The unique multimonth set of simultaneous Earth radiation budget observations and cloud amount estimates taken during the Nimbus 7 satellite mission from June 1979 to May 1980 was used to validate a long-term climate simulation obtained with the latest version of the National Center for Atmospheric Research (NCAR) Community Climate Model. The comprison focused on the temporal variability of the model-generated cloud and radiation fields versus satellite data with the aim to (1) test the model's ability to simulate short-term fluctuations; and (2) examine the impact of the treatment of the interactions between clouds, radiation, and the hydrologic cycle on the model's performance. The Nimbus 7 data set comprised broad-spectral-band observations of the outgoing infrared radiation and planetary albedo taken by the Earth radiation budget scanners and total cloud amount estimates derived from radiances measured by the Temperature Humidity Infrared Radiometer and Total Ozone Mapping Spectrometer using a bispectral threshold method. Model-simulated northern hemisphere summer and winter seasons were obtained from a 15-year time integration including a seasonal cycle.

The comparison helped identify a major difficulty in the simulation of climate with the NCAR Community Climate Model. Although the global distributions of the seasonal average and standard deviation of the model-generated cloud and radiation fields agreed reasonably well with those obtained from satellite observations, the magnitude of the standard deviation of both fields was overestimated by about a factor of 2 over the whole globe. The high frequency of occurrence of clouds is responsible for the increased temporal variability in the model and partially results from reduced interactions between clouds and the hydrologic cycle. In view of the impact of clouds on the atmospheric circulation and its temporal variability, increased fluctuations in cloudiness may affect the sensitivity of the model-simulated climate to external forcings and it is desirable to implement stronger couplings between the various physical processes in the NCAR Community Climate Model. ¿ American Geophysical Union 1990