Planetary-Scale Variations in Winds and UV Brightness at the Venusian Cloud Top: Periodicity and Temporal Evolution

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. The reconstructed planetary-scale vortices were nearly equatorially symmetric and centered at similar to 35 degrees latitude in both hemispheres, which indicated that they were part of a Rossby wave. The amplitude of wind variation associated with the observed Rossby wave packet was amplified gradually over similar to 20 days and attenuated over similar to 50 days. Following the formation of the Rossby wave vortices, brightness variation emerges to form rippling white cloud belts in the 45-60 degrees latitudes of both hemispheres. An similar to 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. Plain Language Summary The Earth's twin planet Venus is mysterious for the fast atmosphere circulation called as the super-rotation. The cloud top atmosphere rotates around the planet with 100 m/s, which corresponds to similar to 4-day circulation. There are many types of atmospheric waves, which are crucial for understanding how the wind blows on the planet. In this study, we analyzed one thousand Venus ultraviolet (UV) images taken by Japanese Venus Climate Orbiter Akatsuki and first captured the continuous temporal evolution of a planetary-scale wave with a period of 5 days. The 5-day wave had large equatorially symmetric vortices at the cloud top. According to the wave evolution, the shape of the dark UV cloud (absorption) features dramatically changed over the planet. Since the UV absorption is important for the radiative energy balance, we open the door for the future investigations of the long-term impacts of planetary-scale waves to the atmospheric dynamics and chemistry