Vertical-Wind-Induced Cloud Opacity Variation in Low Latitudes Simulated by a Venus GCM

Venusian cloud structure and variation are strongly linked to atmospheric dynamics. Past near-infrared measurements have found cloud variation such as zonal-wavenumber-1 cloud marking and cloud discontinuity. However, their formation mechanism is still not well understood. To investigate the Venusian cloud structure and its variation, we have developed a Venus GCM incorporating cloud condensation, evaporation, sedimentation, and simple atmospheric chemistry to represent the H₂SO₄ cycle. The GCM takes into account cloud particles with radii of 0.3, 1.0, 1.26, and 3.13 μm (Modes 1, 2, 2', and 3, respectively) based on past in situ observations. The simulated latitudinal trends of the cloud top and bottom structures are qualitatively consistent with past observations. Zonally averaged cloud mass loading was the largest and smallest in low and middle latitudes, respectively, and maintained by a mechanism similar to that of past 2-D numerical studies. At the equator, the column integrated optical depth at 1 μm varied between 33 and 50, which is in good agreement with past observations. This variation consists of two types of cloud mass loading changes between 46 and 52 km. One is a rapid small-scale variation induced by gravity waves. The other is a quasi-periodic zonal-wavenumber-1 variation coupled with an equatorial Kelvin wave, which is similar to the observed cloud marking. Our results showed that the vertical wind associated with the Kelvin wave is essential for maintaining the quasi-periodic variation, along with the condensation/evaporation by the temperature variation. The vertical-wind-induced cloud generation also suggests a relationship to the cloud discontinuity.