Annual cycle of cloud forcing of the surface radiation budget

The largest climate variation is the annual cycle, except for very long-term changes such as the Milankovich cycles. Thus, an understanding of the annual cycle is a prerequisite for understanding or forecasting climate variability and change. It behooves us to understand the energetics of the atmosphere/ocean system, which is driven by the absorption of solar energy and subsequent emission of longwave radiation. The cloud cover of the Earth, a major component in this system, varies with the annual cycle. This paper quantifies the effect of cloud cover on the annual cycle of radiation at the surface of the Earth, by use of the Surface Radiation Budget data set, which has been developed by the Langley Research Center of NASA in support of the GEWEX program. This data set covers the Earth with a 1-degree quasi-equal area grid for the twelve-year period July 1983 through June 1995. The SRB data set includes monthly-mean values for radiation flux components as upward, downward and net shortwave, longwave and total flux for the all-sky condition and for clear sky. Climatological means are formed for each month of the twelve -year period for each component for all-sky and clear sky. The cloud forcing is defined as the all-sky flux minus the clear-sky flux for any component. In order to examine the time and space variations of the various components, a principal component analysis is performed whereby the time variations are represented by principal components (PCs) and the spatial variations by corresponding empirical orthogonal functions (EOFs). The dominant PCs represent annual and semiannual cycles. The EOFs show the movement of the Intertropical convergence zone along the Equator, with prominent features over India and Southeast Asia, the Philippines and New Guinea, eastern South America and across the South Atlantic Ocean to the Congo Basin.