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VEGA Balloon meteorological measurements
ddv. Space Res. Vol. 10. No. 5. pp. (5)109—(5)124. 1990 Printed in Great Britain. All rights reserved.
0273—1177190 $0.00 4 .50 Copyright © 1989 COSPAR
VEGA BALLOON METEOROLOGICAL MEASUREMENTS D. Crisp,* A. P. Ingersoll,* C. E. Hildebrand** and
R. A. Preston** *f~,45170—25, California Institute of Technology, Pasadena, CA 91125, U.S.A. **~45238—700, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, U.S.A.
ABSTRACT The VEGA Balloons obtained in—situ measurements of pressure, temperature, vertical winds, cloud density, ambient illumination, and the frequency of lightfling during their 48 hour flights in the Venus middle cloud layer (50 to 55 km altitude). In addition, the VLBI tracking experiment provided measurements of balloon positions and horizontal winds along their trajectories. We have used these measurements to develop a comprehensive description of the meteorology of the Venus middle cloud layer. The static stability is usually positive, with values ranging from 0 to 2.0 K/km. There is a 6.5 K offset between the VEGA—i and VEGA—2 temperature profiles. This large horizontal temperature gradient is probably associated with an east—west temperature disturbance that drifts with the prevailing winds. Vertical winds are large (1—3 m/s) and variable, with turbulent episodes lasting about one hour. This turbulence is associated with 2. Cloud density decreases upward heat fluxes that range cloud—free from 0 to regions 350 W/m with altitude. No completely were observed. No lightning was detected. VLBI tracking results indicate zonal wind speeds of 69.4 and 66.0 rn/s for VEGA—i and VEGA—2, respectively. VEGA—i observed little meridional transport, but VEGA—2 measured 2.5 m s1 northward winds, which pushed it almost 500 km toward the equator during its flight. INTRODUCTION Two balloons with instrumented gondolas were deployed in the Venus atmosphere on June ii and i5 of 1985, as part of the Soviet VEGA mission /1/. Each balloon made meteorological measurements for almost 48 hours as it drifted from east to west within the turbulent middle cloud region (50—55 km altitude). Instruments~aboard the gondolas obtained in—situ measurements of pressure, temperature, vertical winds, cloud density, ambient illumination and lightning /2/.
In addition, recent results from the Very Long Baseline Interferornetry (VLBI) Tracking Experiment describe the balloon positions and horizontal winds along
their trajectories /3/. Atmospheric properties measured by the balloons are (5)109
D. Crisp et al.
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Fig. i. VEGA—i in—situ meteorological measurements are shown on a common time scale to facilitate comparisons. Quantities shown include (a) pressure, (b) temperature, (c) atmospheric vertical velocity, (d) ambient illumination, and (e) cloud partic e backscatter.
VEGA Balloon Measurements
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summarized in table 1. Details of the balloon trajectories are give in table 2. These observations provide new insight into the mechanisms responsible for heat, mass and momentum transport at~these levels of the Venus atmosphere. OVERVIEW OF THE METEOROLOGICAL MEASUREMENTS Time series of the in—situ measurements obtained by VEGA Balloon i (VEGA— 1) and VEGA Balloon 2 (VEGA—2) are shown in figures i and 2, respectively.
D. Crisp eta!.
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TABLE 1 VEGA Balloon Meteorological Measurements Quantity
Pressure Temperature Vertical Velocity Cloud Backscatter Illumination Lightning Horizontal Velocity Horizontal Position
Accuracy
Sampling Period
0.5 rnb 0.1 K 0.3 rn/s i0~ m~ srm 2 x iO3 lux (3 thresholds) 1.0 rn/s 30 km
75 second 75 second 75 second 75 second 15 minute 10 minute 30—60 minute 2 hour
The pressure records for both balloons show that they rose rapidly from the 900 mbar level (50 km) to their equilibrium float altitudes near 530 mbar (53.5 km) and then slowly descended during the remainder of the mission as helium slowly diffused through their skin. In addition to this large scale structure, there are several periods when the balloons were pushed away from their equilibrium float altitudes by large vertical winds /1/. Both balloons encountered vertical winds larger than 3 rn/s during their flights. Vertical winds were predominantly downward because the balloons floated near the top of a convecting layer (figure 3). At these altitudes, downdrafts are associated with horizontal convergence, while updrafts are associated with horizontal divergence. The balloons were accelerated toward nearby downdrafts. These large vertical winds and the associated large vertical excursions enhanced the science return from the mission since they caused the balloons to sound a much deeper region of the Venus cloud layer than would have been possible in their absence.
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Fig. 3. Cartoon illustrating the trajectory of the VEGA balloons (dashed line) near the top of a convecting region, with small—scale convection cells being advected along with the prevailing winds. Region A is characterized by updrafts and horizontal divergence. Region B is characterized by downdrafts and horizontal convergence.
