Bidirectional reflectance of oceanic waters: accounting for Raman emission and varying particle scattering phase function

Abstract

The bidirectionality of the upward radiance field in oceanic case 1 waters has been reinvestigated by incorporation of revised parameterizations of inherent optical properties as a function of the chlorophyll concentration (Chl), considering Raman scattering and making the particle phase function shape (beta(rho)) continuously varying along with the Chl. Internal consistency is thus reached, as the decrease in backscattering probability (for increasing Chl) translates into a correlative change in beta(rho). The single particle phase function (previously used) precluded a realistic assessment of bidirectionality for waters with Chl > 1 mg m(-3). This limitation is now removed. For low Chl, Raman emissions significantly affect the radiance field. For moderate Chl (0.1-1 mg m(-3)), new and previous bidirectional parameters remain close. The ocean reflectance anisotropy has implications in ocean color remote-sensing problems, in derivation of coherent water-leaving radiances, in associated calibration-validation activities, and in the merging of data obtained under various geometrical configurations.

Generalized ocean color inversion model for retrieving marine inherent optical properties

“…where b b is the total backscattering coefficient (m −1 ) and a is the total absorption coefficient (m −1 ) [21], but formulations such as u λ b b λ ∕a λ have also been adopted [22]. Common methods for estimating G λ include Gordon et al [21], where G 1 and G 2 are spectrally fixed to 0.0949 and 0.0794 (see [7,23] for alternative coefficients), and the tabulated results of Morel et al [22], where G 1 is estimated using solar and sensor geometries and an estimate of algal biomass and G 2 is set to 0.…”

“…Common methods for estimating G λ include Gordon et al [21], where G 1 and G 2 are spectrally fixed to 0.0949 and 0.0794 (see [7,23] for alternative coefficients), and the tabulated results of Morel et al [22], where G 1 is estimated using solar and sensor geometries and an estimate of algal biomass and G 2 is set to 0. GIOP supports all of these options.…”

Modelling ocean-colour-derived chlorophyll <i>a</i>

“…The bidirectional function Q has values between 3 and 5 sr (Morel et al, 2002) and depends on several variables, including inherent optical properties of the water, wavelength and solar zenith angles (Morel et al, 2002;Voss et al, 2007).…”

“…Appendix A: Assumptions and definitions A1 Value of bi-directional factor, Q The bidirectional function Q has values between 3 and 5 sr (Morel et al, 2002) and depends on several variables, including inherent optical properties of the water, wavelength and solar zenith angles (Morel et al, 2002;Voss et al, 2007). We calculated model reflectance both with a constant value (3 sr) and with time/space/wavelength varying values calculated from the table of Morel et al (2002).…”

“…We calculated model reflectance both with a constant value (3 sr) and with time/space/wavelength varying values calculated from the table of Morel et al (2002). The differences in the relationships between Chl a and blue/green reflectance ratio with variable and uniform Q was almost imperceptible (Fig.…”