Role of a Kelvin wave in the formation of Venusian discontinuous lower clouds

Venus is surrounded by sulfuric acid clouds that are essential to the planet’s climate system. The main cloud deck extends from about 48 km up to approximately 70 km and can be divided into three layers based on the extinction coefficient and particle population. The upper cloud, located around 58­–70 km altitudes, is believed to be of photochemical origin. The middle cloud, located around 50–58 km, is thought to be sustained by the condensation of H₂SO₄ vapor transported from below by convection. The lower cloud, located around 47–50 km, is highly variable and poorly understood in terms of its origin and microphysical properties.

Observations using 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 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.

A Venus GCM reproduced a 5.5-day periodicity in the thickness of the lower cloud driven by a Kelvin wave with a zonal wavenumber of unity (Ando et al. 2021). However, the observed sharp discontinuity was not reproduced in previous models. The present study examines the nucleation of cloud droplets in oscillating winds associated with a Kelvin wave using a simplified microphysical model. The study aims to understand the role of the wave in the formation of the lower cloud and the conditions necessary for the appearance of the observed sharp discontinuity.