EVALUATION OF TOTAL CLOUDINESS AND ITS VARIABILITY IN THE ATMOSPHERICMODEL INTERCOMPARISON PROJECT

Total cloudiness of 29 models participating in the Atmospheric Model I ntercomparison Project is compared with the ISCCP C2 as well as the Ni mbus-7 and Meteor observational estimates. The root-mean-square differ ences between the annual means of the model calculations and the C2 ob servations after global means are removed vary from about twice to nea rly four times the difference between the C2 and Meteor observations. The large differences are in some cases due to the fact that although a model qualitatively has patterns of spatial variations similar to th ose of the observations, the magnitude of those variations is much too small. In other cases the models have produced the approximate magnit ude of the spatial variability of the observations but display sizable errors in the pattern of that variability. Deficiencies with respect to the model simulations of the mean seasonal cycle are also pronounce d. For instance, the differences between the zonal averages of total c loudiness for contrasting seasons suggest that near 60 degrees most mo dels predict minima in cloudiness in summer, whereas observations stro ngly suggest the opposite. In addition, smoothed seasonal cycle analys es suggest that a portion of these deficiencies in some models is the result of a simulated seasonal cycle that leads that of the observatio ns by about two months. However, some models, which appear to have the proper phase of the seasonal cycle, still show large root-mean-square d differences and small correlations when compared with the smoothed s easonal cycle of the C2 observations. The C2 and Meteor observations s how a modest signal in total cloudiness for the only important interan nual variation during the July 1983 through June 1988 observation peri od-the 1986/87 ENSO event. A few models reproduce this event about as well as do the Meteor observations, whereas many models fail to show a ny evidence of it. Overall, models that better reproduce the ENSO resu lts also tend to do well with seasonal variations. No specific differe nces are evident in the physical characteristics of models that are re latively adept at reproducing seasonal and interannual variations and those that perform more poorly. However, there is the general conclusi on that models that have more sophisticated physical processes tend to better simulate the cloud observations.