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Azimuthal Variation of the Microwave Emissivity of Foam
NAVAL RESEARCH LAB WASHINGTON DC REMOTE SENSING DIV
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Successful naval operations require excellent knowledge of the ocean wind speed and direction. In addition, the global ocean wind vector is a key element for weather forecasting and for climate and oceanography studies. WindSat, a satellite-borne multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory, has demonstrated the ability to remotely sense the global ocean wind vector from space.1 The wind direction signal measured by WindSat is about two orders of magnitude smaller than the background scene, and only a little larger than the radiometer noise floor. Therefore, any small uncertainties in the geophysical model used to retrieve the wind direction will introduce errors in the retrieved wind vector. One such uncertainty is the contribution of sea foam on the wind direction signal. Down-looking radiometers, such as WindSat, receive energy emitted from the ocean surface and the atmosphere. The energy from the ocean surface, quantified as the brightness temperature, is related to surface physical temperature by TB es . TW, where TB is the brightness temperature, TW is the physical temperature of ocean water surface, and es is surface emissivity, which depends on the measurement frequency, polarization, incidence angle, and the azimuth angle between wind direction and the direction from which observations are made. The emissivity of the sea surface also depends on physical properties of the water surface such as temperature, salinity, and surface roughness, which is primarily wind-driven. The presence of foam and roughness created by wind and breaking waves greatly increases the surface emission at microwave frequencies. The surface emissivity is sometimes written as es f . eF 1-f.er, where f is the fraction of the surface covered with foam, eF is the emissivity of foam, and eR is the emissivity of the foam-free, rough water surface.
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