Observable effects of convection and gravity waves on the Venus condensational cloud

We present results of a simple two-dimensional model investigating the observable effects that convective motions and gravity waves can have on the condensational Venus cloud. Gravity waves have been observed in the Venus atmosphere in the form of temperature scintillations in the Magellan and Pioneer Venus occultation data. Multiple in situ probes and long-duration remote observations indicate the presence of convective motions in the Venus clouds. Dynamical studies by others have suggested that gravity waves can exist in the stable regions of the Venus atmosphere above the middle clouds and beneath the middle clouds, and likely are triggered by flow past sub-cloud plumes caused by convective overshooting. We find that a simplified treatment of convective kinematics generates variation in the Venus condensational cloud consistent with the observed variability of optical depth and brightness temperature. Specifically, we find that the downdraft regions in our simulated convective cell exhibit a decrease in cloud optical depth of around Δτ∼10. The brightness temperature ranges from about 460 K in the downdraft regions of the simulated convective cells, to about 400 K in the simulated updrafts. We also find that gravity waves launched by obstacles (such as overshooting convective plumes) near the cloud base exhibit horizontal wavelengths comparable to the separation between convective cells, and generate variations in brightness temperature that should be observable by instruments such as VIRTIS on Venus Express. However, a more robust treatment of the atmospheric dynamics is needed to address adequately these interactions between the clouds and the mesoscale dynamics.     

Modeling the effects of shear on the evolution of the holes in the condensational clouds of Venus

Near-infrared brightness temperature contrasts observed on the night side of Venus indicate variations in the size and distribution of particles in the lower and middle cloud decks. McGouldrick and Toon [McGouldrick, K., Toon, O.B., 2007. Icarus 191, 1-24] have shown that these changes can be explained by large-scale dynamics; in particular, that downdrafts may produce optical depth “holes” in the clouds. The lifetimes of these holes are observed to be moderately short, on the order of ten days. Here, we explore a simple model to better understand this lifetime. We have coupled a microphysical model of the Venus clouds with a simple, two-dimensional (zonal, vertical) kinematical transport model to study the effects of the zonal flow on the lifetime of the holes in the clouds. We find that although wind shear may be negligible within the cloud itself, the shear that is present near the top and the bottom of the statically unstable cloud region can lead to changes in the radiative-dynamical feedback which ultimately lead to the dissipation of the holes.     

Morphology of the cloud tops as observed by the Venus Express Monitoring Camera

Since the discovery of ultraviolet markings on Venus, their observations have been a powerful tool to study the morphology, motions and dynamical state at the cloud top level. Here we present the results of investigation of the cloud top morphology performed by the Venus Monitoring Camera (VMC) during more than 3 years of the Venus Express mission. The camera acquires images in four narrow-band filters centered at 365, 513, 965 and 1010 nm with spatial resolution from 50 km at apocentre to a few hundred of meters at pericentre. The VMC experiment provides a significant improvement in the Venus imaging as compared to the capabilities of the earlier missions. The camera discovered new cloud features like bright “lace clouds” and cloud columns at the low latitudes, dark polar oval and narrow circular and spiral “grooves” in the polar regions, different types of waves at the high latitudes. The VMC observations revealed detailed structure of the sub-solar region and the afternoon convective wake, the bow-shape features and convective cells, the mid-latitude transition region and the “polar cap”. The polar orbit of the satellite enables for the first time nadir viewing of the Southern polar regions and an opportunity to zoom in on the planet. The experiment returned numerous images of the Venus limb and documented global and local brightening events. VMC provided almost continuous monitoring of the planet with high temporal resolution that allowed one to follow changes in the cloud morphology at various scales.We present the in-flight performance of the instrument and focus in particular on the data from the ultraviolet channel, centered at the characteristic wavelength of the unknown UV absorber that yields the highest contrasts on the cloud top. Low latitudes are dominated by relatively dark clouds that have mottled and fragmented appearance clearly indicating convective activity in the sub-solar region. At ～50° latitude this pattern gives way to streaky clouds suggesting that horizontal, almost laminar, flow prevails here. Poleward from about 60°S the planet is covered by almost featureless bright polar hood sometimes crossed by dark narrow (～300 km) spiral or circular structures. This global cloud pattern can change on time scales of a few days resulting in global and local “brightening events” when the bright haze can extend far into low latitudes and/or increase its brightness by 30%. Close-up snapshots reveal plenty of morphological details like convective cells, cloud streaks, cumulus-like columns, wave trains. Different kinds of small scale waves are frequently observed at the cloud top. The wave activity is mainly observed in the 65–80° latitude band and is in particular concentrated in the region of Ishtar Terra that suggests their possible orographic origin. The VMC observations have important implications for the problems of the unknown UV absorber, microphysical processes, dynamics and radiative energy balance at the cloud tops. They are only briefly discussed in the paper, but each of them will be the subject of a dedicated study.     

