Equatorial Superrotation in a Slowly Rotating GCM: Implications for Titan and Venus

Abstract

The maintenance of strong superrotating zonal winds in the equatorial Venus atmosphere and the possibility of a similar regime on Titan are examined with a modified version of an efficient terrestrial general circulation model (GCM). The model is simplified by removal of the hydrologic cycle, the diurnal cycle, and all seasonal and geographic variations. We describe a suite of equilibrium simulations in which rotation rate, radiative heating profile, surface drag, and stratospheric drag are varied. The key to superrotation in these experiments is the presence of an upper troposphere cloud which intercepts much of the incoming solar flux and produces a statically stable radiative equilibrium state in the lower and middle troposphere. The radiative heating profile limits the depth of boundary layer convection and detaches the upper level flow from the surface. At Titan's presumed rotation period, the cloud-covered GCM produces equatorial winds in excess of 50/100 m sec-1 with/without stratospheric drag. Superrotation extends throughout most of the atmosphere. Hadley cell interaction with quasi-barotropic eddies which transport momentum equatorward is responsible for the excess angular momentum, confirming certain aspects of proposals by Gierasch (1975) and Rossow and Williams (1979) but with relatively modest Prandtl numbers. The latitudinal profile of zonal wind resembles that for uniform absolute linear momentum. Removal of the cloud decreases static stability, increases vertical convective mixing, and almost completely eliminates equatorial superrotation. Equatorial winds >60 m sec-1 also occur at Venus' rotation period, but only near the upper boundary in the absence of stratospheric drag. In the slowly rotating regime, equatorial winds increase as surface drag increases. Our calculations represent the first GCM simulation of strong superrotation without unrealistic momentum sources. The results suggest that superrotation is not inevitable on slowly rotating planets but is likely if diabatic heating produces a statically stable thermal structure.