論文

The planetary missions including the Venus Climate Orbiter ‘Akatsuki’ provide new information on various atmospheric phenomena. Nevertheless, it is difficult to elucidate their three-dimensional structures globally and continuously only from observations because satellite observations are considerably limited in time and space. We constructed the first ‘objective analysis’ of Venus’ atmosphere by assimilating cloud-top horizontal winds on the dayside from the equator to mid-latitudes, which is frequently obtained from Akatsuki's Ultraviolet Imager (UVI). The three-dimensional structures of thermal tides, found recently to play a crucial role in maintaining the super rotation, are greatly improved by the data assimilation. This result is confirmed by comparison with Akatsuki's temperature observations. The momentum transport caused by the thermal tides and other disturbances are also modified by the wind assimilation and agrees well with those estimated from the UVI observations. The assimilated dataset is reliable and will be open to the public along with the Akatsuki observations for further investigation of Venus’ atmospheric phenomena.

Horizontal diffusion and resolution are important factors to generate and maintain the super-rotation in the general circulation model (GCM) because waves which transport angular momentum are sensitive to them. Here, we investigated how the super-rotation depends on the magnitude of horizontal hyper diffusion using a Venus atmospheric GCM with medium and high resolutions. In both the runs, we found a parameter range where the structure of fully developed super-rotation is almost independent of the magnitude of horizontal diffusion. Spectral analysis shows that unrealistically strong super-rotation is developed when medium-scale disturbances are dissipated by stronger horizontal diffusion. On the other hand, artificially weak super-rotation is also realized because spurious small-scale disturbances are accumulated when the horizontal diffusion is too weak. Graphical Abstract: [Figure not available: see fulltext.]

© 2018 American Meteorological Society. This study proposes that secondary eyewall formation (SEF) of tropical cyclones (TCs) can be attributed to an instability of flow in the free atmosphere coupled with Ekman pumping. Unstable solutions of a 1.5-layer shallow-water system are obtained under fast-wind speed conditions in the free atmosphere. The instability condition derived in the linear model indicates the importance of the ratio of angular velocity to vorticity, and the condition is more likely to be satisfied when the ratio is large and its radial gradient is positive. Thus, fast angular velocity, low absolute vertical vorticity, small negative radial gradient of angular velocity, and large negative gradient of vertical vorticity are favorable. Eigenvalue analyses are performed over a wide range of parameters using a vorticity profile with an infinitesimal secondary maximum. The growth rate increases with vorticity outside the radius of maximum wind (RMW), the radius of the secondary vorticity maximum, its magnitude, and the Rossby number defined by maximum tangential velocity, the RMW, and the Coriolis parameter. Furthermore, the growth rate is positive only between 2 and 7 times the RMW, and it is negative close to or far outside the RMW. These features are consistent with previous studies on SEF. A dimensionless quantity γ obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models;γ increases several hours before a secondary peak of tangential velocity forms, suggesting that the initial process of SEF can be attributed to the proposed mechanism.

©2018. American Geophysical Union. All Rights Reserved. Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65-km altitude at low latitudes equatorward of 35°. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES-Venus, reproduced the local time-dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75- to 85-km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer.

©2018. American Geophysical Union. All Rights Reserved. Thermal tides in the Venus atmosphere are investigated by using a GCM named as AFES-Venus. The three-dimensional structures of wind and temperature associated with the thermal tides obtained in our model are fully examined and compared with observations. The result shows that the wind and temperature distributions of the thermal tides depend complexly on latitude and altitude in the cloud layer, mainly because they consist of vertically propagating and trapped modes with zonal wave numbers of 1–4, each of which predominates in different latitudes and altitudes under the influence of mid- and high-latitude jets. A strong circulation between the subsolar and antisolar (SS-AS) points, which is equivalent to a diurnal component of the thermal tides, is superposed on the superrotation. The vertical velocity of SS-AS circulation is about 10 times larger than that of the zonal-mean meridional circulation (ZMMC) in 60–70 km altitudes. It is suggested that the SS-AS circulation could contribute to the material transport, and its upward motion might be related to the UV dark region observed in the subsolar and early afternoon regions in low latitudes. The terdiurnal and quaterdiurnal tides, which may be excited by the nonlinear interactions among the diurnal and semidiurnal tides in middle and high latitudes, are detected in the solar-fixed Y-shape structure formed in the vertical wind field in the upper cloud layer. The ZMMC is weak and has a complex structure in the cloud layer; the Hadley circulation is confined to latitudes equatorward of 30°, and the Ferrel-like one appears in middle and high latitudes.