DIFFUSE-REFLECTANCE OF OCEANIC WATERS .2. BIDIRECTIONAL ASPECTS

For visible wavelengths and for most of the oceanic waters, the albedo for single scattering (w) over bar is not high enough to generate wit hin the upper layers of the ocean a completely diffuse regime, so that the upwelling radiances below the surface, as well as the water-leavi ng radiances, generally do not form an isotropic radiant field. The no nisotropic character and the resulting bidirectional reflectance are c onveniently expressed by the Q factor, which relates a given upwelling radiance L(u)(theta',phi) to the upwelling irradiance E(u) (theta' is the nadir angle, phi is the azimuth angle, and Q = E(u)/L(u)); in add ition the and function is also dependent on the Sun's position. Anothe r factor, denoted f, controls the magnitude of the global reflectance, R (= E(u)/E(d), where E(d) is the downwelling irradiance below the su rface); f relates R to the backscattering and absorption coefficients of the water body (bb and a, respectively), according to R = f(b(b)/a) . This f factor is also Sun angle dependent. By operating an azimuth-d ependent Monte Carlo code, both these quantities, as well as their rat io (f/Q) have been studied as a function of the water optical characte ristics, namely (w) over bar and eta; eta is the ratio of the molecula r scattering to the total (molecular + particles) scattering. Realisti c cases (including oceanic waters, with varying chlorophyll concentrat ions; several wavelengths involved in the remote sensing of ocean colo r and variable atmospheric turbidity) have been considered. Emphasis h as been put on the geometrical conditions that would be typical of a s atellite-based ocean color sensor, to derive and interpret the possibl e variations of the signal emerging from various oceanic waters, when seen from space under various angles and solar illumination conditions .