VALIDATION OF GENERAL-CIRCULATION MODEL RADIATIVE FLUXES USING SURFACE OBSERVATIONS

The surface radiative fluxes of the ECHAM3 General Circulation Model ( GCM) with T21, T42, and T106 resolutions have been validated using obs ervations from the Global Energy Balance Archive (GEBA, World Climate Program-Water Project A7). GEBA contains the most comprehensive datase t now available for worldwide instrumentally measured surface energy f luxes. The GCM incoming shortwave radiation at the surface has been co mpared with more than 700 long-term monitoring stations. The ECHAM3 mo dels show a clear tendency to overestimate the global annual-mean inco ming shortwave radiation at the surface due to an underestimation of a tmospheric absorption. The model-calculated global-mean surface shortw ave absorption around 165 W m(-2) is estimated to be too high by 10-15 W m(-2). A similar or higher overestimate is present in several other GCMs. Deficiencies in the clear-sky absorption of the ECHAM3 radiatio n scheme are proposed as a contributor to the flux discrepancies. A st and-alone validation of the radiation scheme under clear-sky condition s revealed overestimates of up to 50 W m(-2) for daily maximum values of incoming shortwave fluxes. Further, the lack of shortwave absorptio n by the model clouds is suggested to contribute to the overestimated surface shortwave radiation. There are indications that the incoming l ongwave radiation at the surface is underestimated in ECHAM3 and other GCMs. This largely offsets the overestimated shortwave flux in the gl obal mean, so that the 102 W m(-2) calculated in ECHAM3 for the surfac e net radiation is considered to be a realistic value. A common featur e of several GCMs is, therefore, a superficially correct simulation of global mean net radiation, as the overestimate in the shortwave balan ce is compensated by an underestimate in the longwave balance. Seasona l and zonal analyses show that the largest overestimate in the incomin g shortwave radiation of ECHAM3 is found at low latitudes year round a nd in midlatitude summer, while at high latitudes and in midlatitude w inter the solar input is underestimated. As a result, the meridional g radient of incoming shortwave radiation becomes too large. The zonal d iscrepancies of the fluxes are consistent with differences between the simulated cloud amount and a cloud climatology based on surface obser vations. The shortwave discrepancies are further visible in the net ra diation where the differences show a similar latitudinal dependency in cluding the too strong meridional gradient. On the global and zonal sc ale, the simulated fluxes are rather insensitive to changes in horizon tal resolution. The systematic large-scale model deviations dominate t he effects of increased horizontal resolution.