A Further Study of Dynamics of the Four-Day Circulation in the Venus Atmosphere

Abstract

A steady state model on the four-day circulation in the Venus atmosphere proposed by Matsuda (1980) with use of two-layer model is examined by time integrations of a fivelayer model.
By assuming infinite horizontal viscosity, it is shown that the fast zonal flow is gradually formed in the upper layer over 1, 000 days by the accumulation of angular momentum. This angular momentum is supplied to the lower atmosphere from a slowly rotating planet and is transported upward by meridional circulation.
Stationary solutions are obtained for various values of three external parameters (i.e., horizontal diffusion time, latitudinal differential heating and planetary speed of rotation). Multiple equilibrium states appear in the system when the horizontal diffusion time (normalized by the vertical one) 8 is reduced to 10-2. The super-rotation rate as large as the observed one is realized in some solutions when 8 is reduced to 10-3. The parameter dependence of (multiple) equilibrium states in the present model is analogous to that obtained by the two-layer model in Matsuda (1980). By comparison between the equilibrium states obtained in these two models, the prediction on the stability of multiple equilibrium states (Matsuda (1980)) is verified; the state with the fast zonal flow and the state with the strong meridional cell are stable and the state having the characteristics intermediate between these two states is unstable.
Finally, by assuming two different ways of external heating (sudden heating and gradual heating), it is examined which of the two stable states is attained as a final state; the sudden heating leads to the state with strong meridional cell, while the gradual heating to the state with fast zonal flow.