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Sea Ice Products at NSIDC |
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NSIDC provides software tools to extract and geolocate sea ice data in a polar stereographic projection derived from passive microwave instruments, as well as masking tools that limit the influence of weather effects on sea ice concentrations. All tools are available by ftp.
Geocoordinate tools and masks work with the following polar stereographic gridded passive microwave data sets at NSIDC:
Access geocoordinate and pixel-area tools online.
The geolocation and pixel-area tools described here provide the same functionality for all polar stereographic passive microwave sea ice data sets at NSIDC. They include a FORTRAN routine called "locate," a latitude/longitude grid, and a pixel-area grid. Tools include:
locate.exe_pc: FORTRAN executable file (PC DOS) that allows the user to enter an i,j coordinate and get the corresponding latitude/longitude coordinate, and vice versa
locate.for: a FORTRAN routine that allows the user to ener an i,j coordinate and get the corresponding latitude/longitude coordinate, and vice versa
mapll.for and mapxy.for: subroutines that are associated with the "locate.for" program. These programs need to be compiled, but are not run explicitly. They are called by "locate.for". Thus, the user should compile these programs with locate.for and then use locate to do the conversions.
The following grids can be used to determine the latitude and longitude values and the area of a given pixel for the 25 km grids for either hemisphere. The latitude/longitude grids are in binary format and stored as long word integers (4 byte) scaled by 100,000. Each array location (i,j) contains the latitude or longitude value at the center of the corresponding data grid cells.
The pixel-area grids contain attributes for the 25 km north and south polar stereographic gridded data sets. The arrays are in binary format and stored as long word integers (4 byte) scaled by 1000. Each array location (i,j) contains the real value of the corresponding grid cell.
psn25area_v2.dat: 304 columns x 448 rows, range = [382.068, 664.465]
psn25lats_v2.dat: 304 columns x 448 rows, range = [31.0967, 89.8363]
psn25lons_v2.dat: 304 columns x 448 rows, range = [00.0000, 360.0000]
pss25area_v2.dat: 304 columns x 448 rows, range = [382.068, 664.465]
pss25lats_v2.dat: 316 columns x 332 rows, range = [-39.3649, -89.8368]
pss25lons_v2.dat: 316 columns x 332 rows, range = [000.1651, 359.8350]
Please note that the data ranges given here are latitude and longitude values for the center of each grid cell. The range covered by the full grid extends to the pole (90 degrees latitude) and all longitudes (0 to 360 degrees longitude). Also note that corrected grids for the northern hemisphere were created June 22, 1999 and should replace the three psn25*.dat files that were distributed prior to this date.
To determine the lat/lon values of corresponding (i,j) data grid cells:
Please see our Sea Ice Trends and Climatologies from SMMR and SSM/I page for a description of ocean masks.
The land mask, coastline, and latitude/longitude line files described here can be used for all polar stereographic passive microwave sea ice data sets at NSIDC.
Beginning with the SMMR era, the first masks to be developed were "landmask.ntb" and "landmask.stb". Other masks were later added for use with F8 data ("n3a" and "s3a"). The last set of masks to be added to the series are those beginning with "gsfc" which were originally developed for use with F11 and F13. However, all of the masks provided here will work across all platforms (SMMR through SSM/I F13).
The following are land mask files that are not in HDF format:
landmask.ntb: Northern Hemisphere land mask, 25 km resolution
landmask.stb: Northern Hemisphere land mask, 25 km resolution
For the following files, resolution is indicated by "3a" or "3b" (3a = 85 GHz at 12.5 km resolution; 3b = other channels at 25 km resolution). Note, however, that all sea ice products are at 25 km resolution, so the 12.5 km masks will not apply here.
n3acoast.hdf: Northern Hemisphere coastline mask, 12.5 km resolution
n3bcoast.hdf: Northern Hemisphere coastline mask, 25 km resolution
s3acoast.hdf: Southern Hemisphere coastline mask, 12.5 km resolution
s3bcoast.hdf: Southern Hemisphere coastline mask, 25 km resolution
n3altln.hdf: Northern Hemisphere lat/lon lines, 12.5 km resolution
n3bltln.hdf: Northern Hemisphere lat/lon lines, 25 km resolution
s3altln.hdf: Southern Hemipshere lat/lon lines, 12.5 km resolution
s3bltln.hdf: Southern Hemipshere lat/lon lines, 25 km resolution
n3amask.hdf: Northern Hemisphere land mask, 12.5 km resolution
n3bmask.hdf: Northern Hemisphere land mask, 25 km resolution
s3amask.hdf: Southern Hemisphere land mask, 12.5 km resolution
s3bmask.hdf: Southern Hemisphere land mask, 25 km resolution
gsfc_12n.hdf: Northern Hemisphere land mask, 12.5 km resolution
gsfc_12s.hdf: Southern Hemisphere land mask, 12.5 km resolution
gsfc_25n.hdf: Northern Hemisphere land mask, 25 km resolution
gsfc_25s.hdf: Northern Hemisphere land mask, 25 km resolution
n3bclmsk.hdf: Northern Hemisphere land mask, 25 km resolution
Note on Land Masks
The relatively slight differences in numbers of SSM/I-grid pixels masked as land in the three grids noted above can introduce discrepancies in analyses of time series spanning the SMMR and SSM/I period. One method of addressing this issue is to generate a composite mask in which all pixels mapped as land in any of the masks are coded as land pixels in the composite mask. Use of such a composite mask improves the consistency of the SMMR and SSM/I record at the expense of masking additional ocean areas as land. A composite of the CIA World Data Bank I and II (GSFC-1 and JPL-1) has been produced by J. Maslanik at NSIDC.
