Computer Science
Scientific paper
Nov 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........76b&link_type=abstract
Thesis (PhD). UNIVERSITY OF CALIFORNIA, LOS ANGELES, Source DAI-B 58/10, p. 5440, Apr 1998, 265 pages.
Computer Science
1
Scientific paper
Thermal convection occurs in two distinct layers in Venus' atmosphere. The upper convection layer exists in the cloud region from roughly 48 to 55 km altitude, and the lower convection layer occurs deeper in the atmosphere from 18 to 30 km altitude. A stable layer that supports internal gravity waves exists between these two convection layers. The purpose of this dissertation is to numerically investigate penetrative convection in Venus' atmosphere and to determine the characteristics of convectively generated internal gravity waves. A two- dimensional, nonlinear, fully compressible model of a perfect gas is used to simulate penetrative convection and gravity wave generation in the atmosphere of Venus. Our numerical simulations indicate that cloud-level convection is characterized by cold, narrow downwellings that deeply penetrate the underlying stable layer. Our results show that the Vega balloons drifted in a relatively quiescent part of the convection layer at 54 km altitude. Vertical velocities are three to five times larger in the lower part of the convection layer. Mixing length theory, previously used to parameterize Venus convection, underestimates vertical velocities by a factor of three or more. High Rayleigh number simulations suggest that the cloud-level convection layer may be much thicker in the subsolar region than previously suspected. Convective entrainment of the overlying stable layer by strong downdrafts may explain why cellular features at the cloud tops are predominately found near and downwind of the subsolar point. Strong downward penetrative convection may also be responsible for turbulence observed in Venus' atmosphere at 45 km altitude. Convection in the lower atmosphere is less vigorous than in the cloud region. Cloud-level penetrative convection is therefore the dominant gravity wave source in the stable layer from 30 to 48 km altitude. Convectively generated internal gravity waves have horizontal wavelengths and intrinsic phase speeds similar to cloud- level convection cell sizes and downdraft velocities. However, lower atmosphere convection does interact more strongly with dynamics in the overlying stable layer in the presence of mean wind shear. Vertical wavelengths of simulated gravity waves in the stable layer agree with observations by Pioneer Venus and Magellan spacecraft. Wave-mean flow interaction can be substantial with eastward accelerations as large as 5 m s-1 day-1 in the stable layer. Thus, internal gravity waves generated by cloud-level convection tend to decelerate the Venus superrotation below the clouds.
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