Masahiro Takagi

Based on simulations of the Venusian atmosphere using the AFES‐Venus GCM, we investigated the angular momentum (AM) balance that maintains the atmospheric superrotation (SR) at 35–90 km altitudes. In and above the cloud layer, the Eulerian‐mean meridional circulation consists of Hadley‐like cells at low latitudes and Ferrel‐like cells at middle to high latitudes, while the residual mean meridional circulation (RMMC) consists of a single Hadley‐like cell extending from the equator to the poles. Horizontally integrated AM and Eliassen‐Palm (EP) fluxes show that, at 46–90 km, SR is primarily accelerated by RMMC‐induced upward AM transport. This acceleration is counterbalanced by wave‐induced downward transport; however, the thermal tides significantly accelerate SR at 50–60 and 67–75 km. Thus, SR is maintained by both the meridional circulation and thermal tide mechanisms, which contribute approximately 70% and 30%, respectively, in the cloud layer. Regionally integrated AM and EP fluxes indicate that the RMMC transports substantial AM upward across the 54‐km level at low latitudes, with its upper branch subsequently carrying most of this AM poleward within 54–73 km. Although part of the poleward‐transported AM is returned equatorward by waves, the amount is much smaller than the poleward transport. Instead, wave‐induced downward transport at high latitudes, primarily associated with poleward eddy heat flux, is crucial for the AM budget. Our results provide a more detailed interpretation of the AM balance and clarify the respective roles of the MMC, thermal tides, and planetary‐scale waves than those presented in the so‐called classical theories.

By analyzing radio occultation data obtained by Venus Express in 2006–2013 and Akatsuki in 2016–2023, we investigated long‐term variations in the sulfuric acid vapor mixing ratio and temperature within and below the lower cloud layer (42–54 km altitude) at low latitudes (35S–35N) on Venus. The results show that the sulfuric acid vapor mixing ratio above the cloud base (∼48 km) remained nearly constant from 2006 to 2008, decreased between 2008 and 2018, and increased afterward. A similar long‐term trend was found in temperature. Annual variations of temperature were positively correlated with those of vapor mixing ratio likely because the sulfuric acid vapor mixing ratio within the cloud layer is roughly comparable to the saturation mixing ratio, which is temperature‐dependent. Our findings suggest that the Venusian cloud structure has long‐term variations that are driven by changes in sulfuric acid vapor mixing ratio and temperature. Because sulfuric acid clouds constitute the core of the Venusian climate system, decadal‐scale climate variability may be present on Venus.

Despite decades of exploration, Venus's climate system governing the atmosphere beneath the cloud top (70 km altitude) remains poorly understood due to the persistent thick cloud cover. Here, we present the first detailed characterization of temporal variability in the thermal structure of Venus's low‐latitude atmosphere from the sub‐cloud region to the mesosphere (45–80 km), using 257 vertical temperature profiles obtained from Venus Express and Akatsuki radio occultation measurements in 2006–2024. Remarkable decadal temperature variations of 10 K were observed below 55 km, far exceeding those of 2 K seen at comparable pressure levels in Earth's tropics. In terms of the relative variation, the change on Venus over a decade is 1.4%, which is larger than the corresponding value of 0.6% in Earth's tropical region, indicating that the variability on Venus is more pronounced. The variation in temperature partly correlates with that in the ultraviolet albedo of Venus's cloud, suggesting potential links to the solar heating, cloud physics, and photochemistry. Furthermore, the variations in temperature and geopotential height are nearly in phase with those in zonal wind speed at the cloud top, indicating a strong coupling between the atmospheric dynamics in the upper cloud and the underlying thermal structure. While the mechanisms behind the observed variations remain elusive, our findings reveal that Venus has a more pronounced intrinsic climate variability than Earth. To elucidate Venus's climate system, we need long‐term and continuous observations by successive future missions.

The clouds covering Venus globally, that are primarily composed of ‐O droplets, strongly influence the thermal structure and dynamics of the atmosphere. However, the mechanism governing their growth and long‐term maintenance remains poorly understood. In this study, the bifurcation structure of the droplets' growth dynamics through the co‐condensation of O and is investigated by constructing a box model under the assumption of a monodisperse droplet population. Our analysis reveals that the phase portrait which is a diagram with the masses of O and liquids in the droplet as the axes, showing how each evolves over time, depends on the saturation ratios of the O and gases and that the mass of O in the droplet varies much more rapidly than that of under conditions near the Venusian cloud base. The condition for the stable existence of Venusian cloud droplets is also investigated in terms of the saddle‐node bifurcation. Based on these findings, we simulate the droplets' growth under the thermodynamic conditions near the Venusian cloud base and find that the small cloud droplets, such as Mode 1, may rapidly grow into larger ones, such as Modes 2 or 3, depending on the droplet number density.

