Models

Several GCMs are considered in this study, namely ECHAM3 (Roeckner et al. 1992) from the Max Planck Institute for Meteorology, Hamburg, ARPEGE (Deque et al. 1994) from Meteo-France, Toulouse, and model versions HadAM2b (Stratton 1999) and HadAM3 (Cusack et al. 1998) from the Hadley Centre, Bracknell. All model data stem from AMIP type simulations with prescribed SST and sea ice climatologies. Simulations with the above models were analyzed at various horizontal resolutions, although results are shown here only for the standard resolution (T42 for the spectral models ARPEGE and ECHAM3, (2.5° x 3.75°) for the HadAM gridpoint models), since the calculated radiative fluxes were shown to be insensitive to a change in horizontal resolution (Wild et al. 1995, Wild 1997). Thus the conclusions drawn in this study do not depend on a specific model resolution. All models include broad-band radiation schemes with two-stream approximation, as typically used in GCMs. The ECHAM3 and ARPEGE models further include simple aerosol climatologies based on WMO (1983), while HadAM2b does not include any aerosol effects. From the next generation model version HadAM3 simulations were available both with and without a (simple) aerosol climatology (Cusack et al. 1998), hereafter referred to as HadAM3 and HadAM3-NA, respectively.

For the comparison of model-calculated and observed fluxes, the model fluxes were interpolated to the observation sites using the four surrounding grid points weighted by their inverse spherical distances.

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