15:45 〜 16:00
[PPS04-02] Horizontal structure of Kelvin wave at the Venus cloud top observed by Akatsuki
キーワード：金星、あかつき、惑星波
The mystery of the fast-rotating atmosphere on Venus is well known as the name of the super-rotation. At the cloud top altitude of ~70 km, the super-rotation reaches 100 ms-1, and many types of atmospheric waves are found as periodical signals in winds and temperatures. Planetary-scale waves have been one of the important clues to maintain the super-rotation by transporting angular momentum and heat. Recent works by Takagi et al. [2022; 2023] using a general circulation model (GCM) discovered that Kelvin and Rossby waves exhibit alternating phases of growth and decline, and these temporal changes are associated with the quasi-periodic fluctuations in mean zonal winds. Understanding these planetary-scale waves is essential for revealing the nature of super-rotation and the general atmospheric circulation on Venus. Continuous observations by the Ultraviolet Imager (UVI) onboard the Venus Climate Orbiter Akatsuki enable us to carry out cloud-tracking wind measurements. Transient waves with periods around 4-day and 5-day waves propagating faster and slower than the mean zonal winds were observed in previous explorations (e.g. Pioneer Venus and Venus Express), and they are regarded as Kelvin and Rossby waves, respectively. A prominent 5-day Rossby wave accompanied by the equatorially symmetric planetary-scale vortices with zonal wavenumber 1 was observed by UVI [Imai et al., 2019], and the associated horizontal temperature fluctuation was also captured from the analysis of the Longwave Infrared Camera (LIR) images [Imai et al. in prep].
In this study, we analyzed cloud-tracked winds derived from UVI and temperatures from LIR data from October to November 2018, Akatsuki’s continuous observation period. In this observation period, data were acquired continuously at 2-hour intervals without missing data as much as possible. We found the horizontal wind and temperature fluctuations having ~4-day periodicity, which is possibly due to the Kelvin wave. Zonal wind amplitudes are maximal in the equatorial region and the wave features were trapped at low latitudes within the critical latitudes at ~30oN/S, which are consistent with the equatorial Kelvin mode based on a linearized wave theory. Furthermore, winds with a meridional component extending in the direction of wave travel from the equator to mid-latitudes were also found. While these meridional winds are not consistent with the theoretical Kelvin mode, they are similar to recent GCM predictions by Takagi et al. [2022]. The phases of the winds and temperature fluctuations were also consistent with the GCM. In this presentation, we will report on the first case of the detailed analysis of the Kelvin wave by Akatsuki data, and its characteristics and temporal evolution will be discussed.
In this study, we analyzed cloud-tracked winds derived from UVI and temperatures from LIR data from October to November 2018, Akatsuki’s continuous observation period. In this observation period, data were acquired continuously at 2-hour intervals without missing data as much as possible. We found the horizontal wind and temperature fluctuations having ~4-day periodicity, which is possibly due to the Kelvin wave. Zonal wind amplitudes are maximal in the equatorial region and the wave features were trapped at low latitudes within the critical latitudes at ~30oN/S, which are consistent with the equatorial Kelvin mode based on a linearized wave theory. Furthermore, winds with a meridional component extending in the direction of wave travel from the equator to mid-latitudes were also found. While these meridional winds are not consistent with the theoretical Kelvin mode, they are similar to recent GCM predictions by Takagi et al. [2022]. The phases of the winds and temperature fluctuations were also consistent with the GCM. In this presentation, we will report on the first case of the detailed analysis of the Kelvin wave by Akatsuki data, and its characteristics and temporal evolution will be discussed.