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Planetary-scale 4-day Kelvin-type waves at the cloud top of the Venus atmosphere have been reported from the 1980s, and their significance for atmospheric dynamics has been pointed out. However, these waves have not been reproduced in Venus atmospheric general circulation models (VGCMs). Recently, horizontal winds associated with the planetary-scale waves at the cloud top have been obtained from cloud images taken by cameras onboard Venus orbiters, which could enable us to clarify the structure and roles of Kelvin-type waves. In order to examine this possibility, our team carried out an idealized observing system simulation experiment (OSSE) with a data assimilation system which we developed. The wind velocity data provided by a CCSR/NIES (Center for Climate System Research/National Institute for Environmental Studies) VGCM where equatorial Kelvin-type waves were assumed below the cloud bottom was used as idealized observations. Results show that 4-day planetary-scale Kelvin-type waves are successfully reproduced if the wind velocity between 15&deg, S and 15&deg, N latitudes is assimilated every 6 h at 70 km altitude. It is strongly suggested that the Kelvin-type waves could be reproduced and investigated by the data assimilation with the horizontal wind data derived from Akatsuki ultraviolet images. The present results also contribute to planning future missions for understanding planetary atmospheres.

Abstract Structure of the planetary scale wave, which has been studied for over decades since its discovery in 1980s, is yet to be shrouded in mystery. To clarify this, images by cameras would definitely be necessary. Our team has been assimilating data with AFES LETKF Data Assimilation System for Venus (ALEDAS-V): the first data assimilation system for the Venusian atmosphere, as a pre-experiment before executing the mission. 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. Introduction 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 [1,2]. 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 [3]. However, the equatorial Kelvin wave has not been simulated in any Venusian atmospheric General Circulation Model (VGCM) in the world. We have so far developed AFES-Venus; Atmospheric GCM for the Earth Simulator for Venus, and ALEDAS-V; AFES LETKF Data Assimilation System for Venus, using LETKF; Local Ensemble Transform Kalman Filter, for the first time in the world [4,5]. In this study, we made an observation data regarding the speed of wind, by varying conditions such as altitude, latitudinal range, and frequency of observations, assuming observations with various wavelength cameras. We created the idealized observation data from CCSR/NIES Venus AGCM [6], in which the equatorial Kelvin wave is reproduced at ~70 km by 5.5-day wave forcing from the lowest level (~30 km). Experimental setting AFES-Venus solves dry 3-D Primitive equation on sphere. The physical parameters are based on Venus. The latitude-longitude grids (128 times 64) with 60 vertical layers are used. Solar heating is based on Tomasko (1980) and Crisp (1989), and radiative process is simplified by Newtonian cooling. The simulation starts from zonal wind assuming super rotation and spin up for 4 Earth years. ALEDAS-V uses the LETKF [7] to produce an improved estimate (called analysis) by combining observations and short time ensemble forecasts of AFES-Venus. The number of ensemble members is 31. Assimilation cycle is 6 hourly interval. Observation period and error are set to 1 month and 3 m/s, respectively. We first assimilated the wave observation at several altitudes and different frequency, however, the 4-day wave was not perfectly reproduced. Therefore, we conducted sensitivity experiments of observational domain at an altitude of 70 km, where the wave was mostly reproduced. In this experiment, observation with ultraviolet wavelength camera on Akatsuki for example, is assumed to be used for observations. Results We have been conducting various cases of experiments, though, we will show the results partially here. Figure 1: Longitude-time cross sections of wind at the equator at 70 km (deviations from time averages) (a): free-run-forecast, where data is not assimilated (b): horizontal winds data assimilated in latitudes of S15°- N15°, every 6 hours at 70 km Figure.1 shows Longitude-time cross section of deviation of zonal wind from its time average at the equator at altitude of 70 km. Black arrows refers 4-day waves, and it is approximately 4-days from bottom-left to upper-right of the arrow. In spite of the vague expression in (a), 4-day wave is apparent in (b). The 4-day wave was reproduced by data assimilation. Figure 2: Time variations of temperature (color) and horizontal winds (vector) at 70 km. Horizontal winds data is assimilated in latitudes of S15°- N15°, every 6 hours at 70 km. Figure. 