Nobumasa Komori

Gravity waves play an essential role in planetary atmospheres by transporting momentum and energy far from their source regions. While observed in the Venusian atmosphere, their specific characteristics and generation mechanisms have remained poorly understood. This study investigates the activities of small-scale gravity waves using a Venus general circulation model with ultra-high spatial resolution (horizontal and vertical grid intervals of less than 20 km and 0.25 km, respectively).Our simulations reveal that gravity waves are spontaneously radiated from nearly balanced flows as the super-rotation evolves. In the low-latitudes, thermal tides within the super-rotation act as the primary sources, whereas baroclinic and barotropic waves are essential sources in the mid- to high-latitudes. To isolate these mechanisms, we conducted experiments excluding the diurnal component of solar heating and performed sensitivity tests on the static stability of the cloud layer. We found that higher static stability weakens gravity wave radiation, indicating that the intensity of baroclinic instability is directly linked to the amount of spontaneous wave generation.These small- to medium-scale gravity waves significantly affect the three-dimensional structure of the super-rotation and contribute to material mixing through their breaking processes. Notably, we confirmed for the first time that gravity waves in mid- to high-latitudes can transport zonal momentum vertically down to the lower atmosphere (~50–60 km). Furthermore, vertical propagation leads to a dual effect on the super-rotation: a deceleration in the upper cloud layer (~70 km) and an acceleration above ~80 km. These results demonstrate that momentum transport via spontaneous gravity wave radiation is a critical component of the Venusian atmospheric circulation and must be accounted for in our understanding of its global dynamics.

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.

In Fujisawa et al. (2022) [1], we previously produced an objective analysis of the Venusian atmosphere by assimilating horizontal winds derived from cloud tracking of the UVI camera onboard the Venus orbiter Akatsuki. To produce objective analysis, we used the Venus atmospheric data assimilation system ALEDAS-V (Sugimoto et al., 2017) [2], which is based on the Venus general circulation model AFES-Venus (Sugimoto et al., 2014) [3]. This dataset appropriately corrects both the phase bias of thermal tides and the super-rotation speed in AFES-Venus to be closer to those observed in the real Venusian atmosphere. The dataset was produced by assimilating observations from September to December 2018, a period that includes an intensive observation period of Akatsuki.Akatsuki has accumulated observational data over a long period from 2015 to 2024, and it has been revealed that the super-rotation speed exhibits both faster and slower periods (Horinouchi et al., 2024) [4]. In this study, we selected five epochs during the Akatsuki observation period that exhibit characteristic super-rotation speeds and performed data assimilation for each epoch. As a result, we confirmed that distinct super-rotation speeds corresponding to each epoch, including their meridional asymmetry, are reproduced. In the presentation, we will show the relationship between the reproduced super-rotation speeds and the structure of the atmospheric circulation.[1] Fujisawa, Y., et al. (2022) Sci. Rep. 12, 14577.[2] Sugimoto, N., et al. (2017) Sci. Rep. 7(1), 9321.[3] Sugimoto, N., et al. (2014) J. Geophys. Res. Planets 119, 1950–1968.[4] Horinouchi, T., et al. (2024) J. Geophys. Res. Planets 129, e2023JE008221.

