Non-isotropy of the upward radiance field in typical coastal (Case 2) waters

The in-water radiance field has been computed in typical Case 2 waters by using radiative transfer models and appropriate inherent optical properties (IOPs) combined with realistic boundary conditions. In particular, the bi-directional structure of the subsurface upward flux has been investigated in view of remote sensing applications related to ocean colour. In Case 2 waters, the IOPs are not controlled by the phytoplankton (or chlorophyll) concentration; rather they are essentially determined by the abundance of terrigenous optically active materials, either particulate or dissolved. Based on field data and related IOPs, two extreme situations were selected as representative instances of sediment-dominated and yellow-substance-dominated Case 2 waters. This study shows that even in very turbid natural waters, the upward radiance field is not isotropic and remains Sun-angle dependent. More than 100 successive events are needed to reach a quasi-isotropic, illumination independent, upward radiance field. In contrast, with a high yellow substance content resulting in high absorption (compared to scattering), single scattering prevails in such waters and this leads to strongly featured radiance fields that are heavily dependent on the Sun's position. It is necessary to account for these effects when interpreting water-leaving radiances as detected from space, and, perhaps more importantly, when carrying out at-sea radiometric measurements in support of calibration of remote ocean colour sensors. For this purpose, a practical approach and mean values of relevant coefficients are proposed to describe the bi-directional structure of the upward radiance field in the two extreme situations of strongly scattering or strongly absorbing waters.