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

At the cloud top of the Venus atmosphere, mid-latitude Rossby waves have been observed. They are considered to play an important role in the maintenance of the super-rotation. Here, we reproduce those waves in a general circulation model named AFES-Venus (Atmospheric GCM for the Earth Simulator for Venus [1]) by conducting an observing system simulation experiment with the help of a data assimilation technique (ALEDAS-V: AFES–LETKF Data Assimilation System for Venus [2]). The synthetic observations of horizontal winds associated with the Rossby wave are produced by a linear wave propagation model, and they are assimilated at the cloud top (~70 km) for 30 Earth days in realistic conditions, assuming they are derived from the cloud tracking of the ultra-violet images taken by the Venus climate orbiter “Akatsuki”. It is demonstrated using Eliassen–Palm fluxes that enhanced Rossby wave in the mid-latitudes induces a deceleration in zonal-mean zonal wind. After the stop of data assimilation, the model fields reached another quasi-equilibrium state within about 10 Earth days, indicating that the “memory” of assimilated observations can be kept for a while even new observations are not available. [1] Sugimoto, N., M. Takagi, and Y. Matsuda (2014a), J. Geophys. Res. Planets, 119, 1950–1968.[2] Sugimoto, N., A. Yamazaki, T. Kouyama, H. Kashimura, T. Enomoto, and M. Takagi (2017), Sci. Rep., 7(1), 9321.