8 value used. The importance of backscattering angles close to 180° for water leaving radiance with a fixed backscattering ratio stems from the fact that in the first order of scattering, not all backscattered photons are able to leave the water, with the Fresnel reflection coefficient increasing as the backscattering direction recedes from the zenith until at 48.6° (for flat sea surface),
total internal scattering makes it impossible for the photons to leave the water. This means that for a light source at the zenith, the first order of scattering photons may leave the water only if scattered between 131.4° and 180°. This is why this scattering region (see also Sullivan & Twardowski 2009), as opposed to total backscattering, is so important FG-4592 for reflectance, especially in the small single scattering albedo regime (where a single order of scattering is dominant). For RSR which takes into account only vertical water leaving radiance, the first order of scattering influences RSR only through a single backscattering angle 180° − φ, where φ is the (in-water) source zenith angle. Therefore, the existence
of a scattering peak at 180° translates directly into a RSR peak for ω = 0° (the solar zenith angle in Figure 3 is defined above the water, but obviously the zenith angle of 0° is identical in and above the water). Therefore, the different values of the 180° scattering peak for different phase Sotrastaurin functions (with Henyey-Greenstein having no peak and Petzold having the largest one) seem to be the source of RSR variability close to a solar zenith angle of 0°. Zaneveld (1995), who analytically considered the variability of
the remote sensing reflectance, showed that the approximation of RSR is proportional to the value of the phase function for an angle π – ψ (where ψ is the zenith angle of maximum of radiance). Apart from Petzold’s functions, the values of the water leaving radiance for various phase functions (Figure 3) Staurosporine are arranged in the same way as the scattering angles for values less than 180°. The highest water leaving radiance for the zenith Sun’s position (angular distance from the zenith) from 0 to about 60° is observed for the function FF with n = 1.01, and the lowest value in that range of angles has the function of HG. For larger zenith angles the situation is reversed: phase functions are arranged in the same way for angles 180 – ψ. For ψ from 0 to about 60, the highest phase function values are those for FF with n = 1.01, while the lowest ones are the values of HG. We show that the difference in angular shape between measured and analytical (Fournier-Forand) functions of the same backscattering ratio is not the only source of discrepancy in calculated remote sensing reflectances.