Lower Stable Layer Research Articles

Thermal structure of the Venusian atmosphere from the sub-cloud region to the mesosphere as observed by radio occultation

We present distributions of the zonal-mean temperature and static stability in the Venusian atmosphere obtained from Venus Express and Akatsuki radio occultation profiles penetrating down to an altitude of 40 km. At latitudes equatorward of 75°, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements in that there is a low-stability layer at altitudes of 50–58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. Meanwhile, at latitudes poleward of 75°, a low-stability layer extends down to 42 km, which has been unreported in analyses of previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the Earth’s troposphere, which generally has a deeper low-stability layer at low latitudes than at mid- and high latitudes.

Planetary-scale streak structure reproduced in high-resolution simulations of the Venus atmosphere with a low-stability layer

Cloud patterns are important clues for revealing the atmospheric circulation of Venus. Recently, a planetary-scale streak structure has been discovered in middle- and lower-cloud images of Venus’ night-side taken by IR2, the 2-μm camera, on board the Akatsuki orbiter. However, its formation mechanism has not been investigated. Here we succeed, for the first time, in reproducing the patterns of the observed streak structure, as regions of strong downward flows that develop in high-resolution global simulations of the Venus atmosphere. The streaks are formed in both hemispheres with equatorial symmetry, which is caused by equatorial Rossby-like and Kelvin-like waves with zonal wavenumber one. The low-stability layer that has been suggested by past observations is essential for reproducing the streak structure. The streaks of downward flow result from the interaction of the meridionally tilted phase lines of the Rossby-like waves and the characteristics of baroclinic instability produced around the low-stability layer.

Convective Dynamos with Penetration, Rotation, and Shear

We investigate the dynamo properties of Boussinesq, penetrative convection in the presence of rotation and large-scale velocity shear flows. Several numerical experiments are conducted in a local Cartesian computational domain in which the relative stability of the lower stable layer, the rotation rate, the supercriticality of the convection, and the strength of the imposed shear flow are varied. Once a statistically steady hydrodynamic state is achieved for any set of parameters, a weak seed magnetic field is added and the subsequent dynamo evolution is followed. In all cases studied, the weak seed field is initially amplified exponentially and then eventually saturates in a stationary magnetohydrodynamic (MHD) state. Even in the presence of penetration, rotation, and shear, the field is predominantly small-scale. We analyze the reasons for this and make suggestions as to possible further mechanisms that may lead to large-scale field generation.

Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields

We present the first results from three-dimensional spherical shell simulations of magnetic dynamo action realized by turbulent convection penetrating downward into a tachocline of rotational shear. This permits us to assess several dynamical elements believed to be crucial to the operation of the solar global dynamo, variously involving differential rotation resulting from convection, magnetic pumping, and amplification of fields by stretching within the tachocline. The simulations reveal that strong axisymmetric toroidal magnetic fields (about 3000 G in strength) are realized within the lower stable layer, unlike in the convection zone where fluctuating fields are predominant. The toroidal fields in the stable layer possess a striking persistent antisymmetric parity, with fields in the northern hemisphere largely of opposite polarity to those in the southern hemisphere. The associated mean poloidal magnetic fields there have a clear dipolar geometry, but we have not yet observed any distinctive reversals or latitudinal propagation. The presence of these deep magnetic fields appears to stabilize the sense of mean fields produced by vigorous dynamo action in the bulk of the convection zone.

Gravity Waves and Convection Cells Resulting from Feedback Heating of Venus' Lower Clouds

A radiative-dynamical cloud feedback heating (CFH) is incorporated into a two-dimensional mechanistic model in an equatorial longitude-height plane of Venus. For a typical basic field, the CFH triggered by random disturbance results in the formation of convection cells with various scales in the lowstability layer (∼ 55 km) sandwiched between stable layers, and the convection cells generate vertically propagating gravity waves. The wave-generation mechanism by convection cells resulting from CFH is different from previous mechanisms, in which waves were directly forced by CFH. If the cloud feedback process works in real atmosphere, the convection resulting from the perturbed CFH should be considered as a possible formation mechanism of gravity waves in Venus’ cloud layer.

Blocky Markings and Planetary-Scale Waves in the Equatorial Cloud Layer of Venus

Abstract Linear and nonlinear behaviors of Venus’s planetary-scale waves are examined by using the two-dimensional model in the equatorial longitude–height plane. The zonal wave 1 gravity waves with phase velocities of 79 and 131 m s−1 are predominant at the cloud base and top, respectively, under the conditions that the waves are forced at the atmospheric bottom. These two waves may contribute to the superrotation in the cloud layer. Nonlinear effects of the planetary-scale 5.57-day wave (with the velocity of 79 m s−1) are likely to appear in the low-stability layer (between 50 and 60 km) before the wave reaches the critical level (63 km). The 5.57-day wave with large amplitude is broken into a few blocks in the low-stability layer, because convective motions are induced by phase structure of the planetary-scale wave’s temperature. Blocky markings in near-infrared observation may be associated with the breaking of the planetary-scale 5.57-day wave. When the forcing is increased, the vertically traveling ...

Propagation of Small-Scale Acoustic-Gravity Waves in the Venus Atmosphere

Abstract The amplification and attenuation of small-scale acoustic-gravity waves in Venus's atmosphere is studied with a plane-wave model that realistically simulates height variations in structure and zonal circulation. Forcing for these waves could be convective activity at cloud heights or close to the surface, or turbulence arising from small-scale shear instability of the zonal flow; the model treats both surface forcing and cloud-level forcing by diabatic heating variations in the low-stability layer near the base of the clouds. Waves are attenuated in this cloud-level, low-static-stability layer. Slowly moving waves with small vertical length scales are attenuated by eddy diffusivity. Westward moving waves can undergo critical level absorption. A net enhancement in wave amplitude is ago possible because waves can be trapped between the surface and the base of the low stability layer at about 50 km. Observations of small-scale wave activity at the cloud tops and above can be used to explore uncertai...