Microwave emissivity of ocean foam and its effect on nadiral radiometric measurements

Emissivities at a normal look angle have been computed for two models of ocean foam: a porous dielectric and a series of thin water films. The two models give nearly the same results for low foam densities. They show an emissivity spectrum that is roughly constant from 20 to 60 GHz and that tends to decrease at lower frequencies, depending on the thickness and density of the foam. A whitecap model (with graded density) shows less change of emissivity with frequency. Radiometric measurements are described that show the increased emissivity of rough seas due to white water at frequencies of 10.7, 19.4, 22.2, and 31.4 GHz. The effect of increased emissivity of the surface on radiometrically inferred values of atmospheric parameters such as water vapor and liquid water contents is discussed.