論文

Impacts of temperature assimilation on the Venus atmosphere are investigated by the observing system simulation experiments assuming cross-link radio occultation measurements of the Venus atmosphere (CROVA). Synthetic temperature data are prepared by a Venusian general circulation model (VGCM) with 80% of the solar heating to produce a weaker super-rotation than the default model would. Observational points of radio occultation measurements are realistically determined by a pair of satellites with feasible orbits. Then the vertical temperature profiles in 40–90 km altitudes at those points are assimilated into the original VGCM where the faster super-rotation is reproduced. The results show that the three-dimensional structure of the true atmosphere could be significantly improved not only in temperature but also in horizontal winds, when temperature observations are available with more than 40 points per day using 3 small cube satellites for 30 days. It would be promising to plan CROVA mission for future.

The thermal structure of the upper atmosphere in the Venus polar region is reproduced by the assimilation of horizontal winds in low- and mid-latitudes derived from the Akatsuki ultra-violet images. The obtained structure is well consistent with infrared and radio occultation measurements. In particular, the unique thermal features in the Venus polar vortex such as the cold collar and the warm polar region in our model are realistic: the level where the cold collar is located and the temperature difference between the cold collar and the warm polar region are consistent with data. The reasons why the thermal structure of the upper atmosphere in the Venus polar region is realistic are that the zonal wind distribution around the cloud top level and the structure of the residual mean meridional circulation induced by the thermal tides are improved with respect to the model without data assimilation. Our results also suggest that the thermal structure in the Venus upper polar region is closely related to the atmospheric dynamics.

Long-Infrared camera (LIR) onboard Akatsuki have detected bow-shaped features over mountain regions, suggesting that mountain waves propagate upward to reach the cloud top level. In this study, we investigated how the vertical propagation characteristics of the mountain waves depend on the thickness and static stability of the planetary boundary layer and the zonal wind near the surface by using a two-dimensional non-hydrostatic model. The obtained results were compared with the observations mainly at the cloud top level. The amplitude of waves reproduced in the model is comparable to the observations at the cloud top level when the zonal wind near the surface is relatively fast and the top of the planetary boundary layer is lower than the top of the mountain. The wave amplitude also becomes comparable to the observations when the top of the planetary boundary layer is higher than the top of the mountain, while it depends also on the zonal wind speed near the surface and the static stability in the planetary boundary layer. Our results suggest that the zonal wind speed and the atmospheric structure near the surface vary with local time and might be able to explain why the bow-shaped structure is often seen in the evening time.

We conducted observing system simulation experiments (OSSEs) for radio occultation measurements (RO) among small satellites, which are expected to be useful for future Venus missions. The effectiveness of the observations based on realistic orbit calculations was evaluated by reproduction of the “cold collar”, a unique thermal structure in the polar atmosphere of Venus. Pseudo-temperature observations for the OSSEs were provided from the Venus atmospheric GCM in which the cold collar was reproduced by the thermal forcing. The vertical temperature distributions between 40 and 90 km altitudes at observation points were assimilated. The result showed that the cold collar was most clearly reproduced in the case where the temperature field in high-latitudes was observed twice a day, suggesting that the proposed observation is quite effective to improve the polar atmospheric structure at least. Although the cold collar was also reproduced in the OSSEs for Longwave Infrared Camera (LIR) observations, the result seemed unrealistic and inefficient compared to that obtained in the RO OSSEs. The present study shows that the OSSEs can be used to evaluate observation plans and instruments in terms of reproducibility of specific atmospheric phenomena, and applied to future missions targeting planetary atmospheres.

地球シミュレ―タ向けに高速化された大気大循環モデルを金星版に改変し、局所変換アンサンブルカルマンフィルターを用いて金星大気初のデータ同化システムを開発した。まず、熱潮汐波なしの計算でシステムが機能することを確認した。次に、欧州の金星探査機「Venus Expres」に搭載された金星紫外カメラから導出された風速のデータを用いて、熱潮汐波入りの計算にデータ同化した。その結果、熱潮汐波の位相が観測と整合的に修正されるのみならず、大気大循環に大きなインパクトを与えることが示された。現在、複数の小型衛星による電波掩蔽観測を想定した観測システムシミュレーション実験を実施している。これらの初期成果は、データ同化システムが将来の探査ミッションの立案や大気大循環モデルの改善、さらに金星探査機「あかつき」による最初の解析データの作成に有効であることを示している。

