Polarimetric Observations of the Emissivity of Whitecaps Experiment (POEWEX ’ 04) to Characterize the Azimuthal Variation of the Microwave Emission from Foam Generated by Breaking Waves

WindSat, the first polarimetric microwave radiometer on orbit, and the NPOESS Conical Microwave Imager/Sounder, scheduled for launch in 2010, are both designed to retrieve the ocean surface wind vector from radiometric brightness temperatures. Available observations and models show that the wind direction signal is only 1-3 K peak-to-peak at 19 and 37 GHz, much smaller than the wind speed signal. Therefore, quantitative knowledge of the dependence of the ocean surface emissivity on properties such as surface roughness and wave breaking is critical for wind vector retrieval. The dependence of surface emission on roughness has 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. WindSat, the first polarimetric microwave radiometer on orbit, was launched in January 2003 to demonstrate retrieval of the sea surface wind vector using passive microwave remote sensing. The NPOESS Conical Microwave Imager/Sounde