Observing system simulation experiment to reproduce Kelvin wave in the Venus atmosphere

Abstract:

The planetary scale wave, which is considered as 4-day “equatorial Kelvin wave” is existing at the cloud top in the equatorial region on the Venus atmosphere. The equatorial Kelvin wave is pointed out to have a possibility of contributing to generation and maintenance of what is called “Super Rotation”: the wind circulating Venus 60 times faster than the speed of Venusian rotation. However, the equatorial Kelvin wave has not been simulated in any Venusian atmospheric General Circulation Model in the world, thus its structure is yet to be shrouded in mystery. To clarify this, images by cameras would definitely be necessary. We have so far developed VGCM (AFES-Venus), and the Venus AFES LETKF Data Assimilation System (ALEDAS-V) using the Local Ensemble Transform Kalman Filter (LETKF) for the first time in the world. Therefore, we used data assimilation method for our research, as a pre-experiment before executing the mission. In this study, we made an observation data regarding the speed of wind, by varying the conditions such as observation altitude, observation latitude range, and observation frequency, assuming observations with various wavelength cameras. We created the idealized observation data from CCSR/NIES Venus AGCM, in which the equatorial Kelvin wave is reproduced at ~70 km by 5.5-day wave forcing from the lowest level (~30 km). Results show that you can successfully reproduce 4-day planetary scale wave when assimilating data of wind velocity of latitude S15°- N15°every 6 hours at an altitude of 70 km. This discovery will contribute not only to a mission to observe wind velocity of Venus, but also to proposal regarding missions for further understanding of atmospheric structure on other planets, in the future.