Diffuse reflectance of oceanic waters II Bidirectional aspects

Abstract

For visible wavelengths and for most of the oceanic waters, the albedo for single scattering (?) is not high enough to generate within the upper layers of the ocean a completely diffuse regime, so that the upwelling radiances below the surface, as well as the water-leaving radiances, generally do not form an isotropic radiant field. The nonisotropic character and the resulting bidirectional reflectance are conveniently expressed by the Q factor, which relates a given upwelling radiance L(u) (θ',φ) to the upwelling irradiance E(u) (θ' is the nadir angle, φ is the azimuth angle, and Q = E(u)/L(u)); in addition the Q function is also dependent on the Sun's position. Another factor, denoted f, controls the magnitude of the global reflectance, R (= E(u) /E(d), where E(d) is the downwelling irradiance below the surface); f relates R to the backscattering and absorption coefficients of the water body (b(b) and a, respectively), according to R = f(b(b)/a). This f factor is also Sun angle dependent. By operating an azimuth-dependent Monte Carlo code, both these quantities, as well as their ratio (f/Q) have been studied as a function of the water optical characteristics, namely ? and η; η is the ratio of the molecular scattering to the total (molecular + particles) scattering. Realistic cases (including oceanic waters, with varying chlorophyll concentrations; several wavelengths involved in the remote sensing of ocean color and variable atmospheric turbidity) have been considered. Emphasis has been put on the geometrical conditions that would be typical of a satellite-based ocean color sensor, to derive and interpret the possible variations of the signal emerging from various oceanic waters, when seen from space under various angles and solar illumination conditions.

Diffuse attenuation coefficient of downwelling irradiance: An evaluation of remote sensing methods

“…Because the ratio of R rs (490)/R rs (555) or other spectral ratios of R rs (l) are not sensitive to Sun angle variation Morel and Gentili, 1993], the difference between K d (490) derived from the R rs (l) ratio and the measured K d (490) includes the variation introduced by changes in Sun angle. Applying the simple model of Sathyendranath et al [1989] shows that for the same absorption and backscattering coefficients, K d (490) can vary by 30% if the solar zenith angle varies between 20°a nd 70°.…”

In-water algorithms for ADEOS/OCTS

“…The value of Q was reported to be from 4.4 to >5.4 (Tyler, 1960;Smith, 1974;Austin, 1979;Clark, 1981). In this paper, the value of Q is taken to be 4.5 according to Morel and Gentili (1993).…”

Solar Radiation Modeling and Simulation of Multispectral Satellite Data

“…The transmittance by aerosols is minimum at the short wavelengths and increases slowly as the wavelength increases. (Morel & Gentili, 1993) …”