Scattering Effects on Longwave Fluxes in Broken Cloud Fields

Abstract

General circulation models use the flat black plate approximation to calculate longwave radiative transfer through broken cloud fields. This neglects both cloud geometry and longwave optical properties. It is known that cloud geometry is important in longwave transmission. Since the longwave single scattering albedo is as high as 0.75, it is also necessary to determine the importance of scattering effects. A Monte Carlo simulation was used to compute the upward and downward fluxes for simplified cloud fields with a range of cloud geometries and optical properties. Based on these fluxes, the effective cloud fractions were found. 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 percentage accuracy, it is necessary to model both cloud geometry and scattering.