Thermodynamic control of tropical rainfall

In 1987, Neelin and Held showed how, in the tropics, surface heat fluxes and infrared radiation control atmospheric convergence, and hence rainfall, by means of their joint effect on the supply of moist static energy to the troposphere. They also showed that for a given rate of supply of moist static energy, the strength of convergence is inversely proportional to a 'gross moist stability', which is related to the humidity of the troposphere and to the difference between the height of the environmental minimum in moist static energy and the elevation of maximum vertical mass-flux. The present paper extends Neelin and Held's analysis to the non-equilibrium case by invoking the somewhat speculative hypothesis that rainfall is primarily controlled by the mean saturation-deficit of the troposphere. The relaxation time of the atmosphere to the Neelin-Held equilibrium is found to be a strong function of the existing saturation-deficit under this hypothesis.