A discrete-ordinates treatment of ocean surface roughness for a coupled radiative transfer code

In many coupled radiative transfer models which explicitly account for the rough ocean surface, the Monte Carlo statistical technique is used. Here we present an analytical solution for radiative transfer in such a fully coupled, atmosphere-ocean system with the more computationally efficient discrete-ordinate approach. Our simulations of the effects of surface roughness on radiation in the atmosphere and ocean are compared with satellite and surface measurements. The results show that ocean surface roughness (wind-driven, mirror-like wave facets) has significant effects on the upwelling radiation in the atmosphere and the downwelling radiation in the ocean. As wind speed increases, the angular domain of sunglint broadens, the surface albedo decreases, and the transmission to ocean increases. The downward radiance field in the upper ocean is highly anisotropic, but this anisotropy decreases rapidly as surface wind increases and as depth in ocean increases. The effects of surface roughness on radiation also depend greatly on both wavelength and angle of incidence (i.e., solar elevation); these effects are significantly smaller throughout the spectrum at high sun. The model-observation discrepancies may indicate that the Cox-Munk surface roughness model is not sufficient for high wind conditions.