Relationship between the Longwave Cloud Radiative Forcing at the Surface and the Top of the Atmosphere

Abstract

Attempts to map the global longwave surface radiation budget from space have been thwarted by the presence of clouds. Unlike the shortwave, there is no physical relationship between the outgoing longwave and the surface longwave under cloudy skies. Therefore, there is no correlation between spatial and temporal averages of the outgoing longwave radiation and not longwave radiation at the surface. However, in regions where a particular cloud regime exists preferentially, a relationship between the mean longwave cloud radiative forcing (CRF) at the top of the atmosphere and at the surface can he shown to exist. Results from a general circulation model suggest that this relationship for monthly means is coherent over fairly large geographical areas. For example, in tropical convective areas, the longwave CRF at the top is very large, but at the surface it is quite small because of the high opacity of the lowest layers of the atmosphere. On the other hand, in areas of stratus over cool ocean surfaces, the longwave CRF at the top is very small but at the surface, it is quite substantial. To the extent that the cloudiness simulated in the model mimics the real atmosphere, it may be possible to estimate the monthly mean longwave CRF at the surface from the Earth Radiation Budget Experiment cloud forcing at the top. The net longwave radiation at the surface can then be mapped if monthly means of the clear-sky fluxes are obtained by some independent technique.