Numerical studies of polarized light reflection from wind roughed liquid surfaces

Abstract

Satellite remote measurements taken over the ocean surface, often contain information of near-surface or sub-surface processes, which occur on, or over the ocean. These remote measurements, can enable as to retrieve important parameters such as near surface wind, sea surface temperature and salinity and underwater bathymetry, which, all, in one way or another, depend on how well we understand sea surface reflection and roughness. However, the quality of these retrievals are often limited by the amount of scattering, emission and absorption of electromagnetic radiation caused by atmospheric aerosols which often shows dependency on polarization. From a remote sensing perspective, it is very well known that at off-nadir incidence angles and wavelenghts longer than 3.6 mum, the radiance originated from the sea surface becomes partially vertically polarized. Similarly, at shorter wavelenghts (shorter than 3.6 mum), the total radiance at off-nadir angles can become strongly horizontally polarized , . Since at shorter wavelengths, the reflected skylight becomes more significant; it is important to quantify the contribution that atmospheric polarization has on the top of the atmosphere remotely observed sea surface reflection, specially near areas of enhanced surface reflection such as from the Sun glitter. Furthermore, surface phenomena, such as wind-generated waves, gravity and capillary waves, internal waves, surface currents, density fluctuations etc., modulate the ocean's surface roughness which has a direct effect on sea surface reflection. This modulating effect is represented by the so called the mean square slope (MSS) of ocean surface waves. In general, the MSS is mainly related to short wind waves which are strongly dependent on wind speed, wind direction, surface waves and air sea temperature. To understand these wind and wave related effects on the Sun glint pattern, Salinas [3] have developed an atmospheric radiative transfer model that incorporates a new wind-wave description for the mean square slope of the sea surface. This model solves the scalar radiative transfer equation and includes wind interaction and wave states, such as wave age, as the main factors contributing to surface roughness. In this paper, we extend the above mentioned work and include light polarization effects originating from aerosol and molecular scattering and from the roughness of the surface. Polarization is considered by using the 4-Stokes vector formalism and the 4x4 scattering phase matrix for aerosols and molecules. Simulation results showing polarization effects on the simulated, top of the atmosphere, Sun glint pattern are presented. Initial simulations performed at low wind speed show increasing levels of linear polarization with increasing values for the wave age parameter. However, at higher wind speed, lower levels of linear polarization is obtained for similar values of the wave age parameter. On the other hand, calculations performed at near surface levels, show low linear polarization and a gradual shift of the Brewster angle with increasing wind speed and wave age. These results, although initial, will be carefully analyzed and compared with limiting cases where available. Finally, the present model can have possible applications to the retrieval of wind and wave states, such as wave age, near a Sun glint region and for the study of of ocean surface roughness characteristics arising from air-sea interaction.