereographic projection, the elevation of cells in the ice-concentration grid must be determined by appropriately sampling or interpolating the ETOPO5 data. Because the ETOPO5 grid is finer than the ice-concentration grid, a sampling algorithm was combined with a spatial averaging scheme to derive the desired elevation data. The program that implements this algorithm is called topo.c; its strategy is to find the ice grid cell coordinates of the center of each ETOPO5 cell and to calculate the average depth of all of the ETOPO5 cells that fall within an ice grid cell. The results are two 16-bit images with big- endian byte ordering in which each signed 16-bit word describes the average depth or elevation in meters (sea level is zero) of a cell in the ice-concentration grid.
    Along with the 16-bit depth data files, displayable images are created using depthppm.c in portable pixmap format, and from that intermediate stage, GIF and PICT images are created using the PbmPlus toolkit. Depthppm reduces the complexity of the depth data by assigning one color to a range of depths. The relationship between depth ranges and colors is given in the source-code file depthppm.c.
    The Bourne shell script do_topo carries out these operations.

    Data sources used in this process:

    • ETOPO5

    Date: 1994 (process 4 of 6)
    Create 2-degree by 2-degree gridded ASCII data files.
    One of the principal derivative products of this data set is a set of ice-concentration grids in which the grid cell size and position match those produced by the USGS PRISM project for its reconstruction of the climate of the Pliocene, a warm interval about 3 million years before present. These grids are tab-delimited ASCII files suitable for use in spreadsheet programs; each cell of the spreadsheet represents the ice-concentration value of one grid cell measuring 2 degrees latitude by 2 degrees longitude. This calculation is carried out by the Bourne shell script do_tabs.
    The procedure is to use a list of the geographic coordinates of the grid cell centers as the request file for the program u_rtrv. The result is a table where each row is a grid cell, and columns 3 through 14 represent the ice-concentration profile for each grid cell (columns 1 and 2 reiterate the grid cell coordinates).
    The grid cell centers are given in the files grid_n.2x2 and grid_s.2x2 for the Northern and Southern Hemispheres, respectively. The output of u_rtrv is stored in the files arctic.2x2 and antarctic.2x2.
    By using the UNIX cut command, the columns themselves were isolated in separate files, and the program table.c was used to rearrange each column into a tab-delimited ASCII table suitable for entry into a spreadsheet.

    Date: 10-May-2004 (process 5 of 6)
    Graphical viewer software rewritten using Tcl/Tk to enable the viewer to run on current operating systems: Microsoft Windows and Linux.

    Date: 28-Jul-2004 (process 6 of 6)
    Fixed a serious bug in u_rtrv.c in which the program required the HDF data file name to contain ".s" in order to consider the data to be southern-hemisphere data. This caused pixel coordinates calculated from geographic locations in the south polar region to be incorrect. The gridded data files were affected by this bug, so the files in Derived/grid/south have been rebuilt with corrected information. Thanks to Dr. Simon H.H. Nielsen (post-doc at Univ Florida) for pointing out the problem.

  3. What similar or related data should the user be aware of?

    Schweitzer, Peter N. , 1993, Modern Average Global Sea-Surface Temperature: U.S. Geological Survey Digital Data Series DDS-10.

    Online Links:

    Other_Citation_Details: Library of Congress Cataloging-in-Publication Number 93-9075

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

    Calculation of sea-ice-concentration values from brightness temperatures is described extensively in the user's guides of the source data products and in the NASA publication dealing with the SMMR data.

  2. How accurate are the geographic locations?

    The grids provided in the source data (supplied by NSIDC) are described in the user's guides of the relevant products, and the positional accuracy of the pixels is dealt with both in the user's guides and in the NASA report on the SMMR data.

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    Included in the data set is a table enumerating the days for which sea-ice-concentration data were available in the source material. In general, images were available every 2 days during the time period from 781025 through 870709, when the SMMR was used, and images were available every day during the period 870709 through 911231, when the SSM/I was used. A large gap (871202 through 880113) occurred when the SSM/I became temporarily inoperative. Other gaps in the SSM/I record were not longer than five consecutive days.

  5. How consistent are the relationships among the observations, including topology?

    Included in the data set are the UNIX shell scripts and makefiles that were used to derive the averages and create the tables and images. Source code is provided for all programs written by the author.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints: none
Use_Constraints: none

  1. Who distributes the data set? (Distributor 1 of 1)

    Peter N. Schweitzer
    U.S. Geological Survey
    Mail Stop 954 National Center
    U.S. Geological Survey
    12201 Sunrise Valley Drive
    Reston, VA 20192
    USA

    (703) 648-6533 (voice)
    (703) 648-6252 (FAX)
    pschweitzer@usgs.gov

  2. What's the catalog number I need to order this data set?

    U.S. Geological Survey Digital Data Series DDS-27

  3. What legal disclaimers am I supposed to read?

    Although all data and software published on this CD-ROM have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials or the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data, software, or related materials.

  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 28-Jul-2004
Metadata author:
Peter N. Schweitzer
Mail Stop 954 National Center
U.S. Geological Survey
12201 Sunrise Valley Drive
Reston, VA 20192
USA

(703) 648-6533 (voice)
(703) 648-6252 (FAX)
pschweitzer@usgs.gov

Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

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