The circulation at the nightside lower clouds of Venus with high-precision winds

The study of the atmospheric circulation in the planets and moons of our solar system and exoplanets is critical for the advances in Meteorology. In slowly rotators like Venus, Titan and tidally-locked exoplanets, the Coriolis effect is too weak to balance the pressure gradients and other metric terms (centrifugal forces) generate a different balance called “cyclostrophic”. For reasons yet not fully understood, these slowly rotating bodies tend to exhibit atmospheres which rotate much faster than the solid globe, a phenomenon called atmospheric “superrotation”. Venus constitutes the most extreme case known in our solar system, with its clouds rotating up to 60 times faster than the surface, although numerical models yet fail to accurately reproduce this superrotation, especially at the deeper atmosphere. Besides, despite the richness of information provided by the space missions that explored Venus in the past and the varied and numerous Earth-based observations, accuracy in wind measurements at present is yet insufficient to isolate and characterize the several contributors of the observed speeds of the lower clouds of Venus located within 48-60 km. As a result, the net meridional circulation, influence of the solar tides or the magnitude of transient waves and eddied are yet uncertain.

In this work we intend to provide new insights about the circulation at the deeper atmosphere of Venus obtaining wind speeds with an accuracy better than 1 m·s^-1 taking advantage of the long-term coherence of the lower clouds’ patterns, which enables confidently tracking cloud tracers in images separated by more than 5 hours. These high-precision winds were obtained from sequences of images taken at 1.74, 2.26 and 2.32 µm by space missions Venus Express/VIRTIS-M (2006-2008) and Akatsuki/IR2 (2016), as well as with pairs of images acquired co-ordinately by Earth-based telescopes located at different geographical locations: IRTF/SpeX in Hawaii and HCT/IRCAM in India (May and July 2004, December 2010), IRTF/SpeX in Hawaii and NOT/NOTCam (July 2020). Our results extend those from VIRTIS-M recently published by Gorinov et al. (Atmosphere 2021, 12, 186) and confirm some of their results.