Simulation of inelastic-scattering contributions to the irradiance field in the ocean: variation in Fraunhofer line depths.

Raman scattering and fluorescence are important processes in oceanic optics because of their influence on the natural light field in the water. Monte Carlo simulations are described that verify that measurements of the Fraunhofer line depth in the in-water irradiance can be used to separate the irradiance into elastic and inelastic components, i.e., components that are generated by elastic- and inelastic-scattering processes, respectively. Specifically, the upwelling and downwelling irradiances, including Raman scattering, are simulated for a variety of model oceans. The inherent optical properties of the ocean are derived from a bio-optical model in which the elastic-scattering and the absorption coefficients of the biological material depend only on the phytoplankton pigment concentration, C. The Fraunhofer line at 656 nm is found to fill in, i.e., disappear into, the background continuum rapidly with increasing depth. This indicates a rapid transition from a near-surface light field dominated by elastic s cattering to one composed of irradiance derived entirely from Raman scattering. Conversely the depth of the Fraunhofer line at 486 mm is nearly independent of depth in the water, indicating that Raman scattering never makes a significant contribution to the irradiance there. Between these two extremes, the lines at 518 and 589 nm show variations in line depths that depend significantly on C, e.g., at 518 nm the line fills in with increasing depth at low-C values but not at high-C values.