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)
Initial products of Akatsuki 1-μm camera
The status and initial products of the 1-μm camera onboard the Akatsuki mission to Venus are presented. After the successful retrial of Venus’ orbit insertion on Dec. 2015 (5 years after the failure in Dec. 2010), and after a long cruise under intense radiation, damage in the detector seems small and fortunately insignificant in the final quality of the images. More than 600 dayside images have been obtained since the beginning of regular operation on Apr. 2016 although nightside images are less numerous (about 150 in total at three wavelengths) due to the light scattered from the bright dayside. However, data acquisition stopped after December 07, 2016, due to malfunction of the electronics and has not been resumed since then. The 0.90-µm dayside images are of sufficient quality for the cloud-tracking procedure to retrieve wind field in the cloud region. The results appear to be similar to those reported by previous 1-μm imaging by Galileo and Venus Express. The representative altitude sampled for such dayside images is estimated to be 51–55 km. Also, the quality of the nightside 1.01-µm images is sufficient for a search for active volcanism, since interference due to cloud inhomogeneity appears to be insignificant. The quality of the 0.97-µm images may be insufficient to achieve the expected spatial resolution for the near-surface H2O mixing ratio retrievals.
The jovian anticyclone BA: II. Circulation and interaction with the zonal jets
Abstract In this second part of our study of the large jovian anticyclone BA we present detailed measurements of its internal circulation and numerical models of its interaction with the zonal jets and nearby cyclonic regions. We characterized the flow using high-resolution observations obtained by the Cassini spacecraft in December 2000 (9 months after the genesis of BA as a result of the merger of two large White Ovals), by the ACS camera onboard HST in January 2005 and April 2006 and by the New Horizons spacecraft in February 2007. Cloud motions were derived from high-resolution images using an automatic correlator that provides a large sampling of the motions in images separated by short time intervals (30 min–2 h). The internal wind structure did not change when the oval changed its color reddening in late 2005–early 2006 and all four datasets from 2000 to 2007 consistently show a similar wind regime: an asymmetric intense anticyclonic vortex with faster winds in its Southern portion with mean speeds of 110 m/s and peak velocities of 135 m/s. These speeds are slightly higher than those measured in the three White Ovals predecessors of BA by the Voyagers [Mitchell, J.L., Beebe, R.F., Ingersoll, A.P., Garneau, G.W., 1981. J. Geophys. Res. 86, 8751–8757] and Galileo [Vasavada, A.R., and 13 colleagues, 1998. Icarus 135, 265–275] but not as much as it has been recently reported [Simon-Miller, A.A., Chanover, N.J., Orton, G.S., Sussman, M., Tsavaris, I.G., Karkoschka, E., 2006. Icarus 185, 558–562; Cheng, A.F., and 14 colleagues, 2008. Astronom. J. 135, 2446–2452]. The asymmetry of the velocities in the vortex is a consequence of the interaction of BA with the zonal circulation and emerges as a natural result in high-resolution simulations of the vortex dynamics using the EPIC model.