Estimation of the absorption and backscattering coefficients from inߚwater radiometric measurements

Abstract

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, bb, coefficients in the blue‐green spectral region from measurements of downwelling irradiance, Ed, upwelling irradiance, Eu, and upwelling nadir radiance, Lu. Two relationships are used in this derivation: (i) irradiance reflectance, R = Eu/Ed, is approximately proportional to bb/a and inversely proportional to the average cosine of the underwater light field, µ; and (ii) radiance reflectance, RL = Lu/Ed, is proportional to bb/a but changes little with µ. Accordingly, µ is first estimated from Lu/Eu and then the absorption coefficient is obtained from Gershun's equation. Having determined a, bb is estimated from the relation between RL and bb/a. We also show how the estimation of µ, a, and bb is improved with additional data of the beam attenuation coefficient, c. The retrieval of a and bb from Ed, Eu, and Lu has been tested using field data collected in the Southern California Bight. 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 bb 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 another 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 load of mineral particles.