VEGA Balloon Measurements
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Results from the VLBI tracking experiment and from the ambient illumination sensor show that VEGA—i and VEGA—2 reached the morning terminator at about 36 and 33:45 U.T., respectively. Both balloons encountered enhanced turbulence as they moved into daylight. Unfortunately, the temperature sensor and the balloon envelope were heated by direct sunlight after sunup. These factors seriously compromise the reliably of the measured temperatures and derived vertical wind velocities during this period. We have therefore omitted these periods from all further analysis. TABLE 2 Balloon Trajectories Balloon
Entry Point
Lifetime
Range
VEGA i
+7°il’ Lat i77°48’Lon —6°28’Lat isi°3i’Lon
48.5 hours
109°Lon
47.0 hours
105°Lon
VEGA 2
ATMOSPHERIC STRUCTURE AND STABILITY Variations in pressure and temperature are well correlated throughout the flights of both balloons because these quantities are strong functions of altitude in the middle cloud region. The vertical temperature profiles measured by both balloons were marginally stable, with vertical gradients comparable to the adiabatic lapse rate for an atmosphere composed of 96% CO2 and 4% N2. Figures 4 and 5 show the atmospheric static stability derived from VEGA—i and VEGA—2 measurements, respectively. This quantity is defined as the difference between the observed temperature lapse rate and its adiabatic value: S
=
dT/dz + g/cp
where T is temperature, z is altitude, g is the gravitational acceleration, and c7, is the specific heat at constant pressure. VEGA—i measurements indicate marginally stable conditions, with static stabilities that range from 0 to 2 K/km /4/. VEGA—2 encountered much less stable conditions during its flight. Figure 6 shows the VEGA—i and VEGA—2 temperature data plotted as a function of pressure. The temperatures measured by VEGA—i are about 6.5 K warmer than those measured by VEGA-.2 at the same pressure level. This temperature offset, which was observed at all levels and throughout the flights of the balloons, is comparable to the pole—to—equator temperature difference at these levels. We have searched for systematic errors in instrument calibration or other aspects of the data processing that could introduce such an offset, and found none /5/. The VEGA—i and VEGA—2 temperature measurements have also been confirmed by other simultaneous measurements of engineering temperature sensors in the VEGA Balloon Nephelometers, and by VEGA—2 lauder results. These sensors provide an independent check on the atmospheric temperature observations because they employ different designs and different calibration procedures. The 6.5 K temperature offset is real, and indicates a meridional, zonal, or temporal temperature change in the Venus atmosphere. It is unlikely that this temperature offset is produced by temporal changes in
temperature, because the radiative and dynamical time constants at these levels are much longer than the 4—day period between the flights of the balloons.
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Fig. 4. Atmospheric static stability (K/km) measured by VEGA—i. Each point represents a 30—minute average. The error bars indicate one standard deviation for that period.
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Fig. 5. Same as Figure 4 for VEGA—2.
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VEGA Balloon Measurements
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Vega Bal loon Pressure vs Temperature 34 0
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Fig. 6. VEGA Balloon temperatures are plotted as a function of pressure (squares and triangles). Values for the Pioneer Venus Sounder probe are also shown (solid line). The dashed lines indicate the mean adiabats for VEGA—i and VEGA-2.
This conclusion is supported by the fact that neither balloon temperature profile drifted far from a constant adiabat during the 48—hour flights. Meridional temperature gradients of this magnitude can also be largely ruled out by dynamical considerations /6/. This 6.5 K temperature offset is probably associated with an east—west temperature disturbance that drifts with the prevailing zonal winds. Even though the balloons were deployed symmetrically about the equator, within 14 degrees of latitude of each other (Table 2), they were separated by a much greater east—west distance. Both balloons were deployed near the midnight meridian, but VEGA—2 was deployed four days after VEGA—i. During this period, the prevailing zonal winds (69 m/s) carried that part of the atmosphere sampled by VEGA—i approximately 24000 km away from the midnight meridian. VEGA—2 therefore sampled a region of the atmosphere that was about 135 degrees of longitude away from that sampled by VEGA—i. The 6.5 K temperature difference between the two balloons could easily be accommodated over this large distance. VERTICAL WINDS AND HEAT FLUXES Vertical winds were much stronger and more variable than expected. Time series of the VEGA—i and VEGA—2 vertical winds are shown in figures 1 and 2, respectively. The methods used to derive these winds are described in /4/. Vertical winds with amplitudes of 1 rn/s were common, and winds with amplitudes exceeding 3 rn/s were encountered by both balloons. Time series analysis of
D. Crisp eta!.
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—200 110
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TIME (HOURS U.T.)
Fig. 7. Vertical heat fluxes measured by VEGA—i. Each point represents a 30— minute average. The error bars represent one standard deviation during that period.
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Fig. 8. Same as Figure 7 for VEGA—2. The largest convective heat fluxes were measured while the atmospheric static stability was almost zero (Figure 5).
