Wave-driven cloud formation on Venus

Venus is surrounded by sulfuric acid clouds essential to the planet's climate system. The upper part of the cloud is thought to be of photochemical origin, while the lower part is highly variable and will be more affected by atmospheric dynamics causing condensation and evaporation of sulfuric acid. The specific dynamical processes responsible for the variability are poorly understood.

Observations at near-infrared window wavelengths have revealed large opacity variations, which mostly occur in the lower part of the cloud layer. A significant feature is the planetary-scale dark cloud propagating with a period of 4.9-5.5 days, discovered through ground-based observations (Crisp et al. 1991). The IR2 camera aboard the Venus orbiter Akatsuki observed this phenomenon in more detail and found that the planetary-scale cloud discontinuity (disruption) that spans in the north-south direction characterizes the propagating structure (Satoh et al. 2017; Peralta et al. 2020). The relatively large amplitude near the equator and the zonal propagation faster than the background atmosphere indicate that the cloud opacity variation is mainly induced by a Kelvin wave. McGouldrick et al. (2021) analyzed this feature using spectroscopic data taken by Venus Express VIRTIS. They suggested that the cloud particle microphysical properties and trace gas densities change across the discontinuity, and the changes occur in the lower cloud region. Peralta et al. (2020) and McGouldrick et al. (2021) argued that a front associated with a nonlinear Kelvin wave could be responsible for the discontinuity.

Some Venus GCMs reproduced 5-6 day periodicities in the vertical wind or the thickness of the lower cloud driven by Kelvin waves with a zonal wavenumber of unity (Peralta et al. 2020; Ando et al. 2021). However, the observed sharp discontinuity was not reproduced in the models. The present study proposes mechanisms for the cloud discontinuity from the viewpoint of front generation through hydraulic jump (bore) and associated cloud formation. A simplified dynamical model, in which a Kelvin wave is artificially forced, and a microphysical model are used to reproduce the phenomenon. This study aims to understand the role of the Kelvin wave in the formation of the lower cloud and the conditions necessary for the appearance of the observed sharp discontinuity.