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 u sing radiative transfer models and appropriate inherent optical properties (IOPs) combined with realistic boundary conditions. In particular, the bi-d irectional structure of the subsurface upward flux has been investigated in view of remote sensing applications related to ocean colour. In Case 2 wat ers, the IOPs are not controlled by the phytoplankton (or chlorophyll) conc entration; rather they are essentially determined by the abundance of terri genous optically active materials, either particulate or dissolved. Based o n field data and related IOPs, two extreme situations were selected as repr esentative instances of sediment-dominated and yellow-substance-dominated C ase 2 waters. This study shows that even in very turbid natural waters, the upward radian ce field is not isotropic and remains Sun-angle dependent. More than 100 su ccessive events are needed to reach a quasi-isotropic, illumination indepen dent, upward radiance field. In contrast, with a high yellow substance cont ent resulting in high absorption (compared to scattering), single scatterin g prevails in such waters and this leads to strongly featured radiance fiel ds that are heavily dependent on the Sun's position. It is necessary to acc ount for these effects when interpreting water-leaving radiances as detecte d from space, and, perhaps more importantly, when carrying out at-sea radio metric measurements in support of calibration of remote ocean colour sensor s. For this purpose, a practical approach and mean values of relevant coeff icients are proposed to describe the bi-directional structure of the upward radiance field in the two extreme situations of strongly scattering or str ongly absorbing waters.