Modeling Water Vapor and Clouds as Passive Tracers in an Idealized GCM

This paper introduces an idealized general circulation model (GCM) in which water vapor and clouds are tracked as tracers, but are not allowed to affect circulation through either latent heat release or cloud radiative effects. The cloud scheme includes an explicit treatment of cloud microphysics and diagnoses cloud fraction froma prescribed subgrid distribution of total water. Themodel is capable of qualitatively capturing many largescale features of water vapor and cloud distributions outside of the boundary layer and deep tropics. The subtropical dry zones, midlatitude storm tracks, and upper-tropospheric cirrus are simulated reasonably well. The inclusion of cloudmicrophysics (namely rain re-evaporation) has amodest but significant effect ofmoistening the lower troposphere in thismodel. When being subjected to a uniform fractional increase of saturated water vapor pressure, themodel produces little change in cloud fraction. Amore realistic perturbation, which considers the nonlinearity of the Clausius-Clapeyron relation and spatial structure of CO2-induced warming, results in a substantial reduction in the free-tropospheric cloud fraction. This is reconciled with an increase of relative humidity by analyzing the probability distributions of both quantities, and may help explain partly similar decreases in cloud fraction in full GCMs. The model provides a means to isolate individual processes or model components for studying their influences on cloud simulation in the extratropical free troposphere.