VEGA Balloon Measurements
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the VEGA Balloon temperature and wind data indicate that positive (negative) temperature anamolies on constant pressure surfaces are associated with upward (downward) motions of the gas, suggesting that these winds are primarily a consequence of free convection. This convection may be driven by thermal radiative heating and cooling of the middle cloud layer. Theoretical modeling results presented in /7/ and /8J indicate that the base of the middle cloud deck is heated as the H2S04 cloud droplets absorb thermal radiation that is emitted by the hot lower atmosphere. The top of the middle cloud deck is cooled as thermat radiation is emitted. to space. The thermal radiative heating of the base, and cooling of the top of the middle cloud layer destablizes the temperature structure and promotes free convection. The convective heat fluxes associated with the turbulence encountered by VEGA—i and VEGA—2 are shown in figures 7 and 8, respectively. Methods used to derive these heat fluxes from VEGA balloon pressure, temperature and vertical velocity measurements are described in /4/. VEGA—i heat.fluxes are 2. almost These always are upward, with amplitudes thatarevary between 10 and with 50 W/m values reasonable, because they roughly consistent the globally— averaged net downward solar flux through this layer of the Venus atmosphere /9/. Vertical heat fluxes derived from VEGA—2 measurements also fall in this range for the first 20 hours of its flight, but then increase dramatically to values exceeding 350 W/m2 as the atmosphere became more turbulent. It is impossible assess the consequences of such large vertical heat fluxes on the thermal structure and general circulation of the Venus atmosphere because we cannot determine the horizontal extent of such regions from VEGA Balloon measurements alone. Further observations of the Venus middle cloud layer are needed to address this question. CLOUDS AND LIGHTNING The VEGA—i nephelometer obtained backscatter measurements during the entire flight of that balloon (figure 1). Preliminary results from this experiment are described in /10/. VEGA—2 obtained no backscatter measurements. The VEGA—i backscatter measurements are comparable to results obtained by previous entry probes, except for a period near 43 U.T., when the VEGA—i values were approximately a factor of two larger than anything previously seen. The backscatter is weakly correlated with pressure, indicating that the cloud density decreases with decreasing pressure (increasing altitude). The cloud particle scale height is approximately 3 km in that part of the middle cloud sampled by VEGA—i. In addition to this vertical stratification, there is a large scale (zonal wave number 1 or 2) variation in the cloud density. There is also a weak correlation between large vertical velocities and increased backscatter, indicating larger cloud densities in the presence of turbulence. No completely cloud—free regions were observed. Both balloons included a transient light detector, designed to record the frequency of lightning as the balloons traversed the night side of the planet /1/. No lightning was detected during the 48 hour flights of either balloon. HORIZONTAL TRAJECTORIES AND WINDS Both VEGA balloons floated about 11000 km from the local midnight meridian into the late morning sky, carried by the strong, predominantly east—west winds. VEGA—I and VEGA—2 were deployed 7°11’north and 6°28’south of the equator, respectively (Table 2). We used VLBI techniques /3/ to determine
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VEGA BALLOON TRAJECTORIES 500 a a a a a a ~ a a a
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Fig. 9. VLBI measurements of the north—south deviation from a constant latitude circle during the flights of VEGA—i and VEGA—2.
the balloon trajectories as well as the zonal (east—west) and meridional (north— south) winds in the Venus middle cloud layer. The trajectories for VEGA—i and VEGA—2 are shown in figure 9. Zonal and meridional winds encountered by the balloons are shown in figures 10 13. The. mean wind speeds derived from the VLBI tracking experiment are: —
VEGA—i VEGA—2
west(m/s) 69.4 ±1.5 66.0 ±1.9
north(m/s) 0.2 ±1.3 2.5 ±1.2
VEGA—i initially encountered weak southward (equatorward) winds, which changed to northward winds later in the mission (figure ii). These meridional winds produced north—south displacements in the.VEGA—1 trajectory that never exceeded 50 km (figure 9). VEGA—2 encountered very different conditions. The VEGA—2 meridional winds were consistently northward, (equatorward) with a mean velocity near 2.5 rn/s (figures 9 and 13). These strong winds pushed VEGA—2 approximately 500 km northward during its flight. .
Figures 10 and 12 show that both balloons experienced westward accelerations between 2 and 25 U.T. These times correspond to Venus local times between midnight and 4 A.M. The Balloons decelerated as they crossed the morning terminator (about 35 U.T.). The northward winds also show long—term systematic variations in velocity that are much larger than the expected errors in the VLBI measurements (1 m/s). These large scale horizontal wind fluctuations could be attributed to either a solar—locked (thermal tide) or surface—locked wave—like disturbance (topographic waves?). Unfortunatly the solar longitude changed too little betweeui the VEGA—i and VEGA—2 flights (about 12 degrees) to distinguish between these two possibilities.
VEGA Balloon Measurements
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WAVENU3.~ER 1 AND 2 FITS TO VEGA—i ZONAL VELOCITIES
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Fig. 10. VEGA—i zonal velocities determined from the VLBI tracking experiment (boxes) are shown along with zonal wavenumber 1 (solid line) and wavenumber 2 (dashed line) regression fits (described in text).
WAVENUMBER 1 AND 2 FITS TO VEGA—i ~RIDI0NAL
VELOCITIES
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D. Crisp era!. WAVENUMEER 1
AND 2 FITS TO VEGA—2 ZONAL VELOCITIES
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Fig. 12. Same as Figure 10 for VEGA—2 zonal velocities. WAVENUMBER 1 AND 2 FITS TO VEGA—2 MERIDIONAL VELOCITIES 5 0
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Fig. 13. Same as Figure ii for VEGA—2 meridional velocities.
