We compare cloud-radiative forcing (CRF) at the top-of-the atmosphere from 19 models from the Atmospheric Model Intercomparison Project (AMIP), to observations from the Earth Radiation Budget Experiment (ERBE). With respect to 60 degrees N to 60 degrees S means, a surprising result is that many of the 19 models produce unusually large biases in Net CRF that are all of the same sign (negative), meaning that many of the models significantly overestimate cloud radiative cooling. The primary focus of this study, however, is to demonstrate a diagnostic procedure, using ERBE data, to test if a model might produce, for a given region, reasonable CRF as a consequence of compensating errors caused either by unrealistic cloud vertical structure or cloud optical depth. For this purpose we have chosen two regions, one in the western tropical Pacific characterized by high clouds spanning the range from thin cirrus to deep convective clouds, and the other in the southeastern Pacific characterized by trade cumulus. For a subset of eight models, it is found that most typically produce more realistic regionally-averaged CRF (and its longwave and shortwave components) for the southeastern region as opposed to the western region. But when the diagnostic procedure for investigating cloud vertical structure and cloud optical depth is imposed, this somewhat better agreement in the southeastern region is found to be the result of compensating errors in either cloud vertical structure or cloud optical depth. The comparison with ERBE data also shows large errors in clear-sky fluxes for many of the models.

This research was performed under the auspices of the U.S. Department of Energy (DOE) by the Lawrence Livermore National Laboratory under Contract W-7405-ENG-48.