A GCM Study on the 4-Day and 5-Day Waves in the Venus Atmosphere

The 4-day and 5-day waves observed at the cloud top in the Venus atmosphere are expected to play important roles in the maintenance of the general circulation. However, they have not been reproduced in Venus general circulation models (GCMs) so far, and their structures and excitation mechanisms remain poorly understood. Using an improved Venus GCM, we succeeded in reproducing the planetary-scale 3.3-day and 5.8-day waves, which are considered to correspond to the 4-day and 5-day waves, respectively. We also obtained the 4.8-day wave, whose structure is similar to that of the 5.8-day wave. The three-dimensional structures of these waves consist of a Kelvin wave in low latitudes and one or two Rossby waves in mid- and high latitudes that are connected at the critical latitudes. This fact indicates that they are excited by the Rossby-Kelvin instability. The Kelvin mode of the 3.3-day wave could be observed at the cloud top, while those of the 5.8-day and 4.8-day waves cannot be observed due to the critical levels. The mid-latitude Rossby mode of the 5.8-day wave is dominated by mid-latitude vortices symmetric about the equator at the cloud top; this result is quite consistent with the observations. The equatorward angular momentum flux induced by these waves could contribute to the Venus superrotation. The Rossby modes of the 5.8-day and 4.8-day waves have baroclinic structures in the cloud layer inducing significant poleward heat flux. The planetary-scale structures symmetric about the equator observed in the Venus atmosphere could be explained by these waves.