To analyze these features, and to facilitate comparisons between small—scale horizontal wind fluxuations and the rapidly—varying in—situ measurements of pressure, temperature, and vertical velocity, we attempted to fit the large—scale wind fluctuations with a zonal wave—number 1 solar—locked disturbance of the form: V = V0 + A cos(~)+ B sin(~) where V is either the eastward or northward wind, ~ is the solar longitude (increasing to the east) and V0, A, and B are coefficients to be fit (in units of m/s). If we define the amplitude and solar phase of the disturbance as follows: 2 + B2)”2 IVIp = (A = arctan(B/A)
VEGA Balloon Measurements
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we find
Ju’I
VEGA—i VEGA—2
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Disturbances in the east—west and north—south winds have comparable amplitudes for both balloons, with the VEGA—i values being somewhat larger. The VEGA—2 measurements show significantly more scatter about the best fit curve. A fit that includes both wave number 1 and wave number 2 components is also shown in Figures iO—i3 (dashed lines). These fits do not account for much more of the observed variance in the wind fields than the wave number 1 fits (solid line). The VEGA—i and VEGA—2 westward wind fluxuations have very similar solar phases. Fluxuations in the northward winds appear to be symmetric about the equator, with solar phases that differ by almost 180 degrees. These factors support the hypothesis that the observed wind fluxuations are related to atmospheric thermal tides, but they do not rule out the possible role of surface-fixed effects. We removed the wave-number i component of the horizontal wind fluctuations to facilitate comparisons between these VLBI measurements and the rapidly— varying in—situ meteorological measurements. These comparisons are presented in figures i4 and 15. There, we display measured atmospheric pressures along with the following quantities: S’/R w’ u’
=
v’
=
S/R
= =
aln(p) b c ln(p) d A u cos(~) B,4 sin(~) u0 A,, cos(~) B,, sin(~) v0 —
—
—
u
—
—
—
—
—
v
—
—
—
where p is pressure, S is entropy, R is the gas constant, w~is the atmospheric vertical velocity, ~ is the solar longitude, and A, B, u,,, v0, a, b, c, and d, are constants obtained from regression fits to the VEGA balloon data sets. S’/R and w’ are those parts of the non—dimensional atmospheric entropy and vertical velocity that are not correlated with pressure. u’ and v’ are the residual zonal and meridional winds, with the wave-number 1 feature and a mean value (u..,, v0) removed. In this form, correlations between S’/R and w’ indicate the presence of a vertical heat flux /4/. Similarly, the degree of correlation between u’ or v’ with other.quantities shown here can provide important insight into the heat and momentum transport in the Venus atmosphere. For example, correlations between variations in u’ or v’ with pressure indicate vertical shears in the horizontal winds. If variations in v’ were correlated with those in S’/R, this would indicate a northward heat transport. Correlations between u’ or v’ with w’ indicate a vertical flux of zonal or meridional momentum, respectively. A cursory inspection of Figures 14 and 15 indicates that there is little correlation between u’ or v’ and the in—situ quantities. A more detailed quantitative study confirms this conclusion. Preliminary results from this study are summarized in the following table:
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Fig. 14. VEGA—i in—situ and VLBI measurements are shown on a common time axis to facilitate comparisons~From the top down, these quantities are the residual meridional wind, v’, the residual. zonal wind, u’, the residual vertical velocity, iii’, the residual entropy, S’/R, and the atmospheric pressure, p. These quantities are defined in the text. du/dz(s’) dv/dz(s’) ~ (kgm’s2) Fh(Wm2) VEGA—i 0.1 ±2.7 —0.4 ±1.9 —0.2 ±1.1 49. ±22. VEGA—2 —1.4 ±4.6 3.1 ±5.1 —1.1 ±1.4 21. ±7.5 where du/dz and dv/dz are the vertical shears in the zonal and meridional winds, respectively, ~Fm = pu’w’ is the vertical flux of zonal momentum, and = pcpv’T’ is the meridional heat flux. We obtained non—zero wind shears and momentum fluxes, but their values are smaller than the computed standard deviation. We may be able to improve these statistics once the VLBI data have been processed further to increase the temporal resolution of the sampling. Our preliminary estimates of the meridional heat fluxes appear to be significant, and iiidicate a small northward transport of heat in both hemispheres. We caution the reader that this result is not as robust as the vertical heat flux results reported in /4/, because these correlations are based on a much smaller data set. Thousands of in—situ measurements were used in the vertical heat flux calculation. The results presented here are based on only about 30 VLBI measurements of horizontal winds for each balloon. SUMMARY AND CONCLUSIONS VEGA Balloon meteorological measurements have not provided answers to many of the important questions about the thermal structure, composition, or general circulation of the Venus atmosphere, but they have radically changed our
VEGA Balloon Measurements
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perception of the processes that may be controlling these atmospheric properties. Before the flights of the balloons, the Venus atmosphere was considered to be a relatively calm, horizontally—uniform system,. characterized by a globally— uniform cloud deck, small horizontal temperature gradients, weak vertical mixing, and very little short time—scale variability that could be called “weather.” The VEGA Balloon measurements provide a completely different picture, with large horizontal temperature gradients near the equator, vigorous convection, and weather conditions that can change dramatically on time scales as short as one hour. These measurements have also provided intriguing clues about the possible role of atmospheric thermal tides and topographically—forced waves at levels within the middle cloud deck. Further observational and theoretical investigations are now needed to take advantage of this new insight to improve our understanding of this intriguing planetary atmosphere. ACKNOWLEDGEMENTS We gratefully acknowledge the moral and financial support of Dr. Henry Brinton and the NASA Planetary Atmospheres Program (NAGW—58). This work was also supported by the Jet Propulsion Laboratory. Finally, we wish to acknowledge the assistance of the staffs of the 20 observatories around the world that participated in the VLBI experiment. Contribution number 4715 from the Division of Geological and Planetary Sciences, California Institute of Technology. JASR 10:5-I
