Planetary-scale variations in winds and UV brightness at the Venusian cloud top: Periodicity and temporal evolution

TL;DR

This study analyzes planetary-scale waves at Venus's cloud top, revealing a 5-day Rossby wave and a possible Kelvin wave, with their evolution linked to UV brightness variations and cloud features, based on data from the Akatsuki orbiter.

Contribution

It provides the first detailed temporal evolution analysis of planetary-scale waves and their impact on Venusian cloud brightness using high-resolution time series data.

Findings

Identified a 5-day Rossby wave with phase velocities slower than super-rotation.

Observed amplification and attenuation cycles of Rossby wave vortices over ~20-50 days.

Detected a 3.8-day Kelvin wave potentially causing equatorial dark clusters.

Abstract

Planetary-scale waves at the Venusian cloud-top cause periodic variations in both winds and ultraviolet (UV) brightness. While the wave candidates are the 4-day Kelvin wave and 5-day Rossby wave with zonal wavenumber 1, their temporal evolutions are poorly understood. Here we conducted a time series analysis of the 365-nm brightness and cloud-tracking wind variations, obtained by the UV Imager onboard the Japanese Venus Climate Orbiter Akatsuki from June to October 2017, revealing a dramatic evolution of planetary-scale waves and corresponding changes in planetary-scale UV features. We identified a prominent 5-day periodicity in both the winds and brightness variations, whose phase velocities were slower than the dayside mean zonal winds (or the super-rotation) by >35 m s$^{-1}$. The reconstructed planetary-scale vortices were nearly equatorially symmetric and centered at ~35{\deg} latitude in both hemispheres, which indicated that they were part of a Rossby wave. The amplitude of winds variation associated with the observed Rossby wave packet were amplified gradually over ~20 days and attenuated over ~50 days. Following the formation of the Rossby wave vortices, brightness variation emerges to form rippling white cloud belts in the 45{\deg}-60{\deg} latitudes of both hemispheres. ~3.8-day periodic signals were observed in the zonal wind and brightness variations in the equatorial region before the Rossby wave amplification. Although the amplitude and significance of the 3.8-day mode were relatively low in the observation season, this feature is consistent with a Kelvin wave, which may be the cause of the dark clusters in the equatorial region.