Tropical cloud radiative forcing and the tropical Hadley/Walker circulation: a simple model

The role of cloud radiative forcing (CRF) in the atmospheric large-scale circulation is a poorly understood and challenging problem. Several GCM studies have demonstrated that CRF may be a major driving force for the tropical Hadley/Walker circulation instead of latent heat release. But a theoretical framework for these intriguing results is still lacking. Motivated by Neelin and Held (1987), a new moist dynamic model is constructed based on the linear dynamics of the Gill model and moist static energy equation. With the prescribed apparent total heat source, i.e., the moist adiabtic heat source, the model simulates the gravest baroclinic mode of the climatological low-level winds and convergence.

When forced by the cloudy-sky apparent total heat source, the model can reproduce the fundamental features of the Hadley/Walker circulation, such as the ascending branch in the ITCZ, SPCZ as well as the descending branch in the cold tongue and subtropics. But when the CRF is removed from the model, the Walker circulation disappears and the Hadley cell reverses. These demonstrate the CRF play a pivotal role in the tropical Hadley/Walker circulation in addition to the gross moist stability. As the 1st attempt, our simple model offers a viable theoretical framework to the GCM results. Based on our simple dynamic model as well as the GCM results, we argue that CRF plays a fundamental role in maintaining the Hadley/Walker circulation.