Update: Newly published paper in Nature Geoscience Atmospheric mountain wave generation on Venus and its influence on the solid planet’s rotation rate has open-access links in
Science News, Motherboard, and Science.
There is a stationary gravity wave in Venus's very dense atmosphere. It is intermittent, but has been detected several times. See the NYTimes article Venus Smiled, With a Mysterious Wave Across Its Atmosphere for a discussion of recent observations by JAXA. See Akatsuki and Happy Birthday, Akatsuki!, celebrating it's first Venusian year at Venus.
Considering the extremely high density of the atmosphere near the surface, would it be possible for a suitably shaped spacecraft to "surf" or somehow remain aloft in this wave without propulsion, or even use it to rise high in the atmosphere, taking vertical data (e.g. composition, temperature, radar surface imagery and doppler profiling) and then gently return to the surface?
above: "A sequence of images showing the stationary nature of the bow-shape wave above Venus when it was observed in December 2015. Planet-C" from NYTimes. credit: Planet-C/JAXA
above: "An illustration of how gravity waves travel up mountains and into Venus’s atmosphere. Credit ESA" From How Mountains Obscured by Venus’s Clouds Reveal Themselves.