Zonal-Mean Atmospheric Dynamics of Slowly Rotating Terrestrial Planets

The zonal-mean atmospheric flow of an idealized terrestrial planet is investigated using both numerical simulations and zonally symmetric theories, focusing largely on the limit of low planetary rotation rate. Two versions of a zonally symmetric theory are considered, the standard Held-Hou model, which features a discontinuous zonal wind at the edge of the Hadley cell, and a variant with continuous zonal wind but discontinuous temperature. The two models have different scalings for the boundary latitude and zonal wind. Numerical simulations are found to have smoother temperature profiles than either model, with no temperature or velocity discontinuities even in zonally symmetric simulations. Continuity is achieved in part by the presence of an overturning circulation poleward of the point of maximum zonal wind, which allows the zonal velocity profile to be smoother than the original theory without the temperature discontinuities of the variant theory. Zonally symmetric simulations generally fall between the two sets of theoretical scalings, and have a faster polar zonal flow than either set. Three-dimensional simulations, which allow for the eddy motion that is missing from both models, fall closer to the scalings of the variant model. At very low rotation rates the maximum zonal wind falls with falling planetary rotation rate, and is zero at zero rotation. The low-rotation limit of the overturning circulation, however, is strong enough to drive the temperature profile close to a state of nearly constant potential temperature.