Observational Data

The satellite data used in the present study are ensemble averages of radiative fluxes at the Top of Atmosphere (TOA) from the Earth Radiation Budget Experiment (ERBE, Barkstrom 1990). These are broad band measurements representative for the period 1985 - 1989, with a resolution of 2.5° x 2.5°. The uncertainties in the monthly averaged scanner data are estimated within +/- 5 Wm"2.

The observational data at the surface are retrieved from a database containing the world-wide instrumentally measured surface energy fluxes, the Global Energy Balance Archive (GEBA, Ohmura et al. 1989, Gilgen and Ohmura 1999). This database currently possesses 220,000 monthly mean fluxes for approximately 1600 sites and has been used in a number of studies to assess model and satellite derived estimates of surface energy fluxes (e.g., Garratt 1994, Li et al. 1995, Wild et al. 1995, 1997, 1998, Wild 1999, Arking 1996, Konzelmann et al. 1996, Rossow and Zhang 1995, Cusack et al. 1998). Gilgen et al. (1998) estimated the relative random error (root mean square error / mean) of the incoming shortwave radiation values in the GEBA at 5% for the monthly means and 2% for annual means.

For the assessment of the all-sky shortwave radiation budgets in GCMs, long-term surface observations from 720 GEBA sites together with their collocated TOA fluxes from ERBE are used. The global distribution of these sites is given in Fig. 1. Their temporal and spatial representativity has been investigated in Wild (1997).

For the assessment of the GCM-simulated solar radiation specifically under clear-sky conditions, we are currently preparing an observational dataset of surface clear-sky climatologies at selected sites worldwide. So far, observed clear-sky climatologies of surface insolation have been determined for a number of sites in Germany (Wild and Liepert 1998). The clear-sky insolation climatologies were obtained from composites of cloud-free episodes which were identified on an hourly basis using additional information on cloudiness and sunshine duration. Monthly all-sky climatologies of surface insolation for the same sites and period were available from the Global Energy Balance Archive.



Figure !. Global distribution of the 720 sites with long-term incoming shortwave radiation measurements selected for this analysis from the Global Energy Balance Archive

To determine the amount of shortwave radiation absorbed at the surface, all above mentioned insolation climatologies were combined with the collocated values of a surface albedo climatology provided by the Surface Radiation Budget Project (SRB, Darnell et al. 1992) representative for the period 1985 - 1989. In an attempt to estimate potential errors introduced by the surface albedo, the measured insolation was additionally combined with two alternative sets of albedo climatologies. They did not alter the surface absorption significantly. The clear- and all-sky radiative fluxes at the TOA collocated with the surface measurements at the German sites are again taken from ERBE. Finally, estimates of clear- and all-sky absorption within the atmosphere were obtained from the respective differences between the absorbed radiation implied by satellite observations at the TOA and the absorbed radiation at the surface from the ground measurements.

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