The ω Governor

A theory relating dry Hadley cell strength and width is developed based on the twin constraints of momentum balance in the upper branch and energy balance at mid levels in the updraft. In the latter, the magnitude of diabatic heating limits the updraft velocity, ω, provided the stratification is fixed by processes external to the dynamics; I refer to this as the “ω governor”. Simultaneously in the upper branch of the cell, the Hadley cell must advect enough momentum poleward to offset the momentum sink due to eddy stresses from breaking extratropical Rossby waves. Because the upward mass flux is limited by the ω governor, the Hadley cell must increase its width to increase mass and momentum transport in the upper branch. While the theory lacks predictions for the magnitude of eddy stresses and stratification, given these it makes clear predictions for the scaling of the width and strength of the Hadley cell that differ considerably from other predictions. The proposed theory is tested over a broad range of planetary parameters using dry general circulation model simulations, and is shown to be quantitatively consistent over one to two orders-of-magnitude in planetary radius and rotation rate.