[P-PS04] Advancing the science of Venus in the golden age of exploration

[PPS04-P16] Latitudinal and vertical structures of Venusian planetary-scale waves and their temporal variation studied with Akatsuki LIR

Hiroyuki Koyama^1,5, *Takeshi Imamura¹, Makoto Taguchi², Toru Kouyama³, Takao M. Sato⁴ (1.Graduate School of Frontier Sciences, The University of Tokyo, 2.Rikkyo University, 3.National Institute of Advanced Industrial Science and Technology, 4.Hokkaido Information University, 5.School of Science, The University of Tokyo)

Keywords:Venus, atmospheric wave, Akatsuki

The contribution of planetary-scale waves such as Rossby and Kelvin waves to the maintenance of the superrotation of the Venusian atmosphere is under debate. While model calculations predict contribution of such waves, observational constraints on the three-dimensional structures of the waves and the interaction between these waves and the mean flow are not enough. In this study, we obtain wave spectra from the variation of the cloud-top temperature over a period of 10 Venus years using the brightness temperature data obtained by the Longwave Infrared Camera (LIR) onboard Akatsuki. The latitudinal structures of the waves were compared with that of the background wind, and the temporal variations of the wave periods were also compared with that of the background wind. Near the cloud tops, waves with a period of about 4 days, which propagates faster than the background wind, and waves with a period of 5-6 days, which propagates slower than the background wind, are observed; the former waves will be Kelvin waves, while the latter, Rossby waves. Multiple spectral peaks are observed both for Kelvin and Rossby waves. Though the 5- to 6-day period Rossby waves usually have amplitude maxima at mid-latitudes of both hemispheres, they frequently exhibit additional amplitude maxima near the equator especially when the emission angle of the observation is small. Considering that the contribution function of LIR extends to lower altitudes for smaller emission angles, this result suggests the possibility that coupled Rossby-Kelvin modes arise at altitudes below the cloud top. It was also found that the periods of the planetary-scale waves change with the variation of the period of the background wind at the cloud top. This might be explained by the local excitation of the waves near the cloud top or by the selective attenuation of the vertically-propagating waves excited in the lower atmosphere with periods close to that of the background wind.