Understanding Cloud Processes with an Idealized Model

This paper explores the physical connections between free tropospheric cloud amount, humidity, and circulation in an idealized model. The Held-Suarez (HS) dynamical core (Held and Suarez, 1994, BAMS) is utilized with the addition of passive water and cloud tracers. Outside of the boundary layer and deep tropics, the model captures the large-scale features of cloud amount and humidity including the subtropical dry zones and mid-latitude storm tracks. As passive tracers, clouds do not feed back on the circulation or temperature fields through latent heat release or cloud radiative effects. Due to its capability in representing the free troposphere and the decoupled nature of its cloud feedbacks, this idealized model provides a unique platform to explore the isolated role of various cloud processes. Comparison of saturation adjustment and cloud microphysical schemes highlight the role of cloud processes in redistributing atmospheric moisture. Sensitivity studies within the model's explicit treatment of cloud microphysics reveal the key role of re-evaporation of hydrometeors in moistening the lower troposphere. The magnitude and location of the extratropical cloud maxima is tightly connected to the sub-grid-scale relative humidity distribution assumed by the cloud scheme. Cloud formation in the extratropics interrupts the isentropic transport of moisture from the tropics, drying the polar upper troposphere. Simple alterations to the general circulation in the model demonstrate that details of changes in the temperature gradient and the location of the eddy-driven jet are essential for understanding the simulated cloud response to climate change. The utility of the idealized model for exploring cloud processes is discussed along with further implications of these results.