We have developed the Venusian general circulation model (GCM) named AFES-Venus (Atmospheric GCM for the Earth Simulator for Venus) [1, 2] and the data assimilation system based on the Local Ensemble Transform Kalman Filter (LETKF) named ALEDAS-V (AFES-LETKF data assimilation system for Venus) [3]. Here, we will introduce recent improvements of AFES-Venus and ALEDAS-V and newly obtained results.So far, AFES-Venus reproduced the cold collar in the polar region [4], the planetary-scale streak structure observed by Akatsuki infrared (IR2) camera [5], a fully developed super-rotation [6], and spontaneous gravity waves radiated from the thermal tides [7]. The thermal tides [8] and the planetary-scale short periods (Kelvin and Rossby) waves [9, 10] consistent with observations were also reproduced by improving the profiles of static stability and solar heating. Dependency of the super-rotation on the magnitude of horizontal hyper diffusion was investigated with medium- and high-resolution simulations [11]. Recently we have implemented radiative and cloud microphysical processes into AFES-Venus (Fig.1). We have also investigated disturbances [12] and energy cycles [13] in the Venus atmosphere using Bred vectors.ALEDAS-V improved the horizontal structures of thermal tides with the data assimilation of horizontal winds derived by cloud tracking of ultra-violet images (UVI) from Venus Express [14] and Akatsuki [15]. Although the observed horizontal winds are limited to low latitudes on the day side, the zonal-mean zonal winds and temperature were also modified globally [15]. The first analysis data in which horizontal winds obtained by Akatsuki UVI are assimilated will be released soon. The cold collar was also realistically reproduced in the analysis data [16]. We have also conducted several observing system simulation experiments (OSSEs) assuming Akatsuki Longwave Infrared Camera (LIR) observations [17]. Now we are trying to assimilate temperature obtained by Akatsuki LIR.Fig.1: Zonal mean zonal wind [m s−1] (contours) and static stability [K km−1] (shading) for the original AFES-Venus (left: EXP-DYN) and that with a new radiative transfer process (EXP-RAD).References: [1] Sugimoto+2014 JGR-Planets, 119, 1950–1968. [2] Sugimoto+2014 GRL, 41, 7461–7467. [3] Sugimoto+2017 Sci. Rep.. 7, 9321. [4] Ando+2016 Nature Comm., 7, 10398. [5] Kashimura+2019 Nature Comm., 10, 23. [6] Sugimoto+2019 GRL, 46, 1776–1784. [7] Sugimoto N+2021 Nature Comm., 12, 3682. [8] Suzuki+2022 JGR-Planets, 127, 7243. [9] Takagi+2022 JGR-Planets, 127, 7164. [10] Takagi+2023, JGR-Planets, 128, 7922. [11] Sugimoto+2023 Earth, Planets and Space, 75, 44. [12] Liang+2024 JGR-Planets, 129, 8067. [13] Liang+2025 GRL, 52, 112663. [14] Sugimoto+2019 GRL, 46, 4573–4580. [15] Fujisawa+2022 Sci. Rep., 12, 14577. [16] Ando+2023 JGR-Planets, 128, 7689. [17] Sugimoto+2022 Geoscience Lett., 9, 44.

