Modeling Water Vapor and Clouds in an Idealized GCM

3 This paper introduces an idealized general circulation model (GCM) in which water vapor 4 and clouds are tracked as tracers, but are not allowed to affect circulation either through 5 latent heat release or cloud radiative effects. The cloud scheme includes an explicit treatment 6 of cloud microphysics and diagnoses cloud fraction from a prescribed sub-grid distribution 7 of total water. The model is capable of qualitatively capturing many large-scale features of 8 water vapor and cloud distributions outside of the boundary layer and deep tropics. The 9 subtropical dry zones, mid-latitude storm tracks and upper-tropospheric cirrus are simulated 10 reasonably well. The inclusion of cloud microphysics (namely rain re-evaporation) has a 11 significant effect of moistening the lower troposphere in this model. When being subjected 12 to a uniform fractional increase of saturated water vapor pressure, the model produces little 13 change in cloud fraction. A more realistic perturbation, which considers the non-linearity 14 of the Clausius-Clapeyron relation and spatial structure of CO2-induced warming, results in 15 a substantial reduction in the free-tropospheric cloud fraction. This is reconciled with an 16 increase of relative humidity by analyzing the probability distributions of both quantities. 17 The implications of these results and the utility of the idealized model for understanding 18 cloud feedback are discussed. 19