Assessing the long-term variability of Venus winds at cloud level from VIRTIS-Venus Express

The Venus Express (VEX) mission has been in orbit to Venus for more than 4 years now. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument onboard VEX observes Venus in two channels (visible and infrared) obtaining spectra and multi-wavelength images of the planet that can be used to sample the atmosphere at different altitudes. Day-side images in the ultraviolet range (380 nm) are used to study the dynamics of the upper cloud at 66-72 km while night-side images in the near infrared (1.74 mu m) map the opacity of the lower cloud deck at 44-48 km. Here we present a long-term analysis of the global atmospheric dynamics at these levels using a large selection of orbits from the VIRTIS-M dataset covering 860 Earth days that extends our previous work (Sanchez-Lavega, A. et al. [2008]. Geophys. Res. Lett. 35, L13204) and allows studying the variability of the global circulation at the two altitude levels. The atmospheric superrotation is evident with equatorial to mid-latitudes westward velocities of 100 and 60 m s(-1) in the upper and lower cloud layers. These zonal velocities are almost constant in latitude from the equator to 50 degrees S. From 50 degrees S to 90 degrees S the zonal winds at both cloud layers decrease steadily to zero at the pole. Individual cloud tracked winds have errors of 3-10 m s(-1) with a mean of 5 m s(-1) and the standard deviations for a given latitude of our zonal and meridional winds are 9 m s(-1). The zonal winds in the upper cloud change with the local time in a way that can be interpreted in terms of a solar tide. The zonal winds in the lower cloud are stable at mid-latitudes to the tropics and present variability at subpolar latitudes apparently linked to the activity of the South polar vortex. While the upper cloud presents a net meridional motion consistent with the upper branch of a Hadley cell with peak velocity v = 10 m s(-1) at 50 degrees S, the lower cloud meridional motions are less organized with some cloud features moving with intense northwards and southwards motions up to v = +/- 15 m s(-1) but, on average, with almost null global meridional motions at all latitudes. We also examine the long-term behavior of the winds at these two vertical layers by comparing our extended wind tracked data with results from previous missions. (C) 2011 Elsevier Inc. All rights reserved.