CALCULATION OF SURFACE AND TOP OF ATMOSPHERE RADIATIVE FLUXES FROM PHYSICAL QUANTITIES BASED ON ISCCP DATA SETS .1. METHOD AND SENSITIVITY TO INPUT DATA UNCERTAINTIES

Upwelling and downwelling, shortwave and longwave radiative fluxes are calculated at the top of the atmosphere and at the surface using a complete radiative transfer model and observations of the physical properties of the surface, atmosphere, and clouds based on the International Satellite Cloud Climatology Project (ISCCP) data sets. Results are obtained every three hours over the whole globe for every third month from April 1985 to January 1989. Sensitivity studies are conducted to assess the uncertainties in calculated fluxes caused by the estimated uncertainties in the measurement or specification of the input quantities. Except in the polar regions, uncertainties in cloud properties are no longer the predominant source of radiative flux uncertainty, even at the surface; rather they produce uncertainties similar in magnitude to those caused by atmospheric and surface properties. The largest uncertainty in upwelling shortwave (SW) fluxes (approximate to 10 - 15 W/m(2), regional daily mean) is caused by uncertainties in land surface albedo, whereas the largest uncertainty in downwelling SW at the surface (approximate to 5 - 10 W/m(2), regional daily mean) is related to cloud detection errors. The uncertainty of upwelling longwave (LW) fluxes (approximate to 10 - 20 W/m(2), regional daily mean) depends on the accuracy of the surface temperature for the surface LW fluxes and the atmospheric temperature for the top of atmosphere LW fluxes. The dominant source of uncertainty in downwelling LW fluxes at the surface (approximate to 10 - 15 W/m(2)) is uncertainty in atmospheric temperature and, secondarily, atmospheric humidity; clouds play little role except in the polar regions. The uncertainties of the individual flux components and the total net fluxes are largest over land (15 - 20 W/m(2)) because of uncertainties in surface albedo (especially its spectral dependence) and surface temperature and emissivity (including its spectral dependence). Clouds are the most important modulator of the SW fluxes, but over land areas, uncertainties in net SW at the surface depend almost as much on uncertainties in surface albedo. Although atmospheric and surface temperature variations cause larger LW flux variations, the most notable feature of the net LW fluxes is the changing relative importance of clouds and water vapor with latitude. Uncertainty in individual flux values is dominated by sampling effects because of large natural variations, but uncertainty in monthly mean fluxes is dominated by bias errors in the input quantities.