Estimation of the absorption and backscattering coefficients from in-water radiometric measurements

Numerical simulations of radiative transfer within the ocean surface mixed layer have been used to derive a set of simple equations for estimating the absorption, a, and backscattering, b(b), coefficients in the blue-green spectral region from measurements of downwelling irradiance, E-d, upwelling irradiance, E-a, and upwelling nadir radiance, L-u. Two relationships are used in this derivation: (i) irradiance reflectance, R = E-a/E-d, is approximately proportional to b(b)/a and inversely proportional to the avenge cosine of the underwater light field, mu; and (ii) radiance reflectance, R-L = L-u/E-d, is proportional to b(b)/a but changes little with mu. Accordingly, CL is first estimated from L-u/E-u and then the absorption coefficient is obtained from Gershun's equation. Having determined a, b(b) is estimated from the relation between R-L and b(b)/a. We also show how the estimation of mu, a, and b(b) is improved with additional data of the beam attenuation coefficient, c. The retrieval of a and b(b) from E-d, E-a, and L-u has been tested using field data collected in the Southern California Eight. The a values predicted by the model show good agreement with absorption determinations from the ac-9 instrument and on-board spectrophotometric method, and the modeled b(b) agrees well with the backscattering determinations from the Hydroscat-6 instrument. The algorithm has been derived for a broad range of the ocean inherent optical properties that do not necessarily covary with one Mother like in chlorophyll-based bio-optical models. However, the model should be used with caution if significant departure from the average Petzold particle scattering phase function is expected, for example in waters with high lend of mineral particles.