  A maintenance mechanism of an approximately linear velocity profile of the
Venus zonal flow or superrotation is explored, with the aid of a
Reynolds-averaged turbulence modeling approach. The basic framework is similar
to that of Gierasch (1975) in the sense that the mechanism is examined under a
given meridional circulation. The profile mimicking the observations of the
flow is initially assumed, and its maintenance mechanism in the presence of
turbulence effects is investigated from a viewpoint of the suppression of
energy cascade. In the present work, the turbulent viscosity is regarded as an
indicator of the intensity of the cascade. A novelty of this formalism is the
use of the isotropic turbulent viscosity based on a nonlocal time scale linked
to a large-scale flow structure. The mechanism is first discussed
qualitatively. On the basis of these discussions, the two-dimensional numerical
simulation of the proposed model is performed, with an initially assumed
superrotation, and the fast zonal flow is shown to be maintained, compared with
the turbulent viscosity lacking the nonlocal time scale. The relationship of
the present model with the current general-circulation-model simulation is
discussed in light of a crucial role of the vertical viscosity.
