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著者人数: 16名, Accepted: 2019-04-12, 資料番号: SA1190014000

Vertical temperature profiles at 40–75 km around 80° N in the Venus polar vortex are retrieved over 13 Earth days almost continuously from radio occultation measurements (VeRa) in the Venus Express mission. They show periodical variations with a dominant period of ∼3.1 Earth days. These fluctuations are confined in an altitude range of 45–65 km with a local minimum at ∼58 km altitude, where the static stability abruptly increases with height. The phase of the temperature fluctuations is almost reversed at the 58 km level, and varies little above and below this altitude. A numerical simulation of a Venusian atmospheric general circulation model (GCM) shows that the axi-asymmetric temperature disturbance with zonal wavenumber-1 is predominant at 50–75 km levels in the model atmosphere. The vertical structure of the reproduced disturbance agrees quite well with that retrieved by the radio occultation measurement: amplitude of the temperature fluctuation has a local minimum and its phase is reversed at the altitude (65 km in the model) where the static stability rapidly changes as in the observations. Above and below this altitude, the phase is almost constant in the vertical direction. The relationship among the temperature, horizontal winds and geopotential height associated with the simulated disturbance suggests that the axi-asymmetric temperature disturbance observed in the Venus polar region can be interpreted as neutral barotropic Rossby waves related to barotropic instability in the polar region.

Baroclinic instability in the super-rotation of Venus is investigated by a newly developed atmospheric general circulation model. First, we adopt an idealized super-rotation, i.e., solid-body rotating flow in a weakly stratified layer at cloud level, as an initial basic state in a nominal case. With the evolution of time, baroclinic instability occurs in a weakly stratified layer with large vertical shear of the basic zonal flow. Horizontal wind associated with the baroclinic instability modes is of a few m s−1. The initial structure of the unstable modes is similar to those obtained in previous linear stability analyses. However, it is modified by nonlinear interactions in the later stage, reaching a quasi-steady state. Meridional transport of momentum and heat by these unstable modes accelerates the super-rotation by ~ 0.05 m s−1 day−1 at midlatitudes. Furthermore, the dependence of baroclinic instability on the basic state, i.e., the meridional profiles of zonal flow and the vertical profiles of static stability, are subsequently investigated. For the super-rotation with midlatitude jets at cloud level, the modes are modified from baroclinic to barotropic in the later stage. Typically, their horizontal wind is of O(10) m s−1. Their amplitude is maintained by energy conversion from zonal-mean available potential energy associated with the baroclinic basic state. In the case where static stability is smaller than that in the nominal case, the baroclinic modes transfer angular momentum from midlatitude to the equator near a 70 km level and accelerate the super-rotation by more than 10 m s−1 in the equatorial region.

[1] We investigated generation mechanisms of the local time variation of the wind velocity in the Venusian mesosphere and thermosphere, which has been suggested from recent ground-based CO millimeter/submillimeter and CO2 10 μm observations, using our general circulation model. Our model considers the momentum transport caused by gravity waves with the gravity wave parameterization developed by Medvedev and Klaassen (2000). Our results show that atmospheric circulation distinctly changes from the dayside to the nightside. The subsolar-to-antisolar (SSAS) wind is predominant in the dayside. On the other hand, the retrograde superrotating zonal (RSZ) wind is superposed on the SSAS wind in the nightside. These characteristics are consistent with the previous observations. The westward momentum, which drives the RSZ wind in the nightside at an altitude of 110 km, is supplied by gravity waves in the 115–130 km altitude region. The downward flow that originates in the SSAS wind transports the westward momentum downward in the nightside.

[1] Influence of CO 2 line profiles on vertical temperature distributions in the radiative and radiative-convective equilibria is examined in the Venus atmosphere. The CO 2 opacity obtained by the Voigt (Lorentz) profile without the line cutoff is shown to be excessive since this opacity gives surface temperatures of about 860-1020 K in the radiative-convective equilibrium. On the other hand, the opacity obtained by the extremely sub-Lorentzian profiles of Pollack et al. (1993) and Tonkov et al. (1996) are underestimated; the surface temperature obtained with this opacity remains 600 K even in the radiative equilibrium. In this case, convection does not take place below the cloud layer because of the cloud opacity. It is also shown that Fukabori et al.'s (1986) and Meadows and Crisp's (1996) profiles, both of which have intermediate absorption coefficients, give temperature distributions close to the observed one in the radiative-convective equilibrium. In these cases, the convection layer extends from the surface to 30-50 km altitudes. Then, the temperature distribution below the cloud layer is determined by a dry adiabatic lapse rate and the temperature near the cloud bottom. The surface temperature in the radiative-convective equilibrium is strongly affected by the temperature near the cloud bottom in this situation. The detailed structure of the H 2 SO 4 cloud must be taken into account to construct a realistic radiative transfer model.

