On Solar Energy Disposition: A Perspective from Observation and Modeling

Solar energy reaching our planet is partly reflected to space, partly absorbed in the atmosphere, and partly absorbed at the earth’s surface. This partitioning of the solar energy incident at the top of the atmosphere (TOA), hereafter called solar energy disposition (SED), is determined by the optical properties of the atmospheric column, which, in turn, is influenced by the SED. The key variables of the column that control SED include those associated with the amount, vertical distribution, and optical properties of clouds, moisture and aerosols, as well as surface optical properties. Feedback involving these variables and the SED is important in modeling the climate system response to external perturbations, such as changes in the concentrations of CO 2 and other greenhouse gases. At this point, cloud feedback is the principal contributor to the large uncertainty in climate system response (Cess et al. 1989; Arking 1991). Not only does the SED play an active role in the energetics of the climate system, it is also closely linked to the hydrologic cycle via dynamic and thermodynamic processes (Randall et al. 1989; Stephens and Greenwald 1991; Wielicki et al. 1995). About half the solar energy absorbed at the surface is used to evaporate water, which eventually forms clouds. Latent heat released in cloud formation is a major source of energy driving the atmospheric circulation, especially in the Tropics, and is comparable in magnitude to the solar radiation directly absorbed by the atmosphere. A sensitivity study with a general circulation model (GCM) shows that modifying the partiOn Solar Energy Disposition: A Perspective from Observation and Modeling