An additional issue concerns effects on coastal ocean pixels of contamination by proximity to land. Such proximity can modify the brightness temperatures of coastal ocean pixels, producing false ice concentration values along some coasts. These "pixel mixing" errors are considered in the summer 1996 issue of NSIDC Notes (Issue no. 18). Maslanik et al. (1996) discuss the effects of such land contamination in introducing differences between SMMR and SSM/I time series, and they describe the use of a modified land mask where land areas are extended to mask substantial contamination. The Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I Brightness Temperatures land mask ("N3BCLMSK.HDF") alleviates this problem whereby ocean pixels adjacent to land or one pixel removed from land are designated as "land-contaminated."
Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I
DMSP SSM/I Daily and Monthly Polar Gridded Sea Ice Concentrations
Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations
Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data
Access extraction tools online.
Sample FORTRAN code is also provided for reading and displaying these sea ice concentration data.
The IDL tools provided with this data set are available by ftp. They are similar to the ones provided with the DMSP SSM/I Daily and Monthly Polar Gridded Sea Ice Concentrations, but were modified to read the different format of the GSFC Bootstrap data.
extract_bootstrap_ice.pro: This is an IDL program that extracts a time series of Bootstrap polar sea ice data. The extraction routine works on both the daily and monthly sea ice files, allowing users to select and display sea ice concentrations. For monthly data, users may select which image threshold to view (e.g. either zero, five, 10 or 15 percent ice cut off).
extract_bootstrap_ice,array
When a user types this command, the program compiles and runs and stores the data extracted in the array.
disp_bootstrap_ice_xa.pro: This is an IDL program that will display animations of the sea ice concentration grids (automatically). The routine works on both the daily and monthly sea ice files and allows users to display sea ice concentrations.
disp_bootstrap_ice_xa
When a user types this command in IDL, the program compiles and runs. It displays the number of images requested by the user.
Useful IDL commands:
openw - a procedure that opens a file for write
IDL> openw,1,'n199809av.ic'
writeu - a procedure that writes data (ex: 'seaice') into an opened file
IDL> writeu,1,seaice
loadct - a procedure that loads a color palette by providing a list or loading the palette indicated by the argument.
xloadct - a procedure that opens a new window giving the user a visual choice of colors to use
For a complete description of the Interactive Display Language (IDL), see IDL Reference Guide or IDL User's Guide. Research Systems, Inc., 2995 Wilderness Pl., Suite 203, Boulder, CO 80301, (303) 786-9900.
Access extraction tools online.
Source code includes the following:
hdftor8_alpha - A compiled C executable (at The HDF Group) to extract 8-bit raster image data from HDF files
hdftor8_dec - A compiled C executable (DECstation ULTRIX4.3) to extract 8-bit raster image data from HDF files
hdftor8_hp - A compiled C executable (at The HDF Group) to extract 8-bit raster image data from HDF files
hdftor8_pc - A compiled C executable (at The HDF Group) to extract 8-bit raster image data from HDF files
hdftor8_sgi - A compiled C executable (SGI Challenge IRIX5.2) to extract 8-bit raster image data from HDF files
hdftor8_sun -A compiled C executable (at The HDF Group) to extract 8-bit raster image data from HDF files
The extraction executables listed above are from The HDF Group.
Interactive Data Language (IDL) is a commercial data visualization and analysis software package available from Research Systems, Inc. The package is widely used at NSIDC, though no endorsement of the vendor or product is implied. Procedures developed at NSIDC are provided for the convenience of IDL users.
The IDL tools provided with this data set are available via ftp and consist of procedures that allow the user to read, display, and export F8, F11, and F13 SSM/I sea ice grids. These were revised in April 2001 to provide display and export capability for the Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations. Examples of running the IDL programs are also provided.
extract_ice.pro - This is an IDL program that extracts a time series of SSM/I polar sea ice data. The extraction routine works on both the daily and monthly sea ice files, allowing users to select and display sea ice concentrations from either the NASA Team or Bootstrap algorithms. For monthly data, users may select which image threshold to view (e.g. either zero, five, 10 or 15 % ice cut off).
extract_ice,array
When a user types this command in IDL, the program compiles and runs, and stores the data extracted in the array.
disp_ssmi_ice_xa.pro - This is an IDL program that displays animations of the sea ice concentration grids (automatically). The routine works on both the daily and monthly sea ice files and allows users to display sea ice concentrations derived from either the NASA Team or Bootstrap algorithm. For monthly data, users may select which image threshold to view (e.g. either zero, five, 10 or 15 % ice cut off).
disp_ssmi_ice_xa
When a user types this command in IDL, the program compiles and runs. It displays the number of images requested by the user.
Useful IDL commands:
openw - a procedure that opens a file for write
IDL> openw,1,'n199809av.ic'
writeu - a procedure that writes data (ex: 'seaice') into an opened file
IDL> writeu,1,seaice
loadct - a procedure
that loads a color palette by providing a list or loading the palette
indicated by the
argument
xloadct - a procedure that opens a new window giving the user a visual choice of colors to use
For a complete description of the Interactive Display Language (IDL), see IDL Reference Guide or IDL User's Guide. Research Systems, Inc., 4990 Pearl East Circle, Boulder, CO 80301, (303) 786-9900.
Tools are provided on CD-ROM with the sea ice data (Volumes 1-3). Contact NSIDC User Services to order the CD-ROM series. Tools include IDL and Matlab routines to ingest and read sea ice concentration data. The Matlab procedure has been tested on a SGI-IRIX machine, and on a PC Pentium machine under Windows for Workgroups 3.11. Cross-platform capabilities of Matlab should also permit operation on Macintosh, but this has not been tested.
Corrections to the geocoordinate and pixel-area tools, the IDL routine "showice.pro" described above, and the MATLAB routine, "showice.m" are provided on our ftp site for this particular data set.