2 is time variations of temperature and horizontal winds at an altitude of 70 km. In this figure, high-pass filter with 10-day running mean subtracted is drawn. The black lines in the images characterize borders between contrasting vectors. The locations of two lines in Day 1 and Day 5 are almost the same, respectively. Therefore, it is explicit that 4-day wave is successfully reproduced. Our team also conducted several other experiments and analyzed their results. By drawing latitude-longitude cross section of averaged zonal wind and temperature, we found that the velocity of wind at 65 km – 70 km at S45°- N45° is contrasting by assimilating different latitudenal range data. In another experiment, where we drew time-altitude cross section of wind at the equator, we confirmed that 4-day wave was apparent only when assimilating data. Finally, we investigated latitudinal dependency and observation frequential dependency with root-mean-square-deviation. From these, we learned that there is a strong jet in mid-latitude and also reinforced our hypothesis of the best observation latitude. Summary and Conclusions We sought the reproduction of 4-day wave in Venus atmosphere by assimilating data of an idealized observation with ALEDAS-V. And we discovered that under the following conditions, we can expect to reproduce 4-day wave successfully, using ultraviolet wavelength cameras. First, the altitude of assimilating data would be better to set at 70 km. Also, the most potential frequency for data assimilation would be once in 6 hours, with latitude between S15°- N15°. Our research has an important roll as pre-investigation before executing an actual mission, thus we will carry on a further research to contribute solving the enigma of Venus. Acknowledgements This study was conducted under the joint research project of the Earth Simulator Center and supported by the Japan Science and Technology Agency. References [1] A.D. Del Genio and W.B. Rossow, Icarus, 1982 [2] A.D. Del Genio and W.B. Rossow, J. Atmos. Sci., 1990 [3] M. Yamamoto et al., J. Atmos. Sci., 1997 [4] H. Ando et al., Nature Communications, 2016 [5] N. Sugimoto et al., Scientific Reports, 2017 [6] M. Yamamoto et al., Icarus, 2012 [7] T. Miyoshi and S. Yamane, Mon. Wea. Rev., 2007

Feasibility of radio occultation measurement (RO) among small satellites is investigated by observing system simulation experiment (OSSE) of the Venus data assimilation system in two test cases introducing real orbits. One observes cold collar in polar region. The other observes thermal tides in equatorial region. The reproducibility of the cold collar or thermal tides is investigated by several types of orbits. Results suggests that the cold collar is successfully reproduced with two or three satellites. In addition, zonal wind can be improved by temperature assimilation of the thermal tides with two satellites. Therefore, RO among small satellites would be promising to reproduce global phenomena in the Venus atmosphere. 1. Introduction Venus atmosphere has a global, thick cloud cover at about 48–70 km altitude. Therefore, it is very difficult to observe under the cloud by the images. One of the most useful methods to obtain vertical temperature profiles is RO. For Earth, daily observations have been performed by GNSS (Global Navigation Satellite System). However, Venus has been observed only between one satellite, e.g., Venus Express or Akatsuki [1], and Earth. For multiple satellites, by transmitting and receiving radio waves between them, more observations, multiple points and better time resolution, could be obtained. In the previous study, we have shown that RO among small satellites can reproduce the cold collar with fixed observational points [2]. In order to check feasibility, however, we have to consider realistic orbits of flying multiple satellites around Venus. So far, we have developed Venus atmospheric general circulation model (AGCM) named AFES-Venus (AGCM for the Earth Simulator for Venus) [3]. Recently, using the local ensemble transform Kalman filter (LETKF) [4], we have succeeded in developing the AFES LETKF data assimilation system for Venus (ALEDAS-V) [5]. In this study, we performed OSSE assuming RO among small satellites in two test cases with real orbits targeting the cold collar and thermal tides. 