The Surface Radiation Budget over Oceans and Continents

Abstract

An updated evaluation of the surface radiation budget in climate models (1994?96 versions; seven datasets available, with and without aerosols) and in two new satellite-based global datasets (with aerosols) is presented. All nine datasets capture the broad mean monthly zonal variations in the flux components and in the net radiation, with maximum differences of some 100 W m?2 occurring in the downwelling fluxes at specific latitudes. Using long-term surface observations, both from land stations and the Pacific warm pool (with typical uncertainties in the annual values varying between ±5 and 20 W m?2), excess net radiation (RN) and downwelling shortwave flux density (So?) are found in all datasets, consistent with results from earlier studies [for global land, excesses of 15%?20% (12 W m?2) in RN and about 12% (20 W m?2) in So?]. For the nine datasets combined, the spread in annual fluxes is significant: for RN, it is 15 (50) W m?2 over global land (Pacific warm pool) in an observed annual mean of 65 (135) W m?2; for So?, it is 25 (60) W m?2 over land (warm pool) in an annual mean of 176 (197) W m?2. The effects of aerosols are included in three of the authors? datasets, based on simple aerosol climatologies and assumptions regarding aerosol optical properties. They offer guidance on the broad impact of aerosols on climate, suggesting that the inclusion of aerosols in models would reduce the annual So? by 15?20 W m?2 over land and 5?10 W m?2 over the oceans. Model differences in cloud cover contribute to differences in So? between datasets; for global land, this is most clearly demonstrated through the effects of cloud cover on the surface shortwave cloud forcing. The tendency for most datasets to underestimate cloudiness, particularly over global land, and possibly to underestimate atmospheric water vapor absorption, probably contributes to the excess downwelling shortwave flux at the surface.