陳毅軒 博士

Department of Climate and Space Sciences and Engineering (CLaSP) The University of Michigan at Ann Arbor

Blackbody surface and non-scattering clouds are common approximations in the longwave (LW)radiation scheme in climate models. At least twenty years ago, researchers argued the validity of these approximations, but these arguments have received relatively little attention until recently.

We have implemented realistic surface emissivity and cloud LW scattering into the CESM1.1.1, and used it to investigate the impact of surface spectral emissivity and ice cloud LW scattering on simulated climate. In this talk, I will discuss:
(1) The surface emissivity effect over the Sahara and Sahel. The surface emissivity in these regions can be as low as 0.6-0.7 over the infrared window band while close to unity in other bands, but such spectral dependence has been ignored in climate models. The inclusion of realistic surface emissivity over the Sahara and Sahel, compared to the blackbody surface, increases the surface air temperature over these regions and produces more convective rainfall.The precipitation south of the Sahel is also increased, indicating that the changes of surface emissivity can influence the local climate and beyond.
(2) The ice cloud LW scattering effect on polar climate. Cloud LW scattering is usually neglected in climate models. The traditional rationale is that this scattering is negligible compared to strongLW absorption by clouds and greenhouse gases. This rationale, however, is not valid in the polar regions, in which the atmospheric absorption is weak due to the small amount of water vapor, implying that cloud LW scattering is not negligible anymore. Using the CESM with a slab-ocean model, the scattering effect increases the Arctic (Antarctic) winter surface temperature by around 1.4K (1.4K). Interestingly, this effect becomes much weaker when these a surface temperatures and sea ice are prescribed, only 0.1K (0.4K). These results highlight the importance of the cloud LW scattering effect in the polar regions and the importance of surface-atmosphere coupling when this effect is considered.