2. Experimental setting AFES-Venus solves dry 3-D Primitive equation on sphere. The physical parameters are based on Venus. The latitude-longitude grids (128 times 64) with 60 vertical layers are used. The simulation starts from idealized super rotation and spin up for 4 Earth years. Reproduced zonal wind is in good agreement with observations [6]. ALEDAS-V uses the LETKF [4] to produce an improved estimate (called analysis) by combining observations and short time ensemble forecasts of AFES-Venus. The number of ensemble members is 31. Assimilation cycle is 6 hourly interval. Observational errors and inflation are set to 3.0 K and 10%, respectively. Details are described in [2, 5]. Two experiments are conducted; one is evaluated by the cold collar (experiment 1), and the other is evaluated by the thermal tides (experiment 2). Idealized temperature observations at 40-90 km are prepared by the GCMs assuming RO among small satellites. Experiment 1 uses data from IPSL VGCM because the cold collar is more realistically reproduced [7]. Note that temperature bias between IPSL VGCM and AFES-Venus is corrected. Experiment 2 uses data from AFES-Venus itself but experimental settings are different. Observation points are based on real orbits; Experiment 1 covers polar region and experiment 2 does equatorial region. They are assimilated to AFES-Venus by ALEDAS-V and results are compared with frf (free run forecast). The reproducibility of the cold collar or thermal tides is investigated by several orbits. 3. Results Figure 1 shows the temperature distributions at 30-90°N at ~66 km in experiment 1. Four situations are considered: (a) frf and (b,c,d) has different orbits. p1, p2 and p3 indicate each pair of satellites. Cold collar is most clearly reproduced for (d) in which all 3 satellites proceed RO. Figure 1: Temperature distributions at ~66 km (at 30-90°N seen from the north pole): (a) frf, (b) 1 pair of satellites (p1), (c) 2 pairs (p1 and p3), and (d) 3 pairs (p1, p2 and p3). Figure 2 shows latitude-height cross sections of zonal wind (contour) and difference of zonal wind between frf and the Qt8 cases (color) in which the solar heating is reduced to 80% compared with that used in frf. The 4 types of orbits are considered. The zonal wind above the cloud top (~80 km) is significantly decelerated in (d). It is suggested that temperature observations of the thermal tides can improve zonal wind. Figure 2: Latitude-height cross sections of zonal wind (contour) and difference of zonal wind between frf and the Qt8 cases (color). 4 types of orbits are shown: a2 (a), b1 (b), b2 (c), and c2 (d). c2 has most observations, following b2, b1 and a2. Summary and Conclusions In the present study, we have checked feasibility of the Venus future mission of RO among small satellites by introducing real orbits. In experiment 1, the cold collar is reproduced with 2 or 3 satellites observing polar region. In experiment 2, zonal wind is improved by the temperature observations of the thermal tides in equatorial region. Results suggest that RO with two or three small satellites would be promising to reproduce global phenomena. Furthermore, it is also suggested that this kind of observing system simulation experiment would be very useful to design future missions. Acknowledgements This study and supported by the Japan Science and Technology Agency (JST). References [1] Ando, H. et al.: Thermal structure of Venus atmosphere from sub-cloud region to the mesosphere observed by radio occultation, Scientific Rep., 2020. [2] Sugimoto, N. et al.: Observing system simulation experiment for radio occultation measurements of the Venus atmosphere among small satellites, Journal of JSCE A2: Applied Mechanics, 2019. [3] Sugimoto, N. et al.: Baroclinic instability in the Venus atmosphere simulated by GCM, J. Geophys. Res., Planets, 2014. [4] Miyoshi, T. and S. Yamane: Local ensemble transform Kalman filtering with an AGCM at a T159/L48 resolution. Mon. Wea. Rev. 135, 2007. [5] Sugimoto, N. et al.: Development of an ensemble Kalman filter data assimilation system for the Venusian atmosphere, Scientific Rep., 2017. [6] Sugimoto, N. et al.: Waves in a Venus general circulation model, Geophys. Res. Lett., 2014. [7] Garate-Lopez, I and S. Lebonnois: Latitudinal variation of clouds’ structure responsible for Venus’ cold collar, Icarus, 2018.