Adv. SpaceRes.Vol. 7, No. 12, pp. (12)303—(12)305,1987 0273—1177/87$O.OO+ .50Printedin GreatBritain. All rights reserved. Copyright© 1987COSPAR

IMPLICATIONS OF THE VEGA BALLOONRESULTSFOR VENUS ATMOSPHERICDYNAMICS

R. Young,* The VEGA Balloon ScienceTeam,R. Walterscheid**andG. Schubert***

*NASAAme5ResearchCenter,Moffett Field, CA 94035, U.S.A.* *TheAerospaceCorporation, Los Angeles,CA 90009, U.S.A.*** Universityof California, Los Angeles,CA 90024, U.S.A.

ABSTRACT

The VEGA Venus Balloon Mission returned data on the themodynamic state of the atmosphere to-gether with wind and cloud information. In this invited paper we review possible explana-tions for three aspects of the data: 1) the large amplitude atmospheric vertical winds en-countered by the VEGA balloons; 2) the observed 6.5 K temperature difference consistentlymeasured between the two VEGAballoons; and 3) the apparent influence of surface topographyon atmospheric motions seen by the VEGA—2balloon as it flew over the mountainous terrainknown as Aphrodite.

VERTICAL WINDS AND STATIC STABILITY

The VEGA balloon vertical wind data have been described in /1/ and the implications dis-cussed in /2/. The significance of the vertical wind comes from the fact that vertical mo-mentum and heat transport, which are derived from products of vertical velocity with hori-zontal wind and temperature, respectively, are among the principal quantities that deter-mine the characteristics of the general circulation. In addition, vertical velocities in aregion of relatively low static stability can affect the circulation by generating atmo-spheric waves as vertical motions penetrate surrounding regions of the atmosphere havingrelatively high static stability. Such waves can propagate to other regions of the atmo-sphere and may therefore affect momentum distributions at levels well away from the localregion. The initial balloon float altitudes near 53 km are in the region of the atmosphereidentified by the VEGA—2 lander /3/ and Pioneer Venus /4/ as having nearly neutral staticstability, that is, nearly adiabatic lapse rate of temperature. It is in such neutrallystable regions where one expects relatively large yertical winds. The balloons commonly en-countered yertical winds which exceeded 0.5 m sec , with peak amplitudes reaching 2.5 to3.5 m sec /2/.

Two possible causes for the neutrally stable region in the middle cloud are the following.The first is shear instability, that is, a dynamical instability resulting when the ratioof the square of the Brunt—Vaisalla frequency to the square of vertical gradient of the zo—nal wind is less than 0.25. When this instability occurs, the expected result is a regionof near neutral static stability and small vertical gradient in the zonal wind, even thoughthe initial state may have had large values for both these quantities. A second cause isthermal convection induced by radiative heating of the middle cloud levels by atmosphericlayers below /5/. One way to distinguish these processes is to determine the sign of thevertical heat flux, which is proportional to the product of atmospheric vertical velocityand temperature. Thermal convection is characterized by a net upward heat flux, while theheat flux associated with shear instability is expected to be downward and perhaps near ze-ro. Present estimates of the vertical heat flux implied by the VEGA balloon data indicate anupward heat flux /6/, and hence thermal convection.

If thermal convection is the mechanism producing the vertical winds, order of magnitude es-timates of vertical wind magnitude can be made and compared to the winds actually encountered.Amplitudes of the observed vertical winds are generally consistent with those estimatedfrom thermal convection mixing length theory, which gives an estimate of average values ofquantities associated with convective eddies /2/:

Iwl “- (_~2L.)‘~/3~ j

where w is vertical wind, F is con~ective heat flux, taken to be the global average net sol-ar heat flux at 54 km (40 watts m ), g is the gravitational acceleration, ~ is density atthe float altitude of the balloon, C~, is specific heat at constant pressure, and T is tern-

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(12)304 R. Young et a!.

perature corresponding toe. The justification for the choice of F is given in /2/. Themixing length, 1, was chosen as 5 km (about the thickness of the neutrally stable layer),but a mixing length of 1 km reduces the estimate for twf by less than a factor_1f 2. Thevertical wind velocities are generally consistent with the estimate of 1 m sec

