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. Plain Language Summary: Planetary‐scale waves called 4‐day and 5‐day waves have been observed in the Venus atmosphere, which are expected to play important roles in the global atmospheric motions. However, they have not been reproduced in numerical models so far, and hence their three‐dimensional structures remain completely unknown. In the present study, we found planetary‐scale waves with periods of 3.3, 5.8, and 4.8 days in our numerical simulation. The 3.3‐day and 5.8‐day waves are considered to correspond to the 4‐day and 5‐day waves observed in the Venus atmosphere. The three‐dimensional structures of these waves, in which an equatorial wave and one or two mid‐ and high‐latitude vortex waves are connected, indicate that the waves are excited by dynamical instability called Rossby‐Kelvin instability. The mid‐latitude wave of the 5.8‐day wave is dominated by vortices symmetric about the equator at 70 km altitude, which are quite consistent with the observations. The equatorward angular momentum transport and the poleward heat transport are induced by the 5.8‐day and 4.8‐day waves, suggesting that these waves could contribute to the maintenance of the fast zonal winds on Venus. The observed planetary‐scale structures symmetric about the equator such as cloud patterns could be explained by these waves. Key Points: We succeeded in reproducing the 4‐day and 5‐day waves observed at the Venus cloud top, which are excited by the Rossby‐Kelvin instabilityThese waves induce the equatorward angular momentum flux, which could contribute to the maintenance of the Venus superrotationThese waves could relate to the planetary‐scale structures symmetric about the equator widely observed in the Venus atmosphere [ABSTRACT FROM AUTHOR]