A coupled atmosphere-ocean radiative transfer system using the analytic four-stream approximation

A coupled atmosphere–ocean radiative transfer model based on the analytic four-stream approximation has been developed. It is shown that this radiation model is computational efficient and at the same time can achieve acceptable accuracy for flux and heating rate calculations in the atmosphere and the oceans. To take into account the reflection and transmission of the wind-blown air-water interface, a Monte Carlo method has been employed to simulate the traveling of photons and to compute the reflectance and transmittance of direct and diffuse solar fluxes at the ocean surface. For the ocean part, existing bio-optical models, which correlate the concentration of chlorophyll and the absorption and scattering coefficients of phytoplankton and other matters, have been integrated into this coupled model. Comparing to the values computed by more discrete streams illustrates that the relative accuracies of the surface albedo and total transmission in the ocean determined from the present model are generally within 5%, except in cases of the solar zenith angle larger than 80 degree. Observational data has also been used to validate this model and the results show that the relative differences of downward and upward shortwave fluxes and albedo are within 10% of the observed values. This computationally efficient and physically based radiative transfer model is well suited for consistent flux calculations in a coupled atmosphere-ocean dynamic system.