Waves in the Venus atmosphere are numerically investigated by extending a work of Sugimoto et al. (2014). Fast superrotating zonal flow of 120 m s⁻¹ at the equator is reproduced and maintained by solar heating for more than 10 Earth years. The meridional distribution of the obtained fast zonal flow is quite consistent with observations at the cloud levels. In the cloud layer, baroclinic waves develop continuously with a life cycle of ~25 Earth days at midlatitudes, using available potential energy derived from a baroclinically unstable basic state. Rossby waves observed at the cloud top are generated by the baroclinic waves and induce spatio‐temporal variation of the superrotation with amplitude larger than 25 m s⁻¹. Further, Kelvin waves with a period of ∼ 6.2 days appear in the equatorial region below ~50 km. Momentum and heat transports produced by these waves are discussed.

Baroclinic instability in the super-rotation of Venus is investigated by a newly developed atmospheric general circulation model. First, we adopt an idealized super-rotation, i.e., solid-body rotating flow in a weakly stratified layer at cloud level, as an initial basic state in a nominal case. With the evolution of time, baroclinic instability occurs in a weakly stratified layer with large vertical shear of the basic zonal flow. Horizontal wind associated with the baroclinic instability modes is of a few m s⁻¹. The initial structure of the unstable modes is similar to those obtained in previous linear stability analyses. However, it is modified by nonlinear interactions in the later stage, reaching a quasi-steady state. Meridional transport of momentum and heat by these unstable modes accelerates the super-rotation by ~ 0.05 m s⁻¹ day⁻¹ at midlatitudes. Furthermore, the dependence of baroclinic instability on the basic state, i.e., the meridional profiles of zonal flow and the vertical profiles of static stability, are subsequently investigated. For the super-rotation with midlatitude jets at cloud level, the modes are modified from baroclinic to barotropic in the later stage. Typically, their horizontal wind is of O(10) m s⁻¹. Their amplitude is maintained by energy conversion from zonal-mean available potential energy associated with the baroclinic basic state. In the case where static stability is smaller than that in the nominal case, the baroclinic modes transfer angular momentum from midlatitude to the equator near a 70 km level and accelerate the super-rotation by more than 10 m s⁻¹ in the equatorial region.

We present distributions of the zonal-mean temperature and static stability in the Venusian atmosphere obtained from Venus Express and Akatsuki radio occultation profiles penetrating down to an altitude of 40 km. At latitudes equatorward of 75°, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements in that there is a low-stability layer at altitudes of 50–58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. Meanwhile, at latitudes poleward of 75°, a low-stability layer extends down to 42 km, which has been unreported in analyses of previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the Earth’s troposphere, which generally has a deeper low-stability layer at low latitudes than at mid- and high latitudes.

Cloud patterns are important clues for revealing the atmospheric circulation of Venus. Recently, a planetary-scale streak structure has been discovered in middle- and lower-cloud images of Venus’ night-side taken by IR2, the 2-μm camera, on board the Akatsuki orbiter. However, its formation mechanism has not been investigated. Here we succeed, for the first time, in reproducing the patterns of the observed streak structure, as regions of strong downward flows that develop in high-resolution global simulations of the Venus atmosphere. The streaks are formed in both hemispheres with equatorial symmetry, which is caused by equatorial Rossby-like and Kelvin-like waves with zonal wavenumber one. The low-stability layer that has been suggested by past observations is essential for reproducing the streak structure. The streaks of downward flow result from the interaction of the meridionally tilted phase lines of the Rossby-like waves and the characteristics of baroclinic instability produced around the low-stability layer.