A microwave technique for mapping thin sea ice

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Abstract
A technique is presented for mapping the distribution of new, young and first‐year sea ice in seasonal sea ice zones that utilizes microwave spectral and polarization information from the Defense Meteorological Satellite Program Special Sensor Microwave/Imager. The motivation for this work stems from the need for accurate estimates of open water and thin ice within the Arctic ice pack. The technique utilizes the microwave polarization and spectral characteristics of these three ice types through two microwave radiance ratios: the 19.4 GHz polarization and the spectral gradient ratio, which is a measure of the spectral difference between the 19.4‐GHz and the 37.0‐GHz vertically polarized radiance components. The combined use of the spectral gradient ratio and polarization reduces the low ice concentration bias generally associated with the presence of thin ice types. The microwave polarization, which is sensitive to changes in ice thickness and ice surface characteristics, is used to classify new, young, and first‐year ice types. The polarization varies from about 0.3 for calm ice‐free water, to about 0.15 for new ice, and to 0.03 for thick first‐year ice. On the basis of this parameterization, improved sea ice concentration maps and maps of new, young and first‐year ice type distributions are derived. Examples from the Bering Sea are presented and comparisons are made with analyzed NOAA AVHRR imagery.