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REFERENCES 1. R. Z. Sagdeev, V. M. Linkin, V. V. Kerzhanovich, A. N. Lipatov, A. A. Shurupov, J. E. Blamont, D. Crisp, A. P. Ingersoll, L. S. Elson, R. A. Preston, C. E. Hhldebrand, B. Ragent, A. Seiff, R. E. Young, G. Petit, L. Boloh, Yu. N. Alexandrov, N. A. Armand, R. V. Bakitko, and A. S. Selivanov, Overview of VEGA Venus Balloon in—situ meteorological measurements, Science, 231, 1411—1414 (1986). 2. R. S. Kremnev, V. M. Linkin, A. N. Lipatov, K. M. Pichkadze, A. A. Shurupov, A. V. Terterashvili, R. V. Bakitko, J. E. Blamont, C. Malique, B. Ragent, R. A. Preston, L. S. Elson, and D. Crisp, VEGA Balloon system and instrumentation, Science, 231, 1408—1411 (1986). 3. R. A. Preston, C. E. Hildebrand, G. H. Purcell, J. Ellis, C. T. Stelzried, S. G. Finley, R. Z. Sagdeev, V. M. Linkin, V. V. Kerzhanovich, V. I. Altunin, L. R. Kogan, V. I. .Kostenko, L. I. Matveenko, 5; V. Pogrebenko, I. A. Strukov, E. L. Akim, Uu. N. Alexandrov, N. A. Armand, R. N. Bakitko, A. S. Vyshlov, A. F. Bogomoluv, Yu. N. Gorchankov, A. S. Selivanov; N. M. Ivanov, V. F. Tichonov, J. E. Blamont, L. Boloh, G. Laurans, A. Boischot, F. Biraud, A. Ortega—Molina. C. Rosolen, and G. Petit, Determination of Venus winds by ground—based radio tracking of the VEGA Balloons, Science, 231, i414—i416 (1986). 4. A. P. Ingersoll, D. Crisp, A. Grossman, and the Venus Balloon Science Team, Estimates of convective heat fluxes and gravity wave amplitudes in the Venus middle cloud layer from VEGA Balloon measurements, Adv..Space Re.,., ~, 343— 349 (1987). 5. V. M. Linkin, V. V. Kerzhanovich, A. N. Lipatov,.A. A. Shurupov, A. Seiff, B. Ragent, R. E. Young, A. P. Ingersoll, D. Crisp, L. S. Elson, R. A. Preston, and J. E. Blamont, Thermal structure of the Venus atmosphere in the middle cloud layer, Science, 231, 1420—1422 (1986). 6. J. E. Blamont, R. E. Young, A. Seiff,~B. Ragent, R. Sagdeev, V. M. Linkin, V. V. Kerzhanovich, A. P. Ingersoll, D. Crisp, L. S. Elson, R. A. Preston, G. S. Golitsyn, V. N. Ivanov, Implications of the VEGA Balloon results for Venus atmospheric dynamics, Science, 231, 1422—i425 (1986). 7. J. B. Pollack, 0. B. Toon, and R. Boese, Greenhouse models of Venus’ high surface temperature, J. Geophys. Res., 85, 8223—8231 (1980). 8. D. Crisp, Radiative forcing of the Venus mesosphere II: Thermal fluxes, cooling rates and radiative equilibrium temperatures, Icarus, in press. 9. M. G. Tomasko, L. R. Doose, P. H. Smith, and A. P. Odell, Measurements of the flux ofsunlight in the atmosphere of Venus,. J. Geophys. Res., 85, 8167—8186 (1980). 10. B. Ragent, and the VEGA Balloon Science Team, Results of the VEGA 1 balloon nephelometer experiment, Adv. Space Res., 7, (1987).
0273—1177190 $0.00 4 .50 Copyright © 1989 COSPAR
VEGA BALLOON METEOROLOGICAL MEASUREMENTS D. Crisp,* A. P. Ingersoll,* C. E. Hildebrand** and
R. A. Preston** *f~,45170—25, California Institute of Technology, Pasadena, CA 91125, U.S.A. **~45238—700, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, U.S.A.
ABSTRACT The VEGA Balloons obtained in—situ measurements of pressure, temperature, vertical winds, cloud density, ambient illumination, and the frequency of lightfling during their 48 hour flights in the Venus middle cloud layer (50 to 55 km altitude). In addition, the VLBI tracking experiment provided measurements of balloon positions and horizontal winds along their trajectories. We have used these measurements to develop a comprehensive description of the meteorology of the Venus middle cloud layer. The static stability is usually positive, with values ranging from 0 to 2.0 K/km. There is a 6.5 K offset between the VEGA—i and VEGA—2 temperature profiles. This large horizontal temperature gradient is probably associated with an east—west temperature disturbance that drifts with the prevailing winds. Vertical winds are large (1—3 m/s) and variable, with turbulent episodes lasting about one hour. This turbulence is associated with 2. Cloud density decreases upward heat fluxes that range cloud—free from 0 to regions 350 W/m with altitude. No completely were observed. No lightning was detected. VLBI tracking results indicate zonal wind speeds of 69.4 and 66.0 rn/s for VEGA—i and VEGA—2, respectively. VEGA—i observed little meridional transport, but VEGA—2 measured 2.5 m s1 northward winds, which pushed it almost 500 km toward the equator during its flight. INTRODUCTION Two balloons with instrumented gondolas were deployed in the Venus atmosphere on June ii and i5 of 1985, as part of the Soviet VEGA mission /1/. Each balloon made meteorological measurements for almost 48 hours as it drifted from east to west within the turbulent middle cloud region (50—55 km altitude). Instruments~aboard the gondolas obtained in—situ measurements of pressure, temperature, vertical winds, cloud density, ambient illumination and lightning /2/.
In addition, recent results from the Very Long Baseline Interferornetry (VLBI) Tracking Experiment describe the balloon positions and horizontal winds along
their trajectories /3/. Atmospheric properties measured by the balloons are (5)109
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Fig. i. VEGA—i in—situ meteorological measurements are shown on a common time scale to facilitate comparisons. Quantities shown include (a) pressure, (b) temperature, (c) atmospheric vertical velocity, (d) ambient illumination, and (e) cloud partic e backscatter.
