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.
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.
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.
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.
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.
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.
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].