OCEAN SCALAR IRRADIANCE NEAR-SURFACE MAXIMA

Estimates of the in-water spectral scalar irradiance, E-0, are needed to quantify the photosynthetically available radiation. When a highly scattering, optically thick medium is illuminated at its surface, it i s possible under certain conditions for E-0 to increase with penetrati on depth near the surface, even if there are no internal sources at th e wavelength of interest. Analysis and numerical examples presented he re help to explain and quantify the magnitude and location of potentia l subsurface E-0 peaks in source-free ocean waters and the dependence of the phenomenon on the seawater optical properties and surface condi tions. Peaking in E-0 most likely occurs when the incident illuminatio n is strongly directed at the zenith angle, and the location of the ma ximum is deepest when the asymmetry of the scattering phase function i s large. The presence of surface waves and internal reflection greatly reduces the chance of E-0 peaks being present, making a maximum in E- 0 below the surface possible only if the single scattering albedo, ome ga(0), is >0.95 in homogeneous waters or at potentially smaller values if omega(0) increases with depth.