VEGA Balloon Measurements
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summarized in table 1. Details of the balloon trajectories are give in table 2. These observations provide new insight into the mechanisms responsible for heat, mass and momentum transport at~these levels of the Venus atmosphere. OVERVIEW OF THE METEOROLOGICAL MEASUREMENTS Time series of the in—situ measurements obtained by VEGA Balloon i (VEGA— 1) and VEGA Balloon 2 (VEGA—2) are shown in figures i and 2, respectively.
D. Crisp eta!.
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TABLE 1 VEGA Balloon Meteorological Measurements Quantity
Pressure Temperature Vertical Velocity Cloud Backscatter Illumination Lightning Horizontal Velocity Horizontal Position
Accuracy
Sampling Period
0.5 rnb 0.1 K 0.3 rn/s i0~ m~ srm 2 x iO3 lux (3 thresholds) 1.0 rn/s 30 km
75 second 75 second 75 second 75 second 15 minute 10 minute 30—60 minute 2 hour
The pressure records for both balloons show that they rose rapidly from the 900 mbar level (50 km) to their equilibrium float altitudes near 530 mbar (53.5 km) and then slowly descended during the remainder of the mission as helium slowly diffused through their skin. In addition to this large scale structure, there are several periods when the balloons were pushed away from their equilibrium float altitudes by large vertical winds /1/. Both balloons encountered vertical winds larger than 3 rn/s during their flights. Vertical winds were predominantly downward because the balloons floated near the top of a convecting layer (figure 3). At these altitudes, downdrafts are associated with horizontal convergence, while updrafts are associated with horizontal divergence. The balloons were accelerated toward nearby downdrafts. These large vertical winds and the associated large vertical excursions enhanced the science return from the mission since they caused the balloons to sound a much deeper region of the Venus cloud layer than would have been possible in their absence.
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Fig. 3. Cartoon illustrating the trajectory of the VEGA balloons (dashed line) near the top of a convecting region, with small—scale convection cells being advected along with the prevailing winds. Region A is characterized by updrafts and horizontal divergence. Region B is characterized by downdrafts and horizontal convergence.
VEGA Balloon Measurements
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Results from the VLBI tracking experiment and from the ambient illumination sensor show that VEGA—i and VEGA—2 reached the morning terminator at about 36 and 33:45 U.T., respectively. Both balloons encountered enhanced turbulence as they moved into daylight. Unfortunately, the temperature sensor and the balloon envelope were heated by direct sunlight after sunup. These factors seriously compromise the reliably of the measured temperatures and derived vertical wind velocities during this period. We have therefore omitted these periods from all further analysis. TABLE 2 Balloon Trajectories Balloon
Entry Point
Lifetime
Range
VEGA i
+7°il’ Lat i77°48’Lon —6°28’Lat isi°3i’Lon
48.5 hours
109°Lon
47.0 hours
105°Lon
VEGA 2
ATMOSPHERIC STRUCTURE AND STABILITY Variations in pressure and temperature are well correlated throughout the flights of both balloons because these quantities are strong functions of altitude in the middle cloud region. The vertical temperature profiles measured by both balloons were marginally stable, with vertical gradients comparable to the adiabatic lapse rate for an atmosphere composed of 96% CO2 and 4% N2. Figures 4 and 5 show the atmospheric static stability derived from VEGA—i and VEGA—2 measurements, respectively. This quantity is defined as the difference between the observed temperature lapse rate and its adiabatic value: S
=
dT/dz + g/cp
where T is temperature, z is altitude, g is the gravitational acceleration, and c7, is the specific heat at constant pressure. VEGA—i measurements indicate marginally stable conditions, with static stabilities that range from 0 to 2 K/km /4/. VEGA—2 encountered much less stable conditions during its flight. Figure 6 shows the VEGA—i and VEGA—2 temperature data plotted as a function of pressure. The temperatures measured by VEGA—i are about 6.5 K warmer than those measured by VEGA-.2 at the same pressure level. This temperature offset, which was observed at all levels and throughout the flights of the balloons, is comparable to the pole—to—equator temperature difference at these levels. We have searched for systematic errors in instrument calibration or other aspects of the data processing that could introduce such an offset, and found none /5/. The VEGA—i and VEGA—2 temperature measurements have also been confirmed by other simultaneous measurements of engineering temperature sensors in the VEGA Balloon Nephelometers, and by VEGA—2 lauder results. These sensors provide an independent check on the atmospheric temperature observations because they employ different designs and different calibration procedures. The 6.5 K temperature offset is real, and indicates a meridional, zonal, or temporal temperature change in the Venus atmosphere. It is unlikely that this temperature offset is produced by temporal changes in
temperature, because the radiative and dynamical time constants at these levels are much longer than the 4—day period between the flights of the balloons.
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VEGA Balloon Measurements
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This conclusion is supported by the fact that neither balloon temperature profile drifted far from a constant adiabat during the 48—hour flights. Meridional temperature gradients of this magnitude can also be largely ruled out by dynamical considerations /6/. This 6.5 K temperature offset is probably associated with an east—west temperature disturbance that drifts with the prevailing zonal winds. Even though the balloons were deployed symmetrically about the equator, within 14 degrees of latitude of each other (Table 2), they were separated by a much greater east—west distance. Both balloons were deployed near the midnight meridian, but VEGA—2 was deployed four days after VEGA—i. During this period, the prevailing zonal winds (69 m/s) carried that part of the atmosphere sampled by VEGA—i approximately 24000 km away from the midnight meridian. VEGA—2 therefore sampled a region of the atmosphere that was about 135 degrees of longitude away from that sampled by VEGA—i. The 6.5 K temperature difference between the two balloons could easily be accommodated over this large distance. VERTICAL WINDS AND HEAT FLUXES Vertical winds were much stronger and more variable than expected. Time series of the VEGA—i and VEGA—2 vertical winds are shown in figures 1 and 2, respectively. The methods used to derive these winds are described in /4/. Vertical winds with amplitudes of 1 rn/s were common, and winds with amplitudes exceeding 3 rn/s were encountered by both balloons. Time series analysis of
D. Crisp eta!.
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Fig. 7. Vertical heat fluxes measured by VEGA—i. Each point represents a 30— minute average. The error bars represent one standard deviation during that period.
