Dynamics of the Four-Day Circulation in the Venus Atmosphere

[摘要] In order to make clear the problem of the four-day circulation, we construct a simpleaxisymmetric model. This model contains the mechanism first proposed by Gierasch (1975), i.e., upward transports of angular momentum by a meridional circulation with the aid ofvery large horizontal viscosity which dissipates differential rotation. Further, a suppressing effect on this mechanism due to finiteness of the horizontal eddy viscosity is also involved. The velocity and the temperature field are represented by a few fundamental modes. Terms expressing the nonlinear interactions among the modes are explicitly written in the mode equations. Stationary solutions of this system are obtained mainly by a two-layer model, for both an infinite and a finite horizontal eddy viscosity.First, we determine magnitude of the mean zonal flow (U) as a function of the meridional circulation (V) from angular momentum balance. In the case of an infinite horizontal viscosity, U is simply proportional to V. Its ratio (U/V) is given by the inverse ratio of period of planetary rotation (τΩ) to the time constant of vertical diffusion (τν) (i.e., U/V/≈τΩ.) In the case of a finite horizontal viscosity, U has a maximum value for a certain value of V. Its maximum value is determined by the ratio of horizontal viscosity to vertical one as well as τΩ.Next, associating this U-V relation with the vorticity equation in the zonal direction, we classify types of solutions according to the effect which dominates and balances solenoidal term in the vorticity equation. The types of solution are as follows.Thermal wind balance of the Venus type: The vertical gradient of centrifugal force due to atmospheric rotation dominates.Thermal wind balance of the earth type: The vertical gradient of centrifugal force due to atmospheric rotation coupled with planetary rotation dominates.Direct cell balance: The frictional force associated with the meridional circulation dominates.Kinds of balance are determined on a two-dimensional parameter space of τΩ/τν and the latitudinal differential heating denoted by Gγ. In an infinite horizontal viscosity case, thermal wind balance of the Venus type appears in the whole range of large Gγ. In the case of finite viscosity, solutions of this balance can exist only in a more restricted domain in the τΩ/τυ-Gγ diagram. Gγ of this domain has an upper limit depending τΩ/τυ, and only a direct cell balance can correspond to a Gγ value beyond the upper limit. In a portion of the domain where thermal wind balance of the Venus type is realized, solution of direct cell balance is also obtained as a stable solution. Thus for this parameter range, two utterly different states, a fast zonal motion accompanied by a slow meridional circulation and a strong meridional circulation associated with a slight zonal motion are possible as stable stationary states for the same differential heating. The former corresponds to the four-day circulation, while the latter means a direct cell between day side and night side in actual situation.Results of the numerical experiments by Young & Pollack (1977) are discussed in the light of the present results.

[学科分类] 大气科学