Prograde and retrograde atmospheric rotation of cloud-covered terrestrial planets

Context. Cloud-covered middle-atmospheric dynamics are important in the strong prograde rotation (i.e. superrotation) observed in some terrestrial-planet atmospheres, such as Venus and Titan. In addition, the unified theory and the mechanism driving the general circulation could be extended to as yet unknown extrasolar Earth-sized habitable planet atmospheres. Aims. We elucidate the dynamical process controlling prograde and retrograde atmospheric rotation induced by cloud heating in the middle atmosphere of terrestrial planets. Methods. The terrestrial middle-atmospheric circulation is examined using a general circulation model in which astronomical parameters are varied. Results. Middle-atmospheric circulation is strongly controlled by seasonal variation of the meridional circulation. Prograde and retrograde atmospheric rotation is determined by the direction of vertical angular momentum transport due to the meridional circulation resulting from tilting of the planetary rotation axis. The retrograde rotation rate is limited to -Ω for slowly rotating planets (where Ω is the planetary rotation rate).