Accurate retrieval of ocean wind speed and direction from spaceborne microwave radiometers requires knowledge of how changes in surface properties affect brightness temperatures measured by passive microwave remote sensing instruments. Wind roughening and whitecaps are the key mechanisms allowing spaceborne microwave radiometers to remotely sense wind speed and direction. In particular, the portions of the ocean surface covered by whitecaps exhibit dramatically increased microwave emission. Near-surface radiometric observations are needed to improve quantitative knowledge of the effects of changing surface conditions, including foam and roughness, on microwave emissivity. Boresighted video coverage allows parameterization of whitecap coverage with wind speed and leads to determination of its influence on microwave emission.

The Rough Evaporation Duct (RED) experiment was sponsored by the Office of Naval Research to focus on assessment of the effects of the air-sea boundary layer, including evaporation ducts, on microwave and electro-optical signal propagation near the sea surface. This experiment centered on a suite of instruments, including RF and electro-optical link receivers, aboard the Research Platform (R/P) FLIP, moored 10 km off the northeast shore of Oahu, Hawaii, during August and September of 2001. Two ground meteorological stations, two surface buoys and two research aircraft provided essential supporting measurements of sea and atmospheric conditions. The remote sensing component of the RED experiment consisted of a K-band (18.7 GHz) polarimetric microwave radiometer designed and fabricated at the University of Massachusetts and a boresighted video camera. These instruments were deployed under the face boom of FLIP and scanned over a range of azimuth and elevation angles using a remotely pointed pan-and-tilt positioner with 0.25° accuracy. The 18.7 GHz atmospheric window frequency was chosen to provide quantitative information on sea surface emission in a commonly-used band for satellite radiometer measurements. These and related polarimetric radiometer observations near the ocean surface, combined with in-situ and nearby measurements of wind speed, sea surface temperature, sea surface wave characteristics and air-sea instability, provide data to validate geophysical models of the microwave emission from the sea surface. These models are used for operational retrieval of environmental parameters, such as sea surface temperature and winds, from passive spaceborne measurements, including WindSat, scheduled to be the first polarimetric microwave radiometer in space in early 2003.

In this paper, we provide a comparison of measured sea surface brightness temperatures at a variety of incidence and azimuth angles with geophysical models of ocean surface emission. We also illustrate the dependence of brightness temperatures on sea surface conditions and air-sea interaction over a range of wind speeds from calm to 9 m/s.