The cloud structure of the jovian atmosphere as seen by the Cassini/CIRS experiment

We analyze the thermal infrared spectra of Jupiter obtained by the Cassini-CIRS instrument during the 2000 flyby to infer temperature and cloud density in the jovian stratosphere and upper troposphere. We use an inversion technique to derive zonal mean vertical profiles of cloud absorption coefficient and optical thickness from a narrow spectral window centered at 1392 cm⁻¹ (7.18 μm). At this wavenumber atmospheric absorption due to ammonia gas is very weak and uncertainties in the ammonia abundance do not impact the cloud retrieval results. For cloud-free conditions the atmospheric transmission is limited by the absorption of molecular hydrogen and methane. The gaseous optical depth of the atmosphere is of order unity at about 1200 mbar. This allows us to probe the structure of the atmosphere through a layer where ammonia cloud formation is expected. The results are presented as height vs latitude cross-sections of the zonal mean cloud optical depth and cloud absorption coefficient. The cloud optical depth and the cloud base pressure exhibit a significant variability with latitude. In regions with thin cloud cover (cloud optical depth less than 2), the cloud absorption coefficient peaks at 1.1±0.05 bar, whereas in regions with thick clouds the peak cloud absorption coefficient occurs in the vicinity of 900±50 mbar. If the cloud optical depth is too large the location of the cloud peak cannot be identified. Based on theoretical expectations for the ammonia condensation pressure we conclude that the detected clouds are probably a system of two different cloud layers: a top ammonia ice layer at about 900 mbar covering only limited latitudes and a second, deeper layer at 1100 mbar, possibly made of ammonium hydrosulfide.     

Constraints on the composition of the Venus atmosphere from microwave measurements near 1.35 cm wavelength

The determination of the brightness temperature of Venus near 1.35 cm wavelength is reviewed. The observed brightness temperature is compared with models for the microwave emission based on the physical and chemical structure of the atmosphere as obtained from spacecraft. Upper limits are set on the concentrations of microwave-absorbing minor constituents. In particular, upper limits are determined for SO₂ (180 ppm) and H₂O (0.3%) for a mixing-ratio profile that is uniformly mixed up to the cloud bottom at 50 km and is rapidly depleted ($\text{scale height}\text{? 1}\text{km}\text{)}$ at higher altitudes. The total optical depth of the cloud region at or above 50 km is <0.17 at 1.35 cm wavelength. The SO₂ upper limit is only in marginal agreement with the spacecraft results, and it may be that the latter have been overestimated, or that the distribution of SO₂ is more complex than given by the uniform mixing model.     

Lightning generation in planetary atmospheres

The possibilities of lightning generation on other planets are considered, and the basic conditions that exist in terrestrial clouds during lightning discharges and the various theories of charge separation are reviewed. Recent measurements of cloud structure and whistlers, as well as optical observation of lightning on Jupiter, suggest that charge separation and lightning discharges occur on other planets in ways similar to those in which they occur on Earth. Using these terrestrial ideas, it is concluded that lightning on Venus will probably be found in clouds that are located in regions of convection such as those observed downwind of the subsolar point. It is also possible that if volcanoes on Venus are erupting, they too can produce lightning discharges in their plumes although it seems unlikely that this process can account for the observed rate of discharge. Jovian lightning is most probably generated in the lower water-ice clouds. These clouds are of moderate temperatures and have strong convection and large mass loading, all important ingredients for electrical buildup. Lightning is all but ruled out for Mars, even though some electrification is possible owing to the large dust storms on that planet.     

