Long-Term Variability of Mean Winds and Planetary-Scale Waves Around Venusian Cloud Top Observed With Akatsuki/UVI

Since December 2015, Ultraviolet Imager (UVI) onboard Akatsuki has been observing Venus clouds at the wavelengths of 283 and 365 nm. Horizontal winds near the cloud top derived from the UVI images over similar to 7 earth years are analyzed to elucidate spatial and temporal variability of the superrotation and planetary-scale waves. Zonal winds averaged over the analysis period are asymmetric with respect to the equator, being faster in the southern hemisphere. This asymmetry varied temporarily and was occasionally reverted. Comparison of the winds from the two wavelengths suggests that it is uncertain whether the asymmetry is in the wind distribution or in the sensing altitude for winds. Mean zonal winds representing the superrotation exhibited broad low-frequency variability with spectra resembling the red noise spectra. This is indicative of the presence of internal variability rather than responses to periodical external forcing. Planetary-scale waves with zonal-wavenumber 1 at periods around 4 and 5 days, which have been interpreted as equatorial Kelvin and Rossby waves, respectively, are quantified. While the similar to 5-day waves have nearly constant frequencies, the similar to 4-day waves have variable phase speeds that follow the superrotation speed. This result indicates that the similar to 5-day waves are likely to extend over a large depth below the cloud top and that the similar to 4-day waves are likely to be confined near the cloud top. Their possible generation mechanism through the coupling of Kelvin and Rossby waves is discussed. This study further reports wind variability of 10 to 15-day periodicity, thermal-tide structure, and comparison with minor species observations. Akatsuki is the only currently operational artificial satellite of Venus. It has been conducting imaging observations at two ultraviolet wavelengths since December 2015. By measuring the movement of cloud features in the images, we obtain horizontal winds near the cloud-top altitude where the famous superrotation is maximized. By using wind data over similar to 7 earth years, we show that the superrotation speed varies over time, showing a broad spectrum. This is indicative of internally created variability rather than periodic variability created by external forcing. We also reveal that mean westward winds that characterize the superrotation have some differences between the northern and southern hemispheres and that these differences vary over time. However, the data do not allow us to conclude whether the asymmetry is in the winds or in the sensing altitude. Venus has long been known for having two kinds of planetary-scale waves with periods around 4 and 5 days, but their vertical distributions are unknown. We indicate that the 5-day waves have a deeper vertical distribution than the 4-day waves from the differences in the long-term changes in their frequencies. We further characterize the thermal tides excited by solar heating and suggest wind variability at periods of 10-15 days. Venus' superrotation as measured near the cloud top exhibits rich low frequency variations indicating internal variability Planetary-scale similar to 4- and similar to 5-day waves are suggested to reside in different altitude ranges Temporally varying hemispheric asymmetry is found in zonal winds, which might partly be due to the sensing altitude asymmetry