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 co mplete radiative transfer model and observations of the physical prope rties of the surface, atmosphere, and clouds based on the Internationa l Satellite Cloud Climatology Project (ISCCP) data sets. Results are o btained every three hours over the whole globe for every third month f rom April 1985 to January 1989. Sensitivity studies are conducted to a ssess the uncertainties in calculated fluxes caused by the estimated u ncertainties in the measurement or specification of the input quantiti es. 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 un certainty 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 surf ace (approximate to 5 - 10 W/m(2), regional daily mean) is related to cloud detection errors. The uncertainty of upwelling longwave (LW) flu xes (approximate to 10 - 20 W/m(2), regional daily mean) depends on th e accuracy of the surface temperature for the surface LW fluxes and th e atmospheric temperature for the top of atmosphere LW fluxes. The dom inant source of uncertainty in downwelling LW fluxes at the surface (a pproximate to 10 - 15 W/m(2)) is uncertainty in atmospheric temperatur e and, secondarily, atmospheric humidity; clouds play little role exce pt in the polar regions. The uncertainties of the individual flux comp onents and the total net fluxes are largest over land (15 - 20 W/m(2)) because of uncertainties in surface albedo (especially its spectral d ependence) and surface temperature and emissivity (including its spect ral dependence). Clouds are the most important modulator of the SW flu xes, but over land areas, uncertainties in net SW at the surface depen d almost as much on uncertainties in surface albedo. Although atmosphe ric and surface temperature variations cause larger LW flux variations , the most notable feature of the net LW fluxes is the changing relati ve importance of clouds and water vapor with latitude. Uncertainty in individual flux values is dominated by sampling effects because of lar ge natural variations, but uncertainty in monthly mean fluxes is domin ated by bias errors in the input quantities.