The Hadley Circulation as a Radiative-Convective Instability

Hadley cell simulations over a tropical ocean are reported which suggest that the emission and absorption of thermal radiation by clouds and moisture play a primary role in the dynamics of the Hadley circulation. In particular, inclusion of interactions between clouds and moisture on one hand and radiation on the other causes the tropical atmosphere to become unstable to large scale convective circulations driven by diierential radiative heating between moist and dry regions, even when the underlying sea surface temperature doesn't vary with latitude. Zonally symmetric circulations take the form of an equatorially asymmetric convective cell, with ascent in one hemisphere and descent in the other. This may explain why there is usually only one intertropical convergence zone which is located away from the equator even when the sea surface temperature maximum is on or near the equator. The conclusions of this paper are tentative because the treatments of radiation and cloudiness are highly simpliied. However, if the simulated radiative-convective instability exists in nature, it also may explain the tendency of the tropical atmosphere to divide into large areas which are convec-tively suppressed, punctuated by smaller areas of intense convection.