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

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. The reconstructed planetary‐scale vortices were nearly equatorially symmetric and centered at ~35° 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 ~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 45–60° latitudes of both hemispheres. An ~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.

Vertical Coupling Between the Cloud‐Level Atmosphere and the Thermosphere of Venus Inferred From the Simultaneous Observations by Hisaki and Akatsuki

“…This component is, however, almost absent in the periodogram of the brightness (Figure ). The discrepancy between the periodicities in the wind velocity and the brightness was also reported by Nara et al () and Imai et al () in the same data set. On the other periods, the periodicity associated with the Rossby wave was detected in both the velocity field and the brightness (Imai et al, ; Rossow et al, ).…”

Brightness modulations of our nearest terrestrial planet Venus reveal atmospheric super-rotation rather than surface features

“…We confirm the temporal variation in the signal strength from disk-integrated photometry for each of these periods, associated with Kelvin and Rossby planetary-scale waves, a finding consistent with what has been reported for disk-resolved photometry in previous studies [29][30][31][32] . For example, Imai et al 31 (their Fig. 10) reported a transition from P 1 to P 2 at 365 nm from July to September 2017.…”

“…This is, however, the first instance that both periods are clearly identified in the disk-integrated brightness of Venus and at multiple wavelengths. Based on previous investigations at 365 nm [29][30][31][32] , the P 1 and P 2 periods are associated with an equatorial Kelvin wave and a mid-latitude Rossby wave moving in the direction of the mean zonal flow at phase speeds somewhat faster and slower than the zonal winds, respectively.…”

Spatial and Temporal Variability of the 365‐nm Albedo of Venus Observed by the Camera on Board Venus Express

“…At the cloud top level, there are strong zonal winds, which take 4-5 days to rotate the planet. This caused a 4-to 5-day period in albedo found previously (Rossow et al, 1980;Lee et al, 2015;Imai et al, 2019). However, we do not find this peak in Figure 9.…”