The vertical structure of the thermal tide in the Venusian low‐latitude region (0–30 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 0.30 m 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.

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 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.

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.

Observations of the Venus Climate Orbiter “Akatsuki” provide us with horizontal distributions of the horizontal winds derived from cloud tracking of the Ultraviolet Imager (UVI) and of temperature observed by the Longwave Infrared Camera (LIR). However, these observations are limited in altitude, local time (day or night side), and frequency. Then it is difficult to elucidate the general circulation of the Venus atmosphere, including various temporal and spatial scales, only from observations. In this study, we produced a Venus dataset (analysis) that has high temporal and spatial resolutions by assimilating horizontal winds derived by the Akatsuki observations. At the top of the cloud layer of Venus, there are planetary-scale atmospheric waves that are excited by the solar heating and move with the sun, called the thermal tides. In this presentation, we focused on thermal tides to verify the analysis. We use the Venus atmospheric data assimilation system “ALEDAS-V" (Sugimoto et al., 2017) [1] for assimilation and the Venus atmospheric general circulation model “AFES-Venus" (Sugimoto et al., 2014) [2] for ensemble forecasts. AFES-Venus is a full nonlinear dynamical GCM on the assumption of hydrostatic balance, designed for the Venus atmosphere. ALEDAS-V uses the Local Ensemble Transform Kalman Filter, and is the first data assimilation system for the Venus atmosphere. We assimilated the cloud top (~70km) zonal and meridional winds obtained by tracking morphology, using Akatsuki UVI data (Horinouchi et al., 2021) [3] from September 1st to December 31st, 2018. The assimilation data (analysis) from October 1st to November 30th, 2018, is analyzed, because the root-mean-square-deviations (RMSD) from FR (free run; the case without data assimilation) are stable. Figures (a) and (d) show the observed zonal and meridional winds, respectively. The zonal wind has a local minimum near 11 LT (local time) around the equator (Figure a). The meridional wind is the weakest at the equator and increases with latitude, and the amplitude is maximum around noon (Figure d) in the local time direction. Note that these winds obtained from observations exist only the dayside equatorward of 50° latitudes (Figure a and d). Figures (b) and (e) show the deviations from the zonal means of zonal and meridional winds at an altitude of 70 km in the FR, respectively. For zonal wind, diurnal (zonal wavenumber 1) and semidiurnal (zonal wavenumber 2) tides are dominant at latitudes poleward and equatorward of 30, respectively (Figure b). The zonal wind deviation has a local minimum at 14-15LT, which is ~ 2 hours behind the observation (Figures a and b). The meridional wind deviation is polar and equatorial on the dayside and nightside, respectively (Figure e), and this distribution is consistent with Akatsuki's observation (Figure d). Figures (c) and (f) show the zonal and meridional winds as a result of assimilation, respectively. The zonal wind in the equatorial region have a local minimum near 11 LT. The assimilation improved the semidiurnal tide closer to the observations (Figures a and c). The meridional wind is not so different from FR. This is probably because FR was originally very similar to observations (Figures d and f). These results are consistent with a previous study by Sugimoto et al. (2019) [4]. In addition, while the observed winds exist only on the dayside, the results of assimilation show that the horizontal winds field is modified significantly even on the nightside. It is suggested that spatially limited data assimilation can improve the general circulation of GCM. In the future work, we are planning to release the assimilation dataset as the “objective analysis data” of Venus for the first time in the world. [1] Sugimoto, N., et al. Development of an ensemble Kalman filter data assimilation system for the Venusian atmosphere. Scientific Reports 7(1), 9321 (2017). [2] Sugimoto, N., et al. Baroclinic instability in the Venus atmosphere simulated by GCM. J. Geophys. Res. Planets 119, 1950–1968 (2014). [3] Horinouchi, T., et al. Venus Climate Orbiter Akatsuki Cloud Motion Vector Data Set v1.0, JAXA Data Archives and Transmission System (2021). [4] Sugimoto, N., et al. Impact of data assimilation on thermal tides in the case of Venus Express wind observation. Geophys. Res. Lett. 46, 4573–4580 (2019).