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SMMR Brightness Temperature Land Masks

The SMMR land mask and coastline overlay grids provided in the CD-ROM directory TOOLS, for use with the SMMR brightness temperature grids, are not idenpical to the land mask and overlay grids distributed by NSIDC with the SSM/I brightness temperature and ice concentration grids. This is because the land mask and coastal overlays were developed in two different environments. The SSM/I overlays developed at NSIDC in 1985 used the CIA World Data Base I (WDB-I) coastlines. The SMMR land mask, developed at NASA/GSFC, uses the CIA World Data Base II (WDB-II). A detailed description of the NASA/GSFC process is found in Gloersen et al. 1992, sec. 2.2.3, pp. 22-24. The differences in the land masks result in about a 3 % difference in the number of ocean pixels on the North Polar grid. To facilitate intercomparison of SSM/I and SMMR data, it is suggested that you select either the WDB-I or WDB-II land mask and lat/lon overlay for your work, and use that selection exclusively. Otherwise, areal intercomparisons may lead to inconsistent results.

Land Masks for SMMR CD-ROM

The land mask is created as follows: First, a high-resolution coastline file [adapted from the Central Intelligence Agency (CIA) World Data Base II] is used to produce land boundaries on a high-resolution map with twice the resolution (four times as many pixels) of the projections used here, and with all pixels initially designated as ocean. Next, the high-resolution pixels containing a boundary are designated as coastline, and the areas within the coastline are designated as land. .... The pixels in the higher-resolution map are then grouped into 2 x 2 arrays, corresponding to the single pixels in the lower-resolution map. The determination of the proper designations for the pixels (land, ocean, or coastline) in the lower-resolution map involves performing two classifications of each 2 x 2 higher-resolution array . . . . (Gloersen et al. 1992, sec. 2.2.3, pp. 22-24)

-- one emphasizing ocean and the other land, and then tallying the results over each lower-resolution pixel.