論文

An objective analysis dataset of the Venus atmosphere, ALERA-V version 1.0, has been released. This objective analysis provides the best estimate of the state of the Venus atmosphere produced by combining observations and a forecast model weighted according to their respective levels of uncertainty. The accumulation of frequent observations by the Venus Climate Orbiter ‘Akatsuki’ has, for the first time, enabled an objective analysis of the Venus atmosphere. ALERA-V stands for AFES-LETKF experimental ensemble objective (re)analysis of the Venus atmosphere, and is generated by ALEDAS-V, the AFES-LETKF data assimilation system for the Venus atmosphere. ALEDAS-V uses AFES-Venus (Atmospheric General Circulation Model for the Earth Simulator for Venus) as a forecast model and the LETKF (Local Ensemble Transform Kalman Filter) for data assimilation. The observations of the zonal and meridional winds obtained by a cloud tracking technique from images taken by Akatsuki's UVI (Ultraviolet Imager) are assimilated to produce ALERA-V version 1.0. The dataset consists of atmospheric variables defined on a grid of 128 longitude and 64 latitude points, and 60 vertical levels, with output every 6 Earth hours from September to December 2018, including the intensive observation period of November. ALERA-V is expected to be useful for both scientific and engineering research, such as understanding the dynamical mechanisms of various atmospheric phenomena and planning for future satellite missions, with appropriate consideration of its quality and limitations.

We performed an intercomparison of three eddy-resolving ocean reanalyses—FORA-WNP30, GLORYS12V1, and BRAN2020—for the western North Pacific over the period 1993–2016. While all three products exhibit high consistency in large-scale surface mean fields due to data assimilation, notable discrepancies are found in mesoscale variability and subsurface structures, reflecting differences in model configuration and assimilation schemes. Furthermore, a vertical section analysis along 137°E reveals discernible discrepancies in the representation of subsurface water masses among the products. FORA-WNP30, employing a four-dimensional variational (4D-VAR) scheme, exhibits high spatiotemporal variability in sea surface height (SSH). However, its sea surface salinity (SSS) variability is strongly dampened by a short restoring timescale. GLORYS12V1 retains intense coastal SSS variability due to the absence of strong surface restoring. However, it exhibits lower eddy kinetic energy levels compared to the others, likely due to dissipative viscosity parameterization. BRAN2020 shows systematic differences east of Japan, including reduced SSH variability and lower temporal correlations. These are attributed to its simplified bathymetry (capped at 5000 m), which eliminates topographic steering effects and alters eddy propagation speeds. Additionally, BRAN2020 exhibits a weaker vertical salinity contrast around the North Pacific Intermediate Water (NPIW), suggesting the influence of excessive vertical mixing. Our results highlight that horizontal resolution is not the sole determinant of performance; bathymetry, subgrid-scale parameterizations, and boundary conditions play equally critical roles in reproducing regional ocean dynamics.

Atmospheric gravity waves spontaneously radiated from nearly balanced flows are investigated by the general circulation model of the Venus atmosphere. To resolve small-scale gravity waves, the ultra-high resolution runs with 20- and 0.25-km horizontal and vertical grid intervals are conducted. As the super-rotation evolves over time, gravity waves are spontaneously radiated from thermal tides and baroclinic/barotropic waves in the low-latitudes and the mid- to high-latitudes, respectively. Here, we focus on the generation from baroclinic/barotropic waves by the simulations without diurnal component of the solar heating (c.f. thermal tides are excluded). Then the sensitivities of the static stability are tested. For the cases with larger static stability in the cloud layer, gravity wave radiation is weakened, indicating that the intensity of baroclinic instability is directly related to the amount of spontaneous gravity wave radiation. The small- to medium-scale gravity waves affect the three-dimensional structure of the super-rotation. Especially, we confirmed for the first time that gravity waves can transport zonal momentum vertically to the lower atmosphere (∼50–60 km) in mid- to high-latitudes. The momentum transport caused by spontaneous gravity wave radiation should be considered in the Venus atmosphere.

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◦ S and 15◦ 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.