[1] A nonlinear dynamical model on the sphere has been numerically integrated to investigate a generation mechanism of the Venus atmospheric superrotation by the thermal tides. By using the solar heating exciting the diurnal and semidiurnal tides, the atmospheric superrotation extending from the ground to 80 km is generated. The vertical distributions of the mean zonal flow obtained in our experiments are similar to the observations. Velocity of the mean zonal wind on the equator reaches about 60–100 m s−1 near the cloud top level. A linear theory suggests that the atmospheric superrotation obtained in the present study is generated and maintained by the momentum transport associated with the thermal tides. Namely, the downward transport of zonal momentum that is associated with the downward propagating semidiurnal tide excited in the cloud layer induces the mean zonal flow opposite to the Venus rotation in the lowest layer adjacent to the ground. Surface friction acting on this counter flow provides the atmosphere with the net angular momentum from the solid part of Venus. It is examined how the atmospheric superrotation depends on vertical eddy viscosity and Newtonian cooling. The result shows that magnitude of the atmospheric superrotation is not so sensitive to vertical eddy viscosity but is strongly influenced by Newtonian cooling.

[1] The thermal tides in the lower atmosphere of Venus are examined. It is shown that both the diurnal and semidiurnal tides excited in the cloud layer propagate to the ground, and the existence of thermal tides below the cloud bottom can be attributed almost to the solar heating in the cloud layer. At altitudes of 0–10 km, the atmospheric superrotation is accelerated and decelerated by the momentum transport associated with the diurnal and semidiurnal tides, respectively. The effect of diurnal tide is much smaller than that of the semidiurnal tide, so that the mean zonal flow is accelerated in the direction opposite to the Venus rotation there. It is argued that this momentum transport by the semidiurnal tide is balanced with the surface friction in the atmospheric layer adjacent to the ground, and the net momentum is supplied from the solid Venus to maintain the atmospheric superrotation.

[1] A three-dimensional primitive equation model on a sphere is constructed to investigate stability of a baroclinic flow in cyclostrophic balance and structures of the unstable modes. When the Richardson number is large, the baroclinic instability appears for sufficiently small values of the Rossby number. As the Rossby number increases, only the large scale modes remain unstable and amplitude of these modes becomes confined to the lower layer. When the Richardson number is small, the symmetric instability predominates, while the baroclinic instability modes also exist in a certain parameter range. Unlike the baroclinic instability mode, the amplitude of the symmetric instability mode is confined to low latitudes in the upper layer. Since the Richardson number is small in the Venus cloud layer, it is predicted that the baroclinic instability is suppressed in the midlatitudes, and the symmetric instability appears in the low latitudes.

[1] Stability of a baroclinic basic flow in gradient wind balance is examined for a very wide range of parameters. When the Rossby number is sufficiently small, it is found that the baroclinic instability occurs not only in the basic flow in geostrophic regime, but also in that in cyclostrophic one. As the Rossby number increases, the baroclinic instability disappears in the nearly geostrophic regime. However, the unstable mode appears again when the dynamic balance changes into the cyclostrophic one; its growth rate is fairly larger than that of the baroclinic instability in the geostrophic regime. It is shown that the baroclinic instability problem in the Venus cloud layer (45–70 km) is not so deviated from that in the terrestrial troposphere, while the cyclostrophic balance is essential in a situation of the Venus lower atmosphere, where the unstable baroclinic wave is predicted to appear.

[1] A numerical model is constructed to investigate sensitivity of thermal tides in the Venus atmosphere to distributions of basic mean-zonal wind and Newtonian cooling rate. It is found that the amplitude and spatial structure of the diurnal tide are strongly influenced by the midlatitudinal jets and the Newtonian cooling rates. In all cases examined in this study, the diurnal tide propagates downward in the lower atmosphere below the cloud levels, and reaches the ground. The semidiurnal tide is generally less dependent on these parameters, but its zonal winds are increased in the upper jet regions.