Global mean cloud coverage on Venus in the near-infrared

We present a map of the global mean lower cloud coverage of Venus. This map is the average of 35 nights of 2.26 μm night side observations taken at NASA's Infrared Telescope Facility on Mauna Kea, over the years spanning 2001-2007. The atmosphere of Venus is a very dynamic system, and the lower clouds are constantly changing [Crisp, D., Allen, D.A., Grinspoon, D.H., Pollack, J.B., 1991a. The dark side of Venus: near-infrared images and spectra from the Anglo-Australian Observatory. Science, 253, 1263-1266]. By studying average cloud coverage, the daily variations are suppressed in order to see the underlying persistent cloud pattern. We find a relatively thick but highly variable equatorial band of clouds (±20° in latitude) and more quiescent mid-latitude clouds that are less opaque on average, with persistent cloudiness near the poles. We show that there is enough variation between our daily observations or between observations taken in different months that they cannot be considered individually representative of the global mean. We also compare the cloud coverage map to the topography of Venus and find no definitive correlations with high altitude features.     

The composition and vertical structure of the lower cloud deck on Venus

The opportunity to determine the planetwide temperature and cloud structure of Venus using radio occultation techniques arose with Pioneer Venus. Amplitude and Doppler data provided by the radio occultation experiment offered a unique and powerful means of examining the atmospheric properties in the lower cloud region.Absorption due to gaseous components of the atmosphere was subtracted from the measured absorption coefficient profiles before they were used to compute cloud mass contents. This absorption was found to represent a small part of the total absorption, depending on the latitude. In the main cloud deck, gaseous absorption contributes 10 to 20%, however, at the bottom of the detected absorption layer the sulfuric acid vapor contributes up to 100% due to increased vapor pressures. The clouds are the primary contributing absorbers in the 1- to 3-bar level of the Venus atmosphere. Below about 3 bars, depending on the latitude, absorption due to sulfuric acid vapor dominates.If a cloud particle model consisting of a solid nonabsorbing dielectric sphere with a concentric liquid sulfuric acid coating is invoked, the absorptivity of the particles increases from that of a pure sulfuric acid liquid sphere, and the mass content derived from the absorption coefficient profiles decreases. As the ratio of the core radius to the total radius (q) increases, absorption increases by more than a factor of 10 for high values of q. In the case of pure sulfuric acid droplets, the conductivity is sufficiently high that some of the field is excluded from the interior of the droplet thereby reducing the absorption. When a dielectric core of nonabsorbing material is introduced, the surface charge density is reduced and the absorption increases.The mass contents for all orbits in the equatorial region of Venus were calculated using values of q from 0 to 1. The resulting profiles match the probe mass content profiles at similar locations when a q of 0.97 is chosen.The wavelength dependence of the absorption for the spherical shell model varies with q from 1/λ² for pure liquid to λ^0.2 for a large core. A q of from 0.96 to 0.98 results in a wavelength dependence of 1/λ^1.0 to 1/λ^1.4 which matches the radio occultation absorption wavelength dependence and the microwave opacity wavelength dependence.Mass content profiles using a q of 0.97 were determined for occultations in the polar, collar, midlatitudinal, and equatorial regions assuming q remains constant over the planet. The results show considerable variability in both the level and the magnitude of the lower cloud deck. The cloud layer is lowest in altitude in the polar region. This might be expected as the temperature profile is cooler in the polar region than over the rest of the planet. The mass content is greatest in the polar and collar regions; however, many of the collar profiles were cut off due to fluctuations resulting from increased turbulence in the collar region. The mass contents are least dense in the midlatitude regions. There is a sharp lower boundary at about 1.5 bars in the equatorial and midlatitude regions and at about 2.5 bars in the polar region. Measurements made by the Particle Size Spectrometer and nephelometers also showed sharp lower cloud boundaries at this level.     

Quantification of middle and lower cloud variability and mesoscale dynamics from Venus Express/VIRTIS observations at 1.74 μm

