AZIMUTHAL DEPENDENCE OF THE MICROWAVE EMISSION FROM FOAM GENERATED BY BREAKING WAVES AT 18.7 AND 37 GHz

The need to improve retrieval of the surface wind vector and sea surface temperature (SST) from WindSat and the upcoming NPOESS Conical Microwave Imager Sounder (CMIS) motivated measurements of the microwave emission from breaking waves, both on the open ocean and in a wave basin. Aircraft and satellite measurements have demonstrated that the wind direction dependence of ocean surface brightness temperatures is small, on the order of 1-3 K peak-to-peak. Therefore, the accuracy of wind vector retrieval depends strongly upon quantitative knowledge of the relationship of the ocean surface emissivity to surface properties, such as sea surface wave spectrum and wave breaking. The effects of the surface wave spectrum have been addressed by many studies, but the azimuthal dependence of the microwave emission from breaking waves and foam has not been adequately addressed. Recently, a number of experiments have been conducted to quantify the increase in sea surface microwave emission due to foam. The Polarimetric Observations of the Emissivity of Whitecaps EXperiment (POEWEX´04) was conducted during November 2004 to measure the azimuthal dependence of reproducible breaking waves in order to improve wind vector retrieval from spaceborne radiometric measurements, especially at wind speeds of 7 m/s and higher. The emissivity of breaking waves was shown to vary as a function of azimuth angle at four different WindSat frequencies