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)
Waves in the atmosphere of Venus
The recent Soviet/French instrumented balloon experiments in the Venus atmosphere have shown that turbulent vertical motions with amplitudes of several metres per second exist within the principal cloud deck at about 50 km elevation1. The motions are particularly intense over the Aphrodite mountains, and the experimenters suggest that a disturbance forced by surface flow over the mountains reaches to cloud levels1,2. Here I point out that a patch of turbulence stationary over a surface feature can generate a wake-like pattern of horizontally propagating waves which can explain the general visual appearance of Venus, and which can produce a momentum exchange from mid-latitudes to low latitudes, helping to maintain the Venus atmospheric rotation.
Cellular convection in the atmosphere of Venus
AMONG the most intriguing features of the atmosphere of Venus is the presence of cellular structures near and downwind of the subpolar point. Ultraviolet images from the Mariner 10 and Pioneer Venus missions at cloud-top heights of 60–70 km indicate that these cells are chiefly found only during the afternoon at low latitudes, with dark-rimmed cells having horizontal scales of roughly 500–1,000 km, and bright-rimmed cells having horizontal scales of roughly 200–500 km (refs 1, 2). Images of the subsolar region from the recent Galileo flyby of Venus confirm these earlier observations—cellular features with horizontal scales of 250–1,000 km have been observed3 (Fig. la,b). It has been suggested that the structures are atmospheric convection cells2–5, and, in fact, numerical simulations of three-dimensional compressible convection produce features reminiscent of the cellular structures found in the subsolar Venus atmosphere (Fig. 1c)6. There has been some difficulty, however, in accounting for the sizes of these structures. The convection cells were thought to be located in a neutrally stable cloud layer, roughly 50 km above the surface, with a thickness of only a few kilometres. So broad and thin a convection cell would pose a severe challenge to the dynamics of convection. Here we propose that strongly penetrative convection into the stable regions above and below the neutrally stable cloud layer coupled with penetrative convection from the surface increases the vertical dimensions of the cells, thereby helping to explain their large horizontal extent.