Multiple Equilibrium States Appearing in a Venus-Like Atmospheric General Circulation Model

Abstract

969 IntroductionVenus has a thick CO 2 atmosphere with surface pressure of 9.2 × 10 4 hPa. The surface temperature is about 730 K because of the greenhouse effect imparted by thick CO 2 gas. Sulfuric acid clouds blanket the entire planet at about 47−70 km. An important characteristic of Venus is its very slow rotation: its rotation period is 243 earth days. One Venus Solar day is 117 days considering both daily rotation and revolution. It might therefore be speculated that convection between dayside and nightside dominates the atmospheric circulation, and that the convection pattern moves slowly together with slowly migrating subsolar and antisolar points. However, such a convection has not been observed, and observations have confirmed that fast zonal winds, called super-rotation or four-day circulation, are predominant. The Venusian cloud pattern rotates in the same direction as the rotation of the solid Venus with about four days around Venus along the equator. The zonal wind speed increases with height; its velocity is about 100 m s −1 at the cloud top level (about 70 km). The wind is 60 times faster than the solid planet. The reason why dayside-nightside circulation does not appear but super-rotation (four-day circulation) is dominant remains still unclear. Furthermore, the mechanism producing and maintaining such a fast zonal wind Japan, Vol. 86, No. 6, pp. 969−979, 2008 Journal of the Meteorological Society of AbstractWe investigated the existence of multiple equilibrium states in the Venusian atmospheric general circulation suggested by Matsuda (1980Matsuda ( , 1982 using a Venus-like atmospheric general circulation model. We ran the model from two initial conditions: a state with large zonal wind increasing with height and a motionless state. For the large zonal wind initial state, a strong zonal wind with weak meridional circulation, i.e., super-rotation, appears. However for the motionless initial condition, slow zonal wind with strong meridional circulation relative to the farmer state appears. Each circulation reached a quasi-steady state. These results have the same features suggested by Matsuda. The presence of multiple equilibrium states was sensitive to the horizontal eddy viscosity parameter. For the strong zonal wind state, the acceleration of the zonal mean zonal wind results from the horizontal EP flux divergence from the wavenumber one component on the equator, which is mainly maintained by the Gierasch mechanism (1975). The presence of multiple equilibrium states suggests that an alternative slow zonal wind state could appear in the Venusian atmosphere if an appropriate initial condition or drastic fluctuation is assigned.

Fully Developed Superrotation Driven by the Mean Meridional Circulation in a Venus GCM

“…While the zonal-mean meridional circulation (MMC) has been thought to be important to it (Gierasch, 1975;Matsuda, 1980;Rossow & Williams, 1979), fully developed superrotation has not been reproduced so far in Venus general circulation models (GCMs) driven by a zonal-mean component of the realistic solar heating (Hollingsworth et al, 2007). As confirmed by a series of studies (Hollingsworth et al, 2007;Kido & Wakata, 2008Yamamoto & Takahashi, 2009) and the model intercomparison project (Lebonnois et al, 2013) in which six GCMs are compared under the same simplified thermal forcing based on Lee et al (2007), the fully developed superrotation of~100 m/s is not reproduced from a motionless state by the MMC mechanism with the realistic solar heating. As confirmed by a series of studies (Hollingsworth et al, 2007;Kido & Wakata, 2008Yamamoto & Takahashi, 2009) and the model intercomparison project (Lebonnois et al, 2013) in which six GCMs are compared under the same simplified thermal forcing based on Lee et al (2007), the fully developed superrotation of~100 m/s is not reproduced from a motionless state by the MMC mechanism with the realistic solar heating.…”

Overview of Venus orbiter, Akatsuki

“…Atmospheric general circulation models for Venus-like slowly rotating planets seem to favor this process as the mechanism of maintaining the super-rotation (Del Genio and Zhou, 1996;Takahashi, 2003, 2006;Lee et al, 2007;Kido and Wakata, 2008). However, these models assume either an unrealistically high solar heating rate in the deep atmosphere or an unrealistically large equator-to-pole surface temperature gradient so that a strong Hadley circulation can exist in the deep atmosphere.…”

Numerical simulations of the global circulation of the atmosphere of Venus: Effects of surface relief and solar radiation heating

“…The global circulation models developed for the atmosphere of Venus by other research groups (Young and Pollack, 1977;Seiff et al, 1985;Newman and Leovy, 1992;Del Genio and Zhou, 1996;Yamamoto and Takahashi, 2003a;2003b;2004;2006;Lee et al, 2005;Dowling et al, 2006;Lee, 2006;Herrnstein and Dowling, 2007;Hollingsworth et al, 2007;Takagi and Matsuda, 2007;Kido and Wakata, 2008;Lebonnois et al, 2010;Lee and Richardson, 2010) are based on the numerical solution of the sys tem of geophysical hydrodynamics equations. This equation system is derived from the averaging of the motion of the atmospheric gas over the space on hori zontal scales of about 100 km and on vertical scales of about 10 km for the conditions of the Earth and Venus (see, e.g., Monin, 1988).…”