Influence of Venus topography on the zonal wind and UV albedo at cloud top level: The role of stationary gravity waves
Based on the analysis of UV images (at 365 nm) of Venus cloud top (altitude 67 ± 2 km) collected with Venus Monitoring Camera on board Venus Express (VEX), it is found that the zonal wind speed south of the equator (from 5°S to 15°S) shows a conspicuous variation (from −101 to −83 m/s) with geographic longitude of Venus, correlated with the underlying relief of Aphrodite Terra. We interpret this pattern as the result of stationary gravity waves produced at ground level by the uplift of air when the horizontal wind encounters a mountain slope. These waves can propagate up to the cloud top level, break there, and transfer their momentum to the zonal flow. Such upward propagation of gravity waves and influence on the wind speed vertical profile was shown to play an important role in the middle atmosphere of the Earth by Lindzen (1981) but is not reproduced in the current GCM of Venus atmosphere from LMD. (Laboratoire de Météorologie Dynamique)
Mesoscale modeling of Venus' bow-shape waves
Abstract The Akatsuki instrument LIR measured an unprecedented wave feature at the top of Venusian cloud layer. Stationary bow-shape waves of thousands of kilometers large lasting several Earth days have been observed over the main equatorial mountains. Here we use for the first time a mesoscale model of the Venus's atmosphere with high-resolution topography and fully coupled interactive radiative transfer computations. Mountain waves resolved by the model form large-scale bow shape waves with an amplitude of about 1.5 K and a size up to several decades of latitude similar to the ones measured by the Akatsuki spacecraft. The maximum amplitude of the waves appears in the afternoon due to an increase of the near-surface stability. Propagating vertically the waves encounter two regions of low static stability, the mixed layer between approximately 18 and 30 km and the convective layer between 50 and 55 km. Some part of the wave energy can pass through these regions via wave tunneling. These two layers act as wave filter, especially the deep atmosphere layer. The encounter with these layers generates trapped lee waves propagating horizontally. No stationary waves is resolved at cloud top over the polar regions because of strong circumpolar transient waves, and a thicker deep atmosphere mixed layer that filters most of the mountain waves.
Vertical distribution of SO2 in upper cloud layer of Venus and Origin of U.V.-absorption
It is shown that decline of spherical albedo of Venus toward the ultraviolet can be explained by the presence of two absorbing agents: a) SO2, for which abundance is 1011 cm−3 at height 68 km and scale height is about 1 km; b) some unknown aerosol absorbent, possibly a 1% FeCl3 admixture in a sulfuric acid concentrated solution. A mechanism of aerosol formation is proposed.