We present an analysis of VIRTIS-M-IR observations of 1.74 μm emission from the nightside of Venus. The 1.74 μm window in the near infrared spectrum of Venus is an ideal proxy for investigating the evolution of middle and lower cloud deck opacity of Venus because it exhibits good signal to noise due to its brightness, good contrast between bright and dark regions, and few additional sources of extinction beside the clouds themselves. We have analyzed the data from the first 407 orbits (equivalent to 407 Earth days) of the Venus Express mission to determine the magnitude of variability in the 1.74 μm radiance. We have also performed an analysis of the evolution of individual features over a span of roughly 5–6 h on two successive orbits of Venus Express. We find that the overall 1.74 μm brightness of Venus has been increasing through the first 407 days of the mission, indicating a gradual diminishing of the cloud coverage and/or thickness, and that the lower latitudes exhibited more variability and more brightening than higher latitudes. We find that individual features evolve with a time scale of about 30 h, consistent with our previous analysis. Analysis of the evolution and motion of the clouds can be used to estimate the mesoscale dynamics within the clouds of Venus. We find that advection alone cannot explain the observed evolution of the features. The measured vorticity and divergence in the vicinity of the features are consistent with evolution under the influence of significant vertical motions likely driven by a radiative dynamical feedback. We measure a zonal wind speed of around 65 m/s, and a meridional wind speed around 2.5 m/s by tracking the motion of the central region of the features. But we also find that the measured wind speeds depend strongly on the points chosen for the wind speed analysis.     

Properties of the clouds of Venus,as inferred from airborne observations of its near-infrared reflectivity spectrum

We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H_2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H₂SO₄ and H₂O contained in the cloud material both equal approximately 2× 10^?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10^?4 and 10^?2.     

Observations of a stationary mid-latitude cloud system on Titan

We report the observation of a cloud system on Titan that remained localized near 40°S latitude and 60°W longitude for at least 34 h. Ground-based observations obtained with the SINFONI imaging spectrograph at the Very Large Telescope over four consecutive nights recorded the lifetime and altitude of the unresolved cloud system. Concomitant measurements made by Cassini/VIMS over 3 h resolved changes in the altitude and opacity of individual regions within the system during this time. Clouds are measured from 13 to 37 km altitude with optical depths per pixel ranging from τ=0.13 to 7. Short timescale rise times are consistent with previous measurements of the evolution of mid-latitude clouds; however the long timescale localization of the cloud structure is unexplained. We speculate about the role of mesoscale circulation in relation to cloud formation.     

Voyager 1 imaging and IRIS observations of Jovian methane absorption and thermal emission: Implications for cloud structure

Images from three filters of the Voyager 1 wide-angle camera were used to measure the continuum reflectivity and spectral gradient near 6000 Å and the 6190-Å band methane/continuum ratio for a variety of cloud features in Jupiter's atmosphere. The dark “barge” features in the North Equatorial Belt have anomalously strong positive continuum spectral gradients suggesting unique composition, probably not elemental sulfur. Methane absorption was shown at unprecedented spatial scales for the Great Red Spot and its immediate environment, for a dark barge feature in the North Equatorial Belt, and for two hot spot and plume regions in the North Equatorial Belt. Some small-scale features, unresolvable at ground-based resolution, show significant enhancement in methane absorption. Any enhancement in methane absorption is conspicuously absent in both hot spot regions with 5-μm brightness temperature 255°K. Methane absorption and 5-μm emission are correlated in the vicinity of the Great Red Spot but are anticorrelated in one of the plume hot spot regions. Methane absorption and simultaneously maps of 5-μm brightness temperature were quantitatively compared to realistic cloud structure models which include multiple scattering at 5 μm as well as in the visible. A curve in parameter space defines the solution to any observed quantity, ranging from a shallow atmosphere and thin NH₃ cloud to a deep atmosphere with a thick ammonia cloud. Without additional constraints, such as center-to-limb information, it is impossible to specify the NH₃ cloud optical depth and pressure of a deeper cloud top independently. Variability in H₂ quadrupole lines was also investigated and it was found that the constancy of the 4-0 S(1)-line equivalent width is consistent with the constancy of the methane 6190-Å band equivalent width at ground-based resolution, but the much greater variability of the 3-0 S(1) line is inconsistent with either the methane band or 4-0 S(1) line. In hot spot regions the 255°K brightness temperature requires a cloud optical depth of about 2 or less at 5 μm in the NH₃ cloud layer. To be consistent with the observed 6190-Å methane absorption in hot spot regions, the NH₃ cloud optical depth in the visible is about 7.5, implying that aerosols in hot spot regions have effective radii near 1 μm or less.     

Zonal winds at high latitudes on Venus: An improved application of cyclostrophic balance to Venus Express observations

Recent retrievals of zonal thermal winds obtained in a cyclostrophic regime on Venus are generally consistent with cloud tracking measurements at mