Dynamical implications of the observed thermal contrasts in Venus' upper atmosphere |
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| Authors: | A Seiff |
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| Institution: | Ames Research Center, NASA, Moffett Field, California 94035, USA |
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| Abstract: | The state properties observed by Pioneer Venus experiments in Venus' mesosphere and thermosphere impose constraints on the dynamics at those altitudes and, in fact, suggest a very vigorous dynamics, by virtue of the extremely large day-night pressure contrasts. At both the morning and evening terminators, these are directed to accelerate the flow from the day hemisphere to the night, and are thus consistent with subsolar to antisolar circulation, possibly somewhat unsymmetrical. There is a major vertical contraction of the atmosphere above 100 km as it crosses the terminators, associated with the nightside cooling. Flow across both terminators is thus descending, but at rather gentle angles (～0.003 rad), and there is a consequent downward transport of composition from the dayside to the nightside. The pressure differences and gravitational acceleration in the descending flow are sufficient to generate supersonic speeds in the flow crossing the terminator in the absence of viscosity. However, the equation of continuity cannot be satisfied with such high velocities, given the measured state properties. This is interpreted to be evidence for strong viscous deceleration and dissipation at the 110 to 120-km level, and possibly extending above 120 km. The viscosity required is that of turbulent motion, rather than laminar. It is noteworthy that the basic dynamic models of Dickinson and Ridley are for laminar viscosity. With moderate flow velocities approaching the terminator (～65 m/sec), as measured by A.L. Betz et al. (1977, Proceedings, Symposium on Planetary Atmospheres, pp. 29–33), and for an essentially unaccelerated flow crossing the terminator in the presence of viscous dissipation, as indicated by the continuity equation applied to the data, the observed nightside cooling below 140 km was found to be approximately that given by the 15-μm CO2 band radiative cooling model of R.E. Dickinson (1976, Icarus27, 479–493). This may be an indirect indication that the velocities are indeed low (i.e., less than 100 m/sec) in the subsolar-antisolar circulation, and are kept low by viscous forces. Calculations based on R.E. Dickinson and E.C. Ridley's equations (1977, Icarus30, 163–178) indicate that the radiative cooling continues into the nightside at a level sufficient to approximate the observed cooling zone width. Above 140 km, where CO2 becomes a minor constituent, another cooling mechanism is needed. It is suggested that this could be vertical diffusion with long mean free path, accompanied by exchange of thermal for potential energy. This could become important on the nightside above 140 km, where the mean free path λ ～ 0.5 km, and λg/cp ～ 5°K. Below 100 km, pressures depend primarily on latitude, which, on the basis of similar conditions in the deeper atmosphere, suggests zonal flow in cyclostrophic balance. Under this assumption, pressure differences between 30 and 60° latitude indicate a peak zonal velocity of ～130 m/sec at the cloud tops. The veocity decreases above this level toward zero near 90 km. The wind profile from the north and night probes is generally similar to that obtained earlier from north-day probe pressure differences. The pressure data thus suggest the existence of two dynamical regimes, a primarily subsolar-antisolar regime above 100 km, and a cyclostrophically balanced zonal regime below 100 km, which is an upward continuation of the circulation regime of the atmosphere below the clouds. |
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