Short-wave and long-wave surface radiation budgets in GCMs: a review based on the IPCC-AR4/CMIP3 models

Here I review the developments in the representation of radiation budgets in global climate models (GCMs) from a surface perspective, considering early models up to the latest model generation used in the IPCC fourth assessment report (AR4). As in previous model generations, considerable differences in the simulated global mean radiation budgets are also present in the IPCC-AR4 models, particularly in the atmosphere and at the surface. I use a comprehensive set of surface observations to constrain these uncertainties, and focus on the downward short- and long-wave radiation, which can directly be validated against the surface observations. The majority of the IPCC-AR4 models still shows a tendency to overestimate the short-wave and underestimate the long-wave downward radiation at the surface, each by 6 W m(-2) on average, a long standing problem in many GCMs. A subset of models, however, is now capable of simulating at least one of the short- or long-wave downward components adequately. Model biases in all- and clear-sky fluxes are often similar, suggesting that deficiencies in clear-sky radiative transfer calculations are major contributors to the excessive surface insolation in many of the models. No indication is found that the simulated excessive surface insolation is due to missing cloud absorption.