Wind Speed Effects on Sea Surface Emission and Reflection for the Along Track Scanning Radiometer

Abstract

The emission and reflection properties of a rough sea surface are investigated, with particular emphasis on the wavelengths and viewing geometry relevant to the Along Track Scanning Radiometer. The authors start from Fresnel's equations for a flat water surface and calculate the effect of changing sea state on direct emissivity and reflectivity. The authors then investigate the role of surface-emitted surface-reflected (SESR) radiation, which enhances emissivity at high wind speeds. The effect of foam and whitecaps at high wind speeds is referred to briefly in the appendix but essentially remains an unknown quantity. Radiative transfer calculations that employ emissivity models also have to consider the reflection of downwelling radiance from the atmosphere. Although energy conservation requires that reflectivity is 1 minus emissivity, the variability of the sky brightness with zenith demands that one consider also the angular distribution of the reflected radiance. Additionally, the extended statistical model is used to investigate what one may call surface-reflected surface-reflected (SRSR) radiance. It was found that the SESR and SRSR effects, and the reflection of the anisotropic sky radiance, together act to cancel the first-order effect of reduced emissivity with increasing wind speed, such that the approximation of constant emissivity and specular reflection is essentially valid for the Along Track Scanning Radiometer viewing geometry. Finally, parameterizations are derived for the variable emission and reflection of a rough sea surface that are suitable for fast radiative transfer models.