Validation of the Earth radiation budget as simulated by the Max Planck Institute for Meteorology general circulation model ECHAM4 using satellite observations of the Earth Radiation Budget Experiment

In this study, the Earth radiation budget as simulated by the latest version of the ECHAM general circulation model (ECHAM4) is documented. The Earth radiation budget obtained from the model is evaluated through comparison with Earth Radiation Budget Experiment (ERBE) data. The model simulations generally agree with the satellite-observed spatial distribution and seasonal variation of the radiation budget. The interannual variabilities of the simulated radiative quantities in the tropics are also comparable to the observed data. There are, however, biases in the details. The longitudinal structure of the radiation fields in the tropics is not exactly reproduced. The slight differences in the geographical location of the various radiation quantities are related to the errors in the simulation of the tropical east-west longitudinal circulation and associated cloud fields. The most significant bias in the radiation budget is the simulated shortwave radiative effect from cloud. Consistent underestimation in the shortwave cloud radiative forcing is found over the midlatitude oceans in summer. This bias is due to an underestimation in the total cloud amount. In the tropics the simulated shortwave cloud radiative forcing is persistently larger than that derived from ERBE data. The systematic overestimation in the shortwave cloud radiative forcing at various values of the longwave cloud radiative forcing suggests that this problem is related to the neglect of sub-grid-scale cloud water content variability and its effect on the grid-averaged shortwave radiative flux. Although the tropical interannual variability of cloud radiative forcing is reasonably well reproduced by the model, the major contribution for the simulated anomalies is from the cloud water anomalies, while observations suggest the dominant factor is the total cloud cover anomalies. Lack of observations for various cloud parameters limits the attempt of further validation.