Spatial and temporal variability of global surface solar irradiance

A fast scheme for computing surface solar irradiance using data from the International Satellite Cloud Climatology Project (ISCCP) is described. Daily mean solar irradiances from the fast scheme reproduce the detailed global results from full radiative transfer model calculations to within 6 and 10 W m−2 over the ocean and land, respectively. In particular, the fast scheme reproduces the same dependence of surface irradiance on solar zenith angle which is critical for proper calculation of daily, seasonal, and latitudinal variability. Validation of both model results is limited because globally distributed data sets of high quality are lacking, particularly over the oceans. However, comparison of calculated monthly mean results using 5 months of ISCCP data (July 1983 to July 1984) with climatology from the 1970s at six temperate latitude ocean weather stations shows agreement within published estimates of interannual variability of monthly means at the individual stations. A further test against a 17-day time series at a continental site (43°N, 90°W, October–November 1986; 13–170 W m−2 range of irradiance), where ground and satellite data were spatially and temporally coincident, showed an accuracy of better than 9 W m−2 on a daily basis and less than 4% bias in the 17-day mean. Frequently used bulk formulae for solar irradiance were also evaluated in each of these tests. All suffered in comparison because they did not include a parameterization of the effects of the global variability in mean cloud optical thickness. Data from July (1983 and 1984) and January (1984 and 1985) were used to examine the spatial and temporal variability of surface irradiance and its potential impact on biospheric processes. Results show that the oceans and land experience fundamentally different light regimes, with continents receiving significantly greater irradiance. In summer, major interocean differences in zonally averaged irradiance are found in the northern hemisphere with the Atlantic greater than Pacific by up to 80 W m−2; in the southern hemisphere, interocean differences are small. Regional interannual variability (July 1983 versus 1984) ranged between +100 and −100 W m−2. The variability, perhaps due to the 1982–1983 El Nino event, occurred mostly in the Pacific but extended beyond the tropics over the entire north Pacific basin. The nutrient-rich northern and southern ocean waters are almost perpetually cloud covered; however, there is a correspondence between higher than average surface irradiance and productivity in nutrient-rich areas of the southwest Atlantic and Weddell Sea sector of the circumpolar current. This suggests that solar irradiance must be considered as an important factor governing the productivity of these waters.