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Fig. 8. Same as Figure 7 for VEGA—2. The largest convective heat fluxes were measured while the atmospheric static stability was almost zero (Figure 5).
VEGA Balloon Measurements
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the VEGA Balloon temperature and wind data indicate that positive (negative) temperature anamolies on constant pressure surfaces are associated with upward (downward) motions of the gas, suggesting that these winds are primarily a consequence of free convection. This convection may be driven by thermal radiative heating and cooling of the middle cloud layer. Theoretical modeling results presented in /7/ and /8J indicate that the base of the middle cloud deck is heated as the H2S04 cloud droplets absorb thermal radiation that is emitted by the hot lower atmosphere. The top of the middle cloud deck is cooled as thermat radiation is emitted. to space. The thermal radiative heating of the base, and cooling of the top of the middle cloud layer destablizes the temperature structure and promotes free convection. The convective heat fluxes associated with the turbulence encountered by VEGA—i and VEGA—2 are shown in figures 7 and 8, respectively. Methods used to derive these heat fluxes from VEGA balloon pressure, temperature and vertical velocity measurements are described in /4/. VEGA—i heat.fluxes are 2. almost These always are upward, with amplitudes thatarevary between 10 and with 50 W/m values reasonable, because they roughly consistent the globally— averaged net downward solar flux through this layer of the Venus atmosphere /9/. Vertical heat fluxes derived from VEGA—2 measurements also fall in this range for the first 20 hours of its flight, but then increase dramatically to values exceeding 350 W/m2 as the atmosphere became more turbulent. It is impossible assess the consequences of such large vertical heat fluxes on the thermal structure and general circulation of the Venus atmosphere because we cannot determine the horizontal extent of such regions from VEGA Balloon measurements alone. Further observations of the Venus middle cloud layer are needed to address this question. CLOUDS AND LIGHTNING The VEGA—i nephelometer obtained backscatter measurements during the entire flight of that balloon (figure 1). Preliminary results from this experiment are described in /10/. VEGA—2 obtained no backscatter measurements. The VEGA—i backscatter measurements are comparable to results obtained by previous entry probes, except for a period near 43 U.T., when the VEGA—i values were approximately a factor of two larger than anything previously seen. The backscatter is weakly correlated with pressure, indicating that the cloud density decreases with decreasing pressure (increasing altitude). The cloud particle scale height is approximately 3 km in that part of the middle cloud sampled by VEGA—i. In addition to this vertical stratification, there is a large scale (zonal wave number 1 or 2) variation in the cloud density. There is also a weak correlation between large vertical velocities and increased backscatter, indicating larger cloud densities in the presence of turbulence. No completely cloud—free regions were observed. Both balloons included a transient light detector, designed to record the frequency of lightning as the balloons traversed the night side of the planet /1/. No lightning was detected during the 48 hour flights of either balloon. HORIZONTAL TRAJECTORIES AND WINDS Both VEGA balloons floated about 11000 km from the local midnight meridian into the late morning sky, carried by the strong, predominantly east—west winds. VEGA—I and VEGA—2 were deployed 7°11’north and 6°28’south of the equator, respectively (Table 2). We used VLBI techniques /3/ to determine
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the balloon trajectories as well as the zonal (east—west) and meridional (north— south) winds in the Venus middle cloud layer. The trajectories for VEGA—i and VEGA—2 are shown in figure 9. Zonal and meridional winds encountered by the balloons are shown in figures 10 13. The. mean wind speeds derived from the VLBI tracking experiment are: —
VEGA—i VEGA—2
west(m/s) 69.4 ±1.5 66.0 ±1.9
north(m/s) 0.2 ±1.3 2.5 ±1.2
VEGA—i initially encountered weak southward (equatorward) winds, which changed to northward winds later in the mission (figure ii). These meridional winds produced north—south displacements in the.VEGA—1 trajectory that never exceeded 50 km (figure 9). VEGA—2 encountered very different conditions. The VEGA—2 meridional winds were consistently northward, (equatorward) with a mean velocity near 2.5 rn/s (figures 9 and 13). These strong winds pushed VEGA—2 approximately 500 km northward during its flight. .
Figures 10 and 12 show that both balloons experienced westward accelerations between 2 and 25 U.T. These times correspond to Venus local times between midnight and 4 A.M. The Balloons decelerated as they crossed the morning terminator (about 35 U.T.). The northward winds also show long—term systematic variations in velocity that are much larger than the expected errors in the VLBI measurements (1 m/s). These large scale horizontal wind fluctuations could be attributed to either a solar—locked (thermal tide) or surface—locked wave—like disturbance (topographic waves?). Unfortunatly the solar longitude changed too little betweeui the VEGA—i and VEGA—2 flights (about 12 degrees) to distinguish between these two possibilities.
