Azimuthal Variation of the Emissivity of Foam From C and X Band Polarimetric Measurements

Sea foam increases surface emission and brightness temperature at microwave frequencies. Together with the surface roughness, it is a key component of the signal used to obtain surface wind vector with satellite-borne radiometric and polarimetric instruments. Current knowledge of foam emissivity, however, is incomplete, particularly in regard to azimuthal effects. Since breaking waves on the open ocean are intermittent and highly variable, radiometric measurements of reproducible breaking waves were performed in an outdoor salt water wave tank in 2002 and 2004. The wave tank was configured to create foam-producing breaking waves using an underwater shoaling beach. Four radiometers, operating at frequencies of 6.8, 10.8, 18.7, and 37 GHz, were positioned at several incidence angles and azimuth look angles to record microwave emission from the breaking waves. A bore-sighted video camera recorded images of foam fraction within radiometers footprints. Auxiliary information on the wave field and breaking events (such as void fraction, bubble size spectrum, foam layer thickness, wave height, and subsurface turbulent dissipation) was provided by a suite of additional instruments. The results of wave tank measurements of the azimuthal dependence of foam emissivity at 6.8 and 10.8 GHz are presented, and possible contributors to the observed azimuthal changes of foam emissivity are discussed