Stationary waves and slowly moving features in the night upper clouds of Venus

TL;DR

This study provides the first global measurements of Venus's nightside upper cloud circulation, revealing complex motions including stationary waves and slow-moving features, which differ from the dayside and suggest intricate atmospheric dynamics.

Contribution

It offers new observational data on Venus's nightside upper cloud circulation, highlighting stationary waves and slow motions, expanding understanding of Venus's atmospheric superrotation.

Findings

Zonal motions range from -110 to -60 m/s, similar to dayside but with more variability.

Presence of stationary wave patterns with speeds from -10 to +10 m/s.

Discovery of unexplained slow motions between -50 to -20 m/s.

Abstract

At the cloud top level of Venus (65-70 km altitude) the atmosphere rotates 60 times faster than the underlying surface, a phenomenon known as superrotation. Whereas on Venus's dayside the cloud top motions are well determined and Venus general circulation models predict a mean zonal flow at the upper clouds similar on both day and nightside, the nightside circulation remains poorly studied except for the polar region. Here we report global measurements of the nightside circulation at the upper cloud level. We tracked individual features in thermal emission images at 3.8 and 5.0 $\mathrm{\mu m}$ obtained between 2006 and 2008 by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) onboard Venus Express and in 2015 by ground-based measurements with the Medium-Resolution 0.8-5.5 Micron Spectrograph and Imager (SpeX) at the National Aeronautics and Space Administration Infrared Telescope Facility (NASA/IRTF). The zonal motions range from -110 to -60 m s$^{-1}$, consistent with those found for the dayside but with larger dispersion. Slow motions (-50 to -20 m s$^{-1}$) were also found and remain unexplained. In addition, abundant stationary wave patterns with zonal speeds from -10 to +10 m s$^{-1}$ dominate the night upper clouds and concentrate over the regions of higher surface elevation.