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The following three images of the Bering Sea from 24 April 2001 illustrate the usefulness of MODIS data and sea ice products for monitoring sea ice conditions. The first image (below) was derived from level 1B calibrated visible-band radiances (bands 1, 4, and 3), available from the Goddard Earth Sciences (GES) Distributed Active Archive Center (DAAC). Although sea ice is clearly seen in the middle portion of the image, sea ice typically is not easily distinguished from cloud cover. MODIS sea ice products alleviate this confusion with an algorithm that identifies sea ice on the basis of reflectance characteristics in the visible and near infrared wavelengths, and also estimates ice surface temperature (IST).
Visible image of Bering Sea, 24 April 2001
The image below is a sea ice extent image derived from the MODIS/Terra Sea Ice Extent L2 Swath 1km data set, available from NSIDC. Sea ice extent was determined from reflectance criteria, based on a Normalized Difference Snow Index (NDSI) algorithm adapted for sea ice. The NDSI detects high reflectance of sea ice at visible wavelengths, and low reflectance at infrared wavelengths. Sea ice is clearly distinguished from cloud cover in this image.
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The next image shows IST, derived from the MODIS/Terra Sea Ice Extent L2 Swath 1km data set. Sea ice is identified as any pixel with a surface temperature less than or equal to the freezing point of sea water (271.5 K).
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