6.5 K TEMPERATUREOFFSET

The 6.5 K temperature difference at a given pressure level consistently observed betweenthe two VEGA balloons /7/ implies that: (i) there are measurable hemispheric asyninetriesbetween the northern and southern hemispheres over the approximate 14° latitude separationof the balloons, or (ii) temporal or longitudinal variations, or both, exist in the state ofthe atmosphere. The temperature offset of 6.5 K is much larger than can be accounted forby turbulent mixing processes in the neutrally stable layer. Nor is it likely that the bal-loons are sampling different small scale atmospheric parcels originating in adjacent stat-ically stable layers of the atmosphere, since it would be difficult to maintain a 6.5 Ktemperature difference in this way over a time period of several days. An atmospheric par-cel with a horizontal scale,L , would have a lifetime the order of L/tuI , where w is ahorizontal velocity fluctuation typical of that associated with small scale motions in theneutrally stable region. Since horizontal length scales would be comparable to verticallength scales for mot{ons associated with either thermal convection or shear instability,I~I~Iw/ ~ 1 m sec . Thus the lifetime for a fluid parcel with L < 100 km would be lessthan 1 day. Hence it must be concluded that larger scale (that is, synoptic or planetaryscale) processes are responsible for producing the 6.5 K temperature difference betweenVEGA—i and VEGA—2. However, this difference is not representative of a zonally (east-west)averaged hemispheric asymmetry because the magnitude is much too large. When the tempera-ture difference and latitude separation between the balloons are used to compute the meanmeridional temperature gradient, the vertical derivative of the zonally averaged meridionalmomentum equation implies either vertical shears in the mean zonal wind or mean meridionalvelocities an order of magnitude larger than observed /8,9/.

The remaining possibility is that the balloons are sampling an atmospheric wave, but at a—bout 180° difference in phase. If the wave has a phase speed in the zonal direction ap-proximately the same as the mean zonal wind, the balloons would measure a nearly constanttemperature difference as a function of time. Waves with zonal phase speeds comparable tothe mean zortal wind at balloon float altitudes have been computed theoretically /10,11/,and have been observed in the UV features by the Pioneer Venus orbiter /12/. The wave amp-litude would have to be about 3.5 K if the positions of the balloons corresponded to a 180°phase difference, and the wave would have to have a lifetime of at least several days. Ifthe vertical wavelength of the wave was about an atmospheric scale hei~t, H, and the zonalphase speed relative to the mean zonal wind was approximately 10 m sec , the amplitude of_1the wave zonal velocity estimated from the zonal momentum equation would be about 10 m secwhich is a reasonable value. However, a wave with temperature amplitude 3.5 K and verticalwavelength A ~ H 6 km, has a maximum vertical temperature gradient of ~ 3.7 K/km, andhence the wave would have measureable effects on the static stability, especially in re-gions where the background stability was small. Oscillations of static stability with al-titude between 45 and 60 km altitude are evident in the static stability profiles inferredfrom Pioneer Venus (4) and the VEGA—2lander /13/. Whether or not these oscillations aredue to a vertically propagating wave remains to be determined. In any case, more study isrequired concerning wave dynamics in and near the neutrally stable region of the backgroundatmosphere before a quantitative explanation of the 6.5 K temperature offset can be given.

INFLUENCE OF SURFACE TOPOGRAPHYON ATMOSPHERICMOTIONS

One of the most interesting results returned from the VEGA balloon mission was the indica-tion that atmospheric motions at balloon float altitudes near 53 km are influenced by sur-face topography associated with high mountain ranges /2/. As the VEGA-2 balloon overflewthe mountainous region known as Aphrodite, about 70 south of the equator, it encountered en-hanced vertical winds and relatively large excursions in temperature and pressure /2,3/.At the same time, doppler tracking of the balloon indicated much larger horizontal windvariations than at any other time period during either balloon flight /14/. The VEGA—iballoon, which stayed at approximately 7°North latitude as it circled the planet, did notoverfly any high mountainous regions and did not encounter a set of events analagous tothose seen by VEGA—2over Aphrodite. In /2/ it is suggested that gravity waves, generatedat the surface of Venus by horizontal flow over the surface relief of Aphrodite, propagatevertically and reach cloud level altitudes with sufficient amplitude to account for theVEGA-2 observations. It is well known, for example, that gravity waves generated by sur-face topography are important for the dynamics of the Earth’s stratosphere and mesosphere/15/.

Schubert and Walterscheid /16/ investigated the propagation characteristics of gravitywaves in the Venus atmosphere using a three—dimensional model for flow in a rionrotatirtg,plane-parallel atmosphere. They showed that for vertically varying Venusian static sta-bility and mean zonal wind profiles, certain gravity waves forced at the surface were

Venus Dynamics (12)305

capable of reaching cloud level altitudes, and altitudes well beyond. In their study, wavesgenerated by surface topography, and hence stationary with respect to the surface, were notconsidered, but there is no reason to suspect that stationary waves could not also propogateto high altitudes. In /13/ such waves are considered using the same model as /16/, and itis shown that for the static stability profile implied by the VEGA-2 lander data /17/, sta-tionary waves can indeed reach cloud level altitudes and above with relatively large ampli-tude. The amplification of waves at a particular altitude relative to their amplitudes atthe surface depends on the surface wind speed, the vertical profile of the zonal wind, thehorizontal wavelength of the wave, and, of course, the static stability profile between thesurface and the given altitude. The principal result from /13/ is that vertical winds ofthe magnitude encountered by the VEGA-2 balloon over Aphrodite can be the result of surfaceinduced gravity waves. Thus, gravity waves induced by surface topography may affect the dy-namics of the atmosphere of Venus in ways similar to those that occur in the terrestrial at-mosphere.

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