Asymptotic Solutions to the Axisymmetric Moist Hadley Circulation

In order to construct a simple theory of moist Hadley circulations, the Quasi-Equilibrium Tropical Circulation Model (QTCM) equations are solved asymptotically, in the limit of strong surface friction, for an axisymmetric aquaplanet domain. This model involves only two vertical modes, the barotropic and first baroclinic one, and highly idealized representations of physical processes.

The dry equations, with temperature forced by Newtonian relaxation towards a prescribed radiative-convective equilibrium, are solved first. At leading order, the resulting circulation has angular momentum conserving winds at the top of the troposphere and zero winds at the surface, owing to the cancellation of the barotropic and baroclinic modes there. Weak surface winds are calculated from the first order corrections. The broad features of these solutions are similar to those obtained in previous studies of the dry Hadley circulation.

The moist equations are solved next, with a fixed sea surface temperature at the lower boundary and simple parameterizations of surface fluxes, deep convection, and radiative transfer. The solutions yield the structure of the barotropic and baroclinic winds, as well as the temperature and moisture fields. In addition, we derive expressions for the width and strength of the equatorial precipitating region (ITCZ) and the width of the entire Hadley circulation.