Foam and Roughness Effects on Passive Microwave Remote Sensing of the Ocean

Whitecaps and surface roughness are the two main components of the wind-induced microwave emissivity change of the ocean surface. The resulting difference of the received brightness temperature from that of a flat foamless sea surface at the same sea surface temperature and salinity is used for ocean surface wind vector retrieval using passive microwave remote sensing technology. In other applications such as sea surface salinity retrieval using L-band microwave radiometers, the wind-induced emissivity change is the major source of error, and its correction is important to the accuracy of retrieved salinity. Furthermore, global whitecap distribution using spaceborne radiometer measurements is possible if the contributions of emissivity change from roughness and foam can be separated. This is an important application because whitecaps are the manifestation of surface wave breaking, which is of great importance in air-sea interaction and climate research. This paper describes an analysis of the foam and roughness components of wind-induced emissivity change. The analysis is in good agreement with global WindSat measurements obtained over a broad range of wind speeds. Quantitative results on the variation of the foam and roughness components with respect to wind speed, incidence angle, microwave frequency, and polarization are presented.