We have developed the Venus AFES (atmospheric GCM (general circulation model) for the Earth Simulator) LETKF (local ensemble transform Kalman filter) data assimilation system (VALEDAS) to make full use of observations. In this study, radio occultation measurements among small satellites are evaluated by the observing system simulation experiment (OSSE) of VALEDAS. Idealized observations are prepared by a French Venus Atmospheric GCM in which the cold collar is realistically reproduced. Reproducibility of the cold collar in VALEDAS is tested by several types of observations. The results show that the cold collar is successfully reproduced by assimilating at least 2 or 3 vertical temperature profiles in the polar region every 4 or 6 hours. Therefore, the radio occultation measurements among three satellites in polar orbits would be promising to improve the polar atmospheric structures at about 40–90 km altitudes.

©2019. American Geophysical Union. All Rights Reserved. Fully developed superrotation is reproduced for the first time in very long term simulations with a dynamical Venus general circulation model (GCM) driven by a zonally averaged component of the realistic solar heating only. Starting from a motionless state with temperature distribution including a low static stability layer in the lower cloud layer, the fast superrotating zonal flow of ~100 m/s is established after 500 Earth years. In this experiment, the mean meridional circulation is responsible for generating the superrotation, because effects of thermal tides, topography, radiation process, and cloud physics are excluded in the present simulations. Sensitivity experiments indicate that the vertical eddy viscosity smaller than 0.02 m 2 /s is necessary for the fast superrotation to appear. The low static stability layer also strongly affects the superrotation with high-latitude jets through angular momentum transport by baroclinic/barotropic instability in the cloud layer.

©2019. American Geophysical Union. All Rights Reserved. Impacts of horizontal winds assimilation on thermal tides are investigated by using the Venus atmospheric general circulation model for the Earth Simulator local ensemble transform Kalman filter data assimilation system. The assimilated data are horizontal winds derived from Venus ultraviolet images taken by the Venus Monitoring Camera onboard the Venus Express orbiter. The results show that three-dimensional structures of the thermal tides are significantly improved not only in the horizontal winds but also in the temperature field, even though the observations are available only at the cloud top level on the southern dayside hemisphere approximately once an Earth day. The reproduced temperature fields agree well with recent radio occultation measurements of the Venus Climate Orbiter Akatsuki. The zonal mean fields of the zonal wind and temperature are also improved globally. This study would enable reanalysis of past Venus observations.

© 2019, The Author(s). Cloud patterns are important clues for revealing the atmospheric circulation of Venus. Recently, a planetary-scale streak structure has been discovered in middle- and lower-cloud images of Venus’ night-side taken by IR2, the 2-μm camera, on board the Akatsuki orbiter. However, its formation mechanism has not been investigated. Here we succeed, for the first time, in reproducing the patterns of the observed streak structure, as regions of strong downward flows that develop in high-resolution global simulations of the Venus atmosphere. The streaks are formed in both hemispheres with equatorial symmetry, which is caused by equatorial Rossby-like and Kelvin-like waves with zonal wavenumber one. The low-stability layer that has been suggested by past observations is essential for reproducing the streak structure. The streaks of downward flow result from the interaction of the meridionally tilted phase lines of the Rossby-like waves and the characteristics of baroclinic instability produced around the low-stability layer.

We construct a simple cloud model for a Venus general circulation model (GCM), which includes condensable gases of H2O and H2SO4 vapors, and condensation, evaporation, and sedimentation of sulfuric acid cloud particles. The zonally averaged mass loading of the cloud reproduced in the model takes its maximum and minimum in high and middle latitudes, respectively. This latitudinal distribution is consistent with the infrared measurements. The thick cloud is formed in high latitudes at 43–55 km altitudes by vertical winds associated with disturbances enhanced in the low static stability layer. The moderately thick cloud in low latitudes is attributed mainly to the transport of H2SO4 vapor by the mean meridional circulation. The horizontal cloud distribution in low latitudes has zonal wave numbers 1 and 2 structures, which change in time significantly. These characteristics of the low-latitude cloud would be associated with atmospheric waves in the cloud layer. The mixing ratio of H2O vapor increases with latitude in the cloud layer due to the vertical wind disturbances in the low static stability layer in high latitudes. This latitudinal trend is qualitatively consistent with the infrared measurements. The mixing ratio of H2SO4 vapor increases with latitude in the subcloud layer because a large amount of the cloud is evaporated there due to the sedimentation of cloud particles in the thick lower cloud in the polar region. The present results suggest that the Venus cloud distribution in the lower cloud layer is strongly affected by waves and/or disturbances as well as the mean meridional circulation.

