Improved techniques for evaluating GCM cloudiness applied to the NCAR CCM3

Results show that CCM3 overproduces cloud optical thickness, cloud-top height, and cloud radiative forcing under conditions of synoptic ascent and underproduces cloud cover, cloud-top height, and cloud radiative forcing under conditions of synoptic subsidence. The underproduction of cloudiness in the subsidence regime creates an unrealistic sensitivity of CCM3 low-level cloud cover to changes in circulation. As a result interannual variability of summertime midlatitude North Pacific cloudiness in CCM3 is much more closely coupled to sea level pressure variability than SST variability, opposite the case for observed cloudiness.

This demonstrates small-scale cloud parameterization errors directly and dominantly impact large-scale cloud variability despite the existence of a reasonable climatology.