DEPENDENCE OF CLOUD AMOUNT ON HORIZONTAL RESOLUTION IN THE NATIONAL-CENTER-FOR-ATMOSPHERIC-RESEARCH COMMUNITY CLIMATE MODEL

The dependence of the cloud fraction on horizontal resolution is explored with the National Center for Atmospheric Community Climate Model (CCM1). Using R15, T42, T63 and T106 versions of the CCM, we find that the total cloud amount monotonically decreases with increased horizontal resolution. This decrease is particularly evident in the low-level cloud associated with stable condensation in the tropics. Decreases in low-level and upper-level extratropical clouds with increased resolution are also apparent. Analysis of these model simulations shows that this decrease is due to increased advective drying of the lower atmosphere by stronger subsidence. Stronger upward motion which is directly associated with stronger heating in the convergence region by moist convective adjustment and stable condensation processes is also apparent with increased horizontal resolution. This stronger upward motion affects the upper tropical cloud amount. The upper-level extra-tropical cloud amount is shown to depend strongly on the correction applied to the spectral transport of moisture in the model, the so-called "q fixer" which eliminates negative moisture. At higher resolution this spectral filler plays less of a role at the poles than at the lower resolutions. This study has important implications for the parameterization of cloud activity in large-scale models, since a certain resolved large-scale circulation is necessary for embedding these cloud parameterizations. It also raises questions relating to scale dependence of cloud and precipitation parameterizations.