Venusian middle-atmospheric dynamics in the presence of a strong planetary-scale 5.5-day wave

The middle atmospheric dynamics on Venus are investigated using a middle atmosphere general circulation model. The magnitude of the superrotation is sensitive to the amplitude of the planetary-scale waves. In particular, the critical level absorptions of the forced planetary-scale waves might contribute to the maintenance of the superrotation near the cloud base. In the case of strong 5.5-day wave forcing, the superrotation with zonal wind speed higher than 100ms ^-1 is maintained by the forced wave. Four-day and 5.5-day waves are found near the equatorial cloud top and base, respectively. The planetary-scale waves have a Y-shaped pattern maintained by the amplitude modulation in the presence of strong thermal tides.The polar hot dipole is unstable and its dynamical behavior is complex near the cloud top in this model. The dipole merges into a monopole or breaks up into a tripole when the divergent eddies with high zonal wavenumbers are predominant in the hot dipole region. A cold collar is partly enhanced by a cold phase of slowly propagating waves with zonal wavenumber 1. Although such a complex dipole behavior has not been observed yet, it is likely to occur under a dynamical condition similar to the present simulation. Thus, the dynamical approach using a general circulation model might be useful for analyzing Venus Express and ground-based observation data.