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Abstract-WindSat is a space-based polarimetric microwave radiometer designed to demonstrate the capability to measure the ocean surface wind vector using a radiometer. We describe a nonlinear iterative algorithm for simultaneous retrieval of sea surface temperature, columnar water vapor, columnar cloud liquid water, and the ocean surface wind vector from WindSat measurements. The algorithm uses a physically based forward model function for the WindSat brightness temperatures. Empirical corrections to the physically based model are discussed. We present evaluations of initial retrieval performance using a six-month dataset of WindSat measurements and collocated data from other satellites and a numerical weather model. We focus primarily on the application to wind vector retrievals.

[1] To develop a foam emissivity model, we started a systematic investigation of sea foam properties at microwave frequencies from 1 to 37 GHz. We first examined various permittivity formulae to find the most suitable one for obtaining the dielectric constant of sea foam. This paper presents an investigation of the skin depth of vertically structured foam layers for the first time. Various void fraction profiles representing variations of foam properties within the foam thickness are examined. The skin depth of foam layer thicknesses from 0.2 cm to 10 cm is obtained. The dependence of foam skin depth on frequency and foam layer thickness is investigated and compared to seawater skin depth. It was found that for exponential void fraction profile ranging from 99% at the air-foam interface to 1% at the foam-seawater boundary, the foam skin depth varies from 0.17 cm to no more than 7 cm. Possible variations of the foam skin depth due to permittivity formula choice, seawater temperature and salinity, and shape of void fraction profile, as well as its upper and lower limits, are presented. Analyses of the results help explain the sensitivity of microwave frequencies to foam layer thicknesses, infer the formation of the emissivity signal and its variations from foam-covered surfaces, infer implications for the passive remote sensing of whitecaps, discuss modeling approaches for vertically structured foam layers, and infer qualitatively the relative contributions of different foam structures to the absorption and emission of electromagnetic radiation. These findings give conceptual understanding of foam emissivity.

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Abstract-Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratory's Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, +45 , 45 , left-circular, and right-circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30 to 60 . Surface foam was generated by blowing compressed air through a matrix of gas-permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at +45 , 45 , left-circular, and right-circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.