Cloud scattering impact on thermal radiative transfer and global longwave radiation

摘要

The potential importance of longwave (LW) cloud scattering has been recognized but the actual estimate of this effect on thermal radiation varies greatly among different studies. General circulation models (GCMs) generally neglect or simplify the multiple scattering in the LW. In this study, we use a rigorous radiative transfer algorithm to explicitly consider LW multiple-scattering and apply the GCM to quantify the impact of cloud LW scattering on thermal radiation fluxes. Our study shows that the cloud scattering effect on downward thermal radiation at the surface is concentrated in the infrared atmospheric window spectrum (800-1250cm(-1)). The scattering effect on the outgoing longwave radiation (OLR) is also present in the window region over low clouds but it is mainly in the far-infrared spectrum (300-600 cm(-1)) over high clouds. For clouds with small to moderate optical depth (tau < 10), the scattering effect on thermal fluxes shows large variation with the cloud tau and has a maximum at an optical depth of similar to 3. For opaque clouds, the scattering effect approaches an asymptote and is smaller and less important.