© 2020, The Author(s). We present distributions of the zonal-mean temperature and static stability in the Venusian atmosphere obtained from Venus Express and Akatsuki radio occultation profiles penetrating down to an altitude of 40 km. At latitudes equatorward of 75°, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements in that there is a low-stability layer at altitudes of 50–58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. Meanwhile, at latitudes poleward of 75°, a low-stability layer extends down to 42 km, which has been unreported in analyses of previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the Earth’s troposphere, which generally has a deeper low-stability layer at low latitudes than at mid- and high latitudes.

Crosslink radio occultation (RO) using multiple spacecraft is a promising method to observe the planetary atmosphere with higher spatial and temporal resolution than conventional RO measurements between a spacecraft and a ground station. In this paper, an optimal orbit configuration design method for crosslink RO is discussed. Orbit elements of two spacecraft are explored under two evaluation functions: values of observation and fuel costs for orbit injection. Then, generated Pareto optimal solutions are analyzed based on three categories classified by orbit characteristics, and we proposed an orbit design guideline to enhance observational value against fuel cost. The qualitative understandings for crosslink RO orbit design by this paper can contribute to future mission design.

Gravity waves play essential roles in the terrestrial atmosphere because they propagate far from source regions and transport momentum and energy globally. Gravity waves are also observed in the Venus atmosphere, but their characteristics have been poorly understood. Here we demonstrate activities of small-scale gravity waves using a high-resolution Venus general circulation model with less than 20 and 0.25 km in the horizontal and vertical grid intervals, respectively. We find spontaneous gravity wave radiation from nearly balanced flows. In the upper cloud layer (~70 km), the thermal tides in the super-rotation are primary sources of small-scale gravity waves in the low-latitudes. Baroclinic/barotropic waves are also essential sources in the mid- and high-latitudes. The small-scale gravity waves affect the three-dimensional structure of the super-rotation and contribute to material mixing through their breaking processes. They propagate vertically and transport momentum globally, which decelerates the super-rotation in the upper cloud layer (~70 km) and accelerates it above ~80 km.

At the cloud top of the Venus atmosphere, equatorial Kelvin waves have been observed and are considered to play an important role in the super-rotation. We were able to reproduce the wave in a general circulation model (GCM) by conducting an observing system simulation experiment (OSSE) with the help of a data assimilation system. The synthetic horizontal winds of the Kelvin wave produced by the linear wave propagating model are assimilated at the cloud top (~70 km) in realistic conditions, assuming they are obtained from cloud tracking of ultra-violet images (UVI) taken by the Venus orbiters. It is demonstrated using Eliassen–Palm (EP) fluxes that the reproduced Kelvin wave transports angular momentum and plays an important role in the magnitude and structure of the super-rotation, causing the acceleration and deceleration of zonal wind of ~0.1 m/s day−1. The conditions required in order to reproduce the Kelvin wave have also been investigated. It is desirable to have 24 hourly dayside satellite observations in an equatorial orbit, such as the Akatsuki Venus climate orbiter. The results of this type of data assimilation study will be useful in the planning of future observation missions to the atmospheres of planets.

Impacts of temperature assimilation on Venusian thermal tides are investigated by the observing system simulation experiments assuming Akatsuki Longwave Infrared Camera (LIR) observations. Synthetic temperature data are prepared by a Venusian general circulation model (VGCM) to test if LIR temperature data resolves a discrepancy in the structure of thermal tides between observations and the VGCM. They are assimilated at 70 km altitude with several combinations of frequency and horizontal region. The result shows that the three-dimensional structure of thermal tides is significantly improved not only in temperature but also in horizontal wind, even if observations are available only at a limited frequency of 6-hourly or on the dayside. The zonal–mean zonal wind and temperature fields are also modified at 60–80 km altitudes globally through the vertical momentum transport of thermal tides. It would be promising to assimilate Akatsuki LIR observations to produce realistic objective analysis of the Venus atmosphere.