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

Abstract

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 ∼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 ∼5-day waves have nearly constant frequencies, the ∼4-day waves have variable phase speeds that follow the superrotation speed. This result indicates that the ∼5-day waves are likely to extend over a large depth below the cloud top and that the ∼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. © 2024. The Authors.