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Zusammenfassung:
In the Venus atmosphere, fast zonal wind has been observed, which is called super-rotation, because it is much faster than the rotation period of the solid part of the planet. So far, Venus general circulation models (VGCMs) have been used to study the super-rotation and several studies obtained fully developed super-rotation faster than 100 m/s. It has been suggested that horizontal diffusion and resolution are important factors to generate and maintain the super-rotation in the general circulation model because waves which transport angular momentum are sensitive to them. Here, we investigated how the super-rotation depends on the magnitude of horizontal hyper diffusion using a VGCM (AFES-Venus) with medium and high resolutions (T42L60 and T159L120 where T and L denote the triangular truncation number for spherical harmonics and vertical levels, respectively). In both the runs, we found a parameter range where the structure of fully developed super-rotation is almost independent of the magnitude of horizontal diffusion. Spectral analysis shows that unrealistically strong super-rotation is developed when medium-scale disturbances are dissipated by stronger horizontal diffusion. On the other hand, artificially weak super-rotation is also realized because spurious small-scale disturbances are accumulated when the horizontal diffusion is too weak. We elucidated that the super-rotation reproduced in our VGCM is caused by resolved eddies. Moreover, we could estimate appropriate magnitude of horizontal diffusion by the spectral analysis. This study has broad implications for the planetary atmospheric science, not only in modeling but also in observational aspects.
