U.S. GEOLOGICAL SURVEY DIGITAL DATA SERIES DDS-27

               Monthly Average Polar Sea-Ice Concentration

                         By Peter N. Schweitzer

                              OVERVIEW

    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.

    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.

    This data set represents the results of calculations carried out on
    the NSIDC data and 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 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 (one for each
    month from October 1978 through December 1991 except December 1987,
    and two each for July and August of 1987), 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.

    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.

    The images are provided in three formats: Hierarchical Data Format
    (HDF), a flexible scientific data format developed at the National
    Center for Supercomputing Applications; Graphics Interchange Format
    (GIF); and Macintosh PICT format.  The ice-concentration grids are
    distributed by NSIDC in HDF format.

    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.

    Background information regarding satellite passive microwave
    observations in general, and data from the Scanning Multichannel
    Microwave Radiometer (SMMR) in particular, can be found in the
    excellent book by Gloersen and others (1993).  That work includes
    detailed discussions of many interesting ice features which are
    apparent in these data, such as the North Water polynya formed during
    the thawing of Baffin Bay and the "Odden", an ice tongue that usually
    (but not always) appears off the east coast of Greenland north of
    Iceland in the Greenland Sea during late winter.

    The land mask (that is, the image pixels coded as land) in the source
    data changed in the transition from the SMMR to the SSM/I instruments.
    Consequently, a close comparison of images from the period 1978-1987
    with images from the period 1987-1991 will show small differences in
    the distribution of land pixels.  The composite images use the land
    mask of the SSM/I images.

    Errors in the source data occur primarily along coastlines, where the
    data show ice concentrations for times when ice is unlikely to be
    present.  The source of these errors is probably microwave reflection
    from small land masses and shallow water.  The microwave reflectance
    of land is so much greater than that of water that even small amounts
    of land occurring in a given coastal area can generate a false ice-
    cover signal  (Claire Parkinson, personal communication).

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