VEGA Balloon Measurements
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WAVENU3.~ER 1 AND 2 FITS TO VEGA—i ZONAL VELOCITIES
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To analyze these features, and to facilitate comparisons between small—scale horizontal wind fluxuations and the rapidly—varying in—situ measurements of pressure, temperature, and vertical velocity, we attempted to fit the large—scale wind fluctuations with a zonal wave—number 1 solar—locked disturbance of the form: V = V0 + A cos(~)+ B sin(~) where V is either the eastward or northward wind, ~ is the solar longitude (increasing to the east) and V0, A, and B are coefficients to be fit (in units of m/s). If we define the amplitude and solar phase of the disturbance as follows: 2 + B2)”2 IVIp = (A = arctan(B/A)
VEGA Balloon Measurements
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Disturbances in the east—west and north—south winds have comparable amplitudes for both balloons, with the VEGA—i values being somewhat larger. The VEGA—2 measurements show significantly more scatter about the best fit curve. A fit that includes both wave number 1 and wave number 2 components is also shown in Figures iO—i3 (dashed lines). These fits do not account for much more of the observed variance in the wind fields than the wave number 1 fits (solid line). The VEGA—i and VEGA—2 westward wind fluxuations have very similar solar phases. Fluxuations in the northward winds appear to be symmetric about the equator, with solar phases that differ by almost 180 degrees. These factors support the hypothesis that the observed wind fluxuations are related to atmospheric thermal tides, but they do not rule out the possible role of surface-fixed effects. We removed the wave-number i component of the horizontal wind fluctuations to facilitate comparisons between these VLBI measurements and the rapidly— varying in—situ meteorological measurements. These comparisons are presented in figures i4 and 15. There, we display measured atmospheric pressures along with the following quantities: S’/R w’ u’
=
v’
=
S/R
= =
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where p is pressure, S is entropy, R is the gas constant, w~is the atmospheric vertical velocity, ~ is the solar longitude, and A, B, u,,, v0, a, b, c, and d, are constants obtained from regression fits to the VEGA balloon data sets. S’/R and w’ are those parts of the non—dimensional atmospheric entropy and vertical velocity that are not correlated with pressure. u’ and v’ are the residual zonal and meridional winds, with the wave-number 1 feature and a mean value (u..,, v0) removed. In this form, correlations between S’/R and w’ indicate the presence of a vertical heat flux /4/. Similarly, the degree of correlation between u’ or v’ with other.quantities shown here can provide important insight into the heat and momentum transport in the Venus atmosphere. For example, correlations between variations in u’ or v’ with pressure indicate vertical shears in the horizontal winds. If variations in v’ were correlated with those in S’/R, this would indicate a northward heat transport. Correlations between u’ or v’ with w’ indicate a vertical flux of zonal or meridional momentum, respectively. A cursory inspection of Figures 14 and 15 indicates that there is little correlation between u’ or v’ and the in—situ quantities. A more detailed quantitative study confirms this conclusion. Preliminary results from this study are summarized in the following table:
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Fig. 14. VEGA—i in—situ and VLBI measurements are shown on a common time axis to facilitate comparisons~From the top down, these quantities are the residual meridional wind, v’, the residual. zonal wind, u’, the residual vertical velocity, iii’, the residual entropy, S’/R, and the atmospheric pressure, p. These quantities are defined in the text. du/dz(s’) dv/dz(s’) ~ (kgm’s2) Fh(Wm2) VEGA—i 0.1 ±2.7 —0.4 ±1.9 —0.2 ±1.1 49. ±22. VEGA—2 —1.4 ±4.6 3.1 ±5.1 —1.1 ±1.4 21. ±7.5 where du/dz and dv/dz are the vertical shears in the zonal and meridional winds, respectively, ~Fm = pu’w’ is the vertical flux of zonal momentum, and = pcpv’T’ is the meridional heat flux. We obtained non—zero wind shears and momentum fluxes, but their values are smaller than the computed standard deviation. We may be able to improve these statistics once the VLBI data have been processed further to increase the temporal resolution of the sampling. Our preliminary estimates of the meridional heat fluxes appear to be significant, and iiidicate a small northward transport of heat in both hemispheres. We caution the reader that this result is not as robust as the vertical heat flux results reported in /4/, because these correlations are based on a much smaller data set. Thousands of in—situ measurements were used in the vertical heat flux calculation. The results presented here are based on only about 30 VLBI measurements of horizontal winds for each balloon. SUMMARY AND CONCLUSIONS VEGA Balloon meteorological measurements have not provided answers to many of the important questions about the thermal structure, composition, or general circulation of the Venus atmosphere, but they have radically changed our
VEGA Balloon Measurements
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perception of the processes that may be controlling these atmospheric properties. Before the flights of the balloons, the Venus atmosphere was considered to be a relatively calm, horizontally—uniform system,. characterized by a globally— uniform cloud deck, small horizontal temperature gradients, weak vertical mixing, and very little short time—scale variability that could be called “weather.” The VEGA Balloon measurements provide a completely different picture, with large horizontal temperature gradients near the equator, vigorous convection, and weather conditions that can change dramatically on time scales as short as one hour. These measurements have also provided intriguing clues about the possible role of atmospheric thermal tides and topographically—forced waves at levels within the middle cloud deck. Further observational and theoretical investigations are now needed to take advantage of this new insight to improve our understanding of this intriguing planetary atmosphere. ACKNOWLEDGEMENTS We gratefully acknowledge the moral and financial support of Dr. Henry Brinton and the NASA Planetary Atmospheres Program (NAGW—58). This work was also supported by the Jet Propulsion Laboratory. Finally, we wish to acknowledge the assistance of the staffs of the 20 observatories around the world that participated in the VLBI experiment. Contribution number 4715 from the Division of Geological and Planetary Sciences, California Institute of Technology. JASR 10:5-I
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