  The zonal-mean atmospheric flow of an idealized terrestrial planet is
analyzed 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 because of 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. Three-dimensional
simulations that allow for eddy motion fall closer to the scalings of the
variant model. At very low rotation rates the maximum zonal wind falls with
falling planetary rotation rate, even in the three-dimensional simulations, and
collapses completely at zero rotation. Nevertheless, the low-rotation limit of
the overturning circulation is strong enough to drive the temperature profile
close to a state of nearly constant potential temperature.
