Estimation of the absorption and backscattering coefficients from in-waterradiometric 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 sp ectral region from measurements of downwelling irradiance, E-d, upwelling i rradiance, E-a, and upwelling nadir radiance, L-u. Two relationships are us ed in this derivation: (i) irradiance reflectance, R = E-a/E-d, is approxim ately proportional to b(b)/a and inversely proportional to the avenge cosin e 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 o btained from Gershun's equation. Having determined a, b(b) is estimated fro m the relation between R-L and b(b)/a. We also show how the estimation of m u, a, and b(b) is improved with additional data of the beam attenuation coe fficient, c. The retrieval of a and b(b) from E-d, E-a, and L-u has been te sted using field data collected in the Southern California Eight. The a val ues predicted by the model show good agreement with absorption determinatio ns from the ac-9 instrument and on-board spectrophotometric method, and the modeled b(b) agrees well with the backscattering determinations from the H ydroscat-6 instrument. The algorithm has been derived for a broad range of the ocean inherent optical properties that do not necessarily covary with o ne Mother like in chlorophyll-based bio-optical models. However, the model should be used with caution if significant departure from the average Petzo ld particle scattering phase function is expected, for example in waters wi th high lend of mineral particles.