SCATTERING EFFECTS ON LONGWAVE FLUXES IN BROKEN CLOUD FIELDS

General circulation models use the hat black plate approximation to ca lculate longwave radiative transfer through broken cloud fields. This neglects both cloud geometry and longwave optical properties. It is kn own that cloud geometry is important in longwave transmission. Since t he longwave single scattering albedo is as high as 0.75, it is also ne cessary to determine the importance of scattering effects. A Monte Car lo simulation was used to compute the upward and downward fluxes for s implified cloud fields with a range of cloud geometries and optical pr operties. Based on these fluxes, the effective cloud fractions were fo und. The results show that scattering can have a significant effect on fluxes and effective cloud fractions. The effects are largest for low cloud upward fluxes and high cloud downward fluxes. To attain high pe rcentage accuracy, it is necessary to model both cloud geometry and sc attering.