Feasibility Study of Radio Occultation Measurement Among Small Satellites by Data Assimilation for Designing the Future Venus Atmosphere Observing Mission

We have succeeded in reproducing the super rotation, baroclinic instability waves and a planetary-scale streak structure consistent with the recent Akatsuki observations, using a AGCM for Venus (AFES-Venus), which is based on AFES (Atmospheric GCM For the Earth Simulator). Recently, introducing the Local Ensemble Transform Kalman Filter (LETKF), we have developed the Venus AFES LETKF Data Assimilation System (ALEDAS-V). In this study, we conduct feasibility studies for designing the radio occultation measurement mission among small satellites of Venus using this data assimilation system. In the Venus atmosphere, there is a unique phenomenon called "cold collar", where the temperature at 60-80$^\circ$ latitudes is lower than that of polar region at the altitude of about 65 km. The idealized observations are provided from French Venus atmospheric GCM (IPSL VGCM), in which the cold collar is realistically reproduced, and the observation places are located near the pole, and their conditions such as time interval, places and numbers are changed. We investigate the effectiveness of radio occultation measurement among small satellites by the reproducibility of the cold collar. As a result, the cold collar is reproduced if the observations are 2-3 vertical profiles 4-6 hourly, and it is expected that radio occultation measurement among small satellites will be promising to reproduce the polar atmospheric structures by 3 small satellites. In addition, it is also important to improve not only the polar region but the global temperature distributions in the GCM by the data assimilation of real observations for reproduction and understanding of the general circulation including "super rotation". In this study, we prepare idealized temperature profiles from an AFES-Venus nature run considering radio occultation measurement on realistically simulated orbits of three satellites, and assimilate them to AFES-Venus artificial runs forced by different profiles of the basic temperature and stability. We will report how the global temperature structure reproduced in the model depends on number and/or frequency of the observations.