SIMULATIONS OF SUPERROTATION ON SLOWLY ROTATING PLANETS - SENSITIVITYTO ROTATION AND INITIAL CONDITION

We use a simplified terrestrial general circulation model as a nonline ar process model to investigate factors that influence the extent of e quatorial superrotation in statically stable atmospheres on slowly rot ating planets such as Titan and Venus, The possibility of multiple equ ilibria is tested by running the same model to equilibrium from vastly different initial conditions, The final state is effectively independ ent of initial state, reinforcing the impression that equatorial super rotation is inevitable on slowly rotating planets with stable radiativ e equilibrium structures, Of particular interest is the fact that at T itan rotation, the model equilibrates with strong prograde winds even when initialized with strong retrograde winds, This suggests that reli able remote sensing inferences of latitudinal temperature gradients on Titan can unambiguously be interpreted as evidence for superrotation, We also demonstrate for the first time that significant equatorial su perrotation can be produced at Venus' rotation rate in such models, gi ven sufficient numerical precision, The strength of superrotating zona l winds increases with rotation rate in the slowly rotating regime whe n other parameters are held fixed, However, the efficiency of superrot ation relative to the angular momentum of an atmosphere corotating wit h the solid planet increases with decreasing rotation rate instead, be cause the Hadley cell strengthens and expands poleward, This allows fo r the formation of stronger high latitude jets, which ultimately serve as the source for equatorial superrotation via barotropic instability , Estimates of relevant parameter settings for Triton and Pluto tentat ively imply that their atmospheres may marginally be in the superrotat ing regime, but only if temperature decreases with height near the sur face. (C) 1996 Academic Press, Inc.