Low Clouds over the Western Pacific Ocean in ARM Observations and CAM5 Simulations

Abstract:

Low clouds observed at the Manus Island of the tropical western Pacific are compared to those simulated by CAM5 during YOTC. Here, low clouds are defined as clouds with their tops below the freezing level and bases within the boundary layer. The comparisons are made in terms of the general occurrence of low clouds, its mean vertical profiles, fractions of precipitating vs. non-precipitating events, diurnal cycle, and monthly time series. Other types of clouds are included to put the comparison in a broader context. The results show that the model overproduces total clouds and their precipitation fraction but underestimates low clouds in general. The model, however, produces excessive low clouds in a thin layer between 954-930 hPa, which coincides with excessive humidity near the top of the mixed layer. This suggests that the erroneously excessive low clouds stem from deficiencies in parameterization of both cloud and turbulence mixing. The model also fails to produce the observed diurnal cycle in low clouds, not exclusively due to the model coarse grid spacing that does not resolve Manus Island.