At the cloud-top level, around 65–70 km on Venus, there exists a zonal wind with a velocity of about 100 m/s, which is a remarkable phenomenon known as super-rotation. Planetary-scale waves with a zonal wavenumber of 1 and periods of about 4-day and 5-day have also been observed at this altitude and have been interpreted as equatorial Kelvin waves and Rossby waves, respectively (Del Genio and Rossow, 1990 [1]; Kouyama et al., 2015 [2]; Kajiwara et al., 2021 [3]). The intensity of these waves has been shown to vary over time based on long-term observational data of the horizontal wind distribution obtained from cloud tracking by the Ultraviolet Imager (UVI) onboard the Venus orbiter "Akatsuki" (Imai et al., 2019 [4]; Horinouchi et al., 2024 [5]). Numerical model studies suggest that these waves are excited by the Rossby-Kelvin instability and that angular momentum transport by these waves contribute to the long-term variations in super-rotation (Takagi et al., 2022 [6]). We have previously assimilated horizontal winds obtained from Akatsuki cloud tracking into the world’s first Venus data assimilation system and successfully produced the Venus objective analysis dataset, ALERA-V v1.0 (Fujisawa et al., 2024 [7], 2025 [8]). In addition, in an observation system simulation experiment using the same system, we assimilated synthetic data, including Kelvin waves, and confirmed that Kelvin waves can be reproduced through data assimilation (Sugimoto et al., 2021 [9], 2022 [10]). In this study, we will prepare objective analysis dataset for a period different from that of Fujisawa et al. (2024 [7], 2025 [8]). The goal of this study is to prepare this dataset, which has fewer temporal and spatial restrictions than the observational data, in order to extract a 4-day wave that suggests a Kelvin wave-like structure. This wave was difficult to extract in Imai et al. (2019) [4] due to its weak signal, and it is expected that the structure of this wave can be clarified using the objective analysis dataset. “ALEDAS-V” (AFES-LETKF data assimilation system for the Venus atmosphere; Sugimoto et al., 2017) [11] is used for assimilation, and "AFES-Venus" (Atmospheric General Circulation Model for the Earth Simulator for Venus; Sugimoto et al., 2014) [12] is used for ensemble forecasts. AFES-Venus is a fully nonlinear dynamical general circulation model that assumes hydrostatic equilibrium and is designed for the Venus atmosphere. ALEDAS-V, which uses a local ensemble transform Kalman filter, is the first data assimilation system for the Venus atmosphere. Zonal and meridional winds were assimilated at the cloud top altitude of 70 km using cloud tracking data obtained from the UVI 365-nm images (Horinouchi et al., 2024) [13]. The settings, other than the assimilation period, are the same as those in Fujisawa et al. (2022). The assimilation period is from June 1 to December 31, 2017, during which the existence of Kelvin waves was suggested by Imai et al. (2019) [4]. The objective analysis (assimilated case) is compared with the free run (case without data assimilation). The figure shows the power spectral density of the zonal winds at an altitude of 70 km over a 120-day period starting from July 1, 2017. Panels (a) and (b) represent the free run and objective analysis, respectively. The free run shows signals with periods of 5.8-day and 7.5-day (Panel a), which are comparable to the results from a numerical model by Takagi et al. (2022) [6]. Takagi et al. (2022) [6] indicated that the 5.8-day wave is dominated by a Rossby mode with a peak amplitude at mid-latitudes at an altitude of 70 km. They also noted that the 6.5-7.6-day wave, which is close to the 7.7-day period, is dominated by an antisymmetric structure centered on the equator. The results of the free run in this study show signals at mid-latitudes (50-60 degrees latitude) and high latitudes (70-90 degrees latitude), which differ from the characteristics observed by Takagi et al. (2022) [6]. Future work will need to consider whether these differences are due to variations in the settings of the numerical model. The objective analysis shows signals with periods of 3.7-day and 5.1-day (Panel b). These can be compared with those of Imai et al. (2019) [4], who analyzed the observational data from Akatsuki. Imai et al. (2019) [4] revealed that the 3.5-4.0-day mode shows Kelvin wave characteristics with amplitude confined to the equatorial region, while the 5.0-5.5-day mode shows Rossby wave characteristics with amplitude at latitudes higher than 35 degrees. The 3.7-day and 5.1-day signals in the objective analysis of this study are in good agreement with the characteristics described by Imai et al. (2019) [4]. This indicates that the objective analysis successfully captured the short-period disturbances. In the future, we plan to continue analyzing the structure of these waves. Figure. Power spectral density of zonal winds for high-frequency components: (a) free run, (b) objective analysis. [1] Del Genio, A. D. & Rossow, W. B. (1990) J. Atmos. Sci., 47(3), 293–318.[2] Kouyama, T., et al. (2025) Icarus, 248, 560–568.[3] Kajiwara, N., et al. (2021) J. Geophys. Res. Planets, 126(12), e2021JE007047.[4] Imai, M., et al. (2019) J. Geophys. Res. Planets, 124(10), 2635–2659. [5] Horinouchi, T., et al. (2024) J. Geophys. Res. Planets, 129(3), e2023JE008221.[6] Takagi, M., et al. (2022) J. Geophys. Res. Planets, 127(4), e2021JE007164.[7] Fujisawa, Y., et al. (2022) Sci. Rep. 12, 14577.[8] Fujisawa, Y., et al. (2025) ALERA-V version 1.0: an experimental objective analysis dataset of Venus atmosphere (1.0) [Data set]. Zenodo. [9] Sugimoto, N., et al. (2021) Atmosphere, 12, 14.[10] Sugimoto, N., et al. (2022) Atmosphere, 13, 182. [11] Sugimoto, N., et al. (2017) Sci. Rep. 7(1), 9321. [12] Sugimoto, N., et al. (2014) J. Geophys. Res. Planets 119, 1950–1968.[13] Horinouchi, T., et al. (2024) Venus Climate Orbiter Akatsuki...

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.

Data assimilation of the Venus atmosphere has become more feasible due to the long-term observations by the Venus climate orbiter “Akatsuki”. We have produced the first objective analysis for the Venus atmosphere by assimilating horizontal winds derived by the cloud tracking of the Akatsuki Ultraviolet Imager (UVI) [1]. Here, we assimilate the temperature derived by the Akatsuki Longwave Infrared Camera (LIR), using the AFES-LETKF data assimilation system for Venus "ALEDAS-V" [2], which is based on the atmospheric general circulation model for Venus "AFES-Venus" [3] and local ensemble transform Kalman filter “LETKF”. In this experiment the altitude for the assimilation is set to 65 km regardless of the emission angle for simplicity, though the altitude observed by LIR depends on the emission angle (the angle between the zenith and observer directions as seen from the observation point). The influence of the emission angle is investigated for two cases; the one is the emission angle limited to 40 degrees or less and the other one is that limited to 75 degrees or less. [1] Fujisawa, Y., Sy. Murakami, N. Sugimoto, M. Takagi, T. Imamura, T. Horinouchi, G. L. Hashimoto, M. Ishiwatari, T. Enomoto, T. Miyoshi, H. Kashimura and Y.-Y. Hayashi, Y.-Y. (2022), Sci. Rep. 12, 14577.[2] Sugimoto, N., A. Yamazaki, T. Kouyama, H. Kashimura, T. Enomoto, and M. Takagi (2017), Sci. Rep., 7(1), 9321.[3] Sugimoto, N., M. Takagi, and Y. Matsuda (2014a), J. Geophys. Res. Planets, 119, 1950–1968.