A quasi-periodic variation of the lower cloud amount induced by atmospheric waves was reproduced in our Venus general circulation model for the first time. This result agrees well with previous 2.3-μm infrared nightside measurements. The cloud amount variation is mainly caused by a temperature fluctuation with a period of ∼5.5 Earth days near the cloud base, which is associated with a Kelvin-like gravity wave with a zonal wavenumber-1 structure. Our result might be able to explain the mechanism of the periodical variation of the brightness in the Venusian low-latitudes shown by the previous 2.3-μm infrared nightside measurements. It is also suggested that the temporal variation of the mass loading due to the temperature fluctuation is related to the abrupt appearance and disappearance of the large cloud particle (Mode 3) and explains the sharp discontinuity of the lower cloud found by the recent Akatsuki and ground-based infrared nightside measurements.

金星探査機「あかつき」の観測データを最大限に活用するために，我々の研究グループでは金星大気の全球的な流れを地球シミュレータ上で計算する数値モデル「AFES-Venus」と，局所アンサンブルカルマンフィルタを用いて観測データをAFES-Venusへと同化するデータ同化システム「ALEDAS-V」を開発してきました．前回からの進展として，AFES-Venusでは，超長時間積分による静止状態からのスーパーローテーションの再現，超高解像度計算による熱潮汐波からの大気重力波の自発的な放射の発見，雲物理過程の導入，低安定度層などの温度構造の再現，加熱や安定度の改良によるケルビン波や熱潮汐波の現実的な再現，などに成功しました．また，ALEDAS-Vでは，「あかつき」やVenus Expressの紫外画像で得られた水平風を同化し，金星大気初の現実的かつ時空間的に偏りのないデータセットの公開への準備が進めているほか，仮想的な観測データを同化することにより観測の「有効性」を検証するための観測システムシミュレーション実験なども実施しています．これらの成果を簡単に紹介し，今後の展望についての報告をしたいと思います．

Venusian cloud structure and variation are strongly linked to atmospheric dynamics. Past near-infrared measurements have found cloud variation such as zonal-wavenumber-1 cloud marking and cloud discontinuity. However, their formation mechanism is still not well understood. To investigate the Venusian cloud structure and its variation, we have developed a Venus GCM incorporating cloud condensation, evaporation, sedimentation, and simple atmospheric chemistry to represent the H2SO4 cycle. The GCM takes into account cloud particles with radii of 0.3, 1.0, 1.26, and 3.13 μm (Modes 1, 2, 2', and 3, respectively) based on past in situ observations. The simulated latitudinal trends of the cloud top and bottom structures are qualitatively consistent with past observations. Zonally averaged cloud mass loading was the largest and smallest in low and middle latitudes, respectively, and maintained by a mechanism similar to that of past 2-D numerical studies. At the equator, the column integrated optical depth at 1 μm varied between 33 and 50, which is in good agreement with past observations. This variation consists of two types of cloud mass loading changes between 46 and 52 km. One is a rapid small-scale variation induced by gravity waves. The other is a quasi-periodic zonal-wavenumber-1 variation coupled with an equatorial Kelvin wave, which is similar to the observed cloud marking. Our results showed that the vertical wind associated with the Kelvin wave is essential for maintaining the quasi-periodic variation, along with the condensation/evaporation by the temperature variation. The vertical-wind-induced cloud generation also suggests a relationship to the cloud discontinuity.

The equatorial jet in the Venusian lower cloud layer (47–55 km altitudes) and its quasi-periodic variation are found in a general circulation model (GCM). The equatorial jet is produced by the 5.8-day wave and destroyed by the 7-day wave, and its quasi-periodic variation with a timescale of about 280 days is caused by the alternating development of these waves in the GCM. The 5.8-day wave, which is excited by the Rossby-Kelvin instability, produces the equatorward angular momentum (AM) flux, and accelerates the zonal-mean zonal wind in the equatorial region. The 7-day wave, newly found in the present study, is a planetary-scale wave antisymmetric about the equator, although it has not yet been observed. It is excited by the coupling among the lower-altitude equatorial Rossby mode, the mid-latitude Rossby mode, and the high-latitude Rossby mode. In the growth period, the 7-day wave produces the poleward (equatorward) AM flux around the equatorward (poleward) critical latitude. As a result, the zonal-mean zonal wind is decelerated (accelerated) in the equatorial region and high latitudes (mid-latitudes). In the regrowth period, the lower-altitude equatorial Rossby mode disappears due to the disappearance of the equatorial jet, but the high-latitude Rossby mode is still enhanced by the coupling with the mid-latitude Rossby mode. These results could provide a possible explanation of the equatorial jet inferred from the recent Akatsuki observation, although it should be investigated by further observations.

The Lorenz energy cycle was often used to analyze energy conversions related to instabilities in planetary atmospheres using zonal means as basic states. Alternatively, the bred vector (BV) energy equations use the control run as basic states and detect longitudinal dependency of the energy conversions. Additionally, it quantifies contributions from baroclinic and barotropic instabilities separately. We apply this method to understand energy conversions of the Venus atmosphere. BVs are obtained from breeding cycles emphasizing perturbation growths in the cloud layer. The BV potential energy in the pressure coordinate is newly derived. Energy conversions at different latitudes in the cloud layer are examined. Results show that baroclinic conversions are stronger at higher latitudes and exceed barotropic conversions at mid- to high-latitudes. Thermal tides increase energy conversions in the morning hemisphere at mid-latitudes. This study offers new insights into energy conversions of the Venus atmosphere, with potential applications to other planetary atmospheres.

The vertical structure of the thermal tide in the Venusian low-latitude region (0 (Formula presented.) –30 (Formula presented.) latitudes) was investigated by the Akatsuki radio occultation measurements. The result shows that the phase of the diurnal tide little varies in the vertical direction, while that of the semidiurnal tide tilts toward earlier local times with increasing altitude above 65 km and tilts in the same direction with decreasing altitude below 50 km. This indicates that the semidiurnal tide is excited between 50 and 65 km altitudes and propagates upward above these heights and downward below. The vertical momentum flux associated with the semidiurnal tide in low latitudes was calculated above 58 km, and the associated acceleration rate was estimated to be (Formula presented.) 0.30 m (Formula presented.) (Formula presented.) near the cloud top, which is comparable to those expected in the previous numerical studies. Our results observationally confirmed the simultaneous upward and downward propagations of the thermal tide, which can account for the vertical shear of the Venusian atmospheric superrotation in low latitudes, supporting the previous theoretical predictions.

金星大気中の波を大気大循環モデル (GCM) によって再現し，大気スーパーローテーション (SR) の時間変動との関係を調べました．雲頂付近 (高度約70 km) で観測される4日波・5日波はロスビー・ケルビン (RK) 不安定によって励起され，赤道ケルビン波と中高緯度ロスビー波が臨界緯度で結合した構造をもちます．どちらも角運動量を赤道向きに輸送し低緯度のSR (赤道ジェット) を強化します．5日波 (GCM中では5.8日波) の作る下部雲層の赤道ジェットが発達すると，赤道反対称な構造をもつ7日波が現れ，角運動量を赤道域から中緯度に輸送することで赤道ジェットを解消します．5日波と7日波が交互に成長・減衰を繰り返すことにより，280日程度の時間スケールをもつ赤道ジェットの準周期的な変動が生じます．これらの結果はあかつき観測とも整合的です．

The 4-day and 5-day waves observed at the cloud top in the Venus atmosphere are expected to play important roles in the maintenance of the general circulation. However, they have not been reproduced in Venus general circulation models (GCMs) so far, and their structures and excitation mechanisms remain poorly understood. Using an improved Venus GCM, we succeeded in reproducing the planetary-scale 3.3-day and 5.8-day waves, which are considered to correspond to the 4-day and 5-day waves, respectively. We also obtained the 4.8-day wave, whose structure is similar to that of the 5.8-day wave. The three-dimensional structures of these waves consist of a Kelvin wave in low latitudes and one or two Rossby waves in mid- and high latitudes that are connected at the critical latitudes. This fact indicates that they are excited by the Rossby-Kelvin instability. The Kelvin mode of the 3.3-day wave could be observed at the cloud top, while those of the 5.8-day and 4.8-day waves cannot be observed due to the critical levels. The mid-latitude Rossby mode of the 5.8-day wave is dominated by mid-latitude vortices symmetric about the equator at the cloud top; this result is quite consistent with the observations. The equatorward angular momentum flux induced by these waves could contribute to the Venus superrotation. The Rossby modes of the 5.8-day and 4.8-day waves have baroclinic structures in the cloud layer inducing significant poleward heat flux. The planetary-scale structures symmetric about the equator observed in the Venus atmosphere could be explained by these waves.

Distributions of temperature and static stability in the Venus atmosphere consistent with recent radio occultation measurements are reproduced using a general circulation model. A low-stability layer is maintained at low- and mid-latitudes at 50–60 km altitude and is sandwiched by high- and moderate-stability layers extending above 60 and below 50 km, respectively. In the polar region, the low-stability layer is located at 46–63 km altitude and the relatively low-stability layer is also found at 40–46 km altitude. To investigate how these thermal structures form, we examine the dynamical effects of the atmospheric motions on the static stability below 65 km altitude. The results show that the heat transports due to the mean meridional circulation and disturbances are important in low-latitudes. The mid- and high-latitudes above ∼47 km are destabilized by radiative processes and stabilized by the disturbances, which are mainly associated with baroclinic Rossby-type waves. Below ∼47 km altitude, the polar region is destabilized by the dynamical processes which induce the appreciable equatorward heat transport, which might be related to Rossby waves in the sub-cloud region, although the suppression of convective adjustment at cloud heights leads to stabilization at 43–47 km.

In order to resolve discrepancy between recent observational and numerical studies on the thermal tides in the Venusian atmosphere, we investigated by means of a general circulation model how the thermal tides are affected by the static stability in and above the upper cloud layer by using three different distributions of the static stability. The results show that the vertical structure of the semidiurnal tide, which propagates vertically, is strongly affected by the static stability. The diurnal tide, which has an equivalent barotropic structure in 62–73 km altitudes, becomes weaker with the higher static stability although its vertical structure is almost unchanged. The horizontal distribution of the thermal tides with the realistic static stability distribution, which is consistent with radio occultation measurements, agrees with the observations at the cloud top. The meridional angular momentum flux associated with the thermal tides is equatorward in low latitudes near the altitude where the equatorial zonal-mean wind takes its maximum. This result is consistent with the recent Akatsuki UVI observations, suggesting that the thermal tides could contribute to the maintenance of the superrotation in the equatorial region near the cloud top. In the most realistic case, the zonal-mean zonal wind is effectively accelerated at rates of 0.2–0.5 m s−1 day−1 in low latitudes at altitudes of 52–76 km by both the meridional and vertical angular momentum transports. The thermal tides also induce significant meridional heat flux, which cannot be ignored in the dynamical effect on the zonal-mean zonal wind.

Barotropic, baroclinic, and Rossby-Kelvin instabilities exist in the Venus atmosphere, as revealed by previous studies using numerical models. This study aims to deepen our understanding of the instabilities in the Venus atmosphere using the breeding of growing modes (BGM) method for the first time. We conducted a 1-year model simulation as the control run. First, we conducted identical twin experiments in which random perturbations were added to the control run at the initial time, and they grew freely. Perturbations in the upper layer (UL, 70–100 km altitudes) grow faster at the beginning but saturate earlier at a low value compared to those in the lower layer (LL, 40–70 km altitudes). Next, we implemented the BGM method and conducted multiple breeding cycle experiments with different rescaling amplitudes and intervals, the two parameters of the BGM method. The bred vectors (BVs) from these experiments identified instabilities in various regions. With large rescaling amplitudes, the structure and amplitude of the BVs in the LL closely resembled the deviations in the control run, indicating the growth of BVs due to barotropic, baroclinic, and Rossby-Kelvin instabilities. Composite mean analysis shows larger BV amplitudes in the morning hemisphere at the 60–70 km altitudes in the mid-latitudes, indicating enhanced baroclinic instability by the thermal tide. Finally, we estimated the intrinsic predictability related to baroclinic instability for Venus is >1 month, which would be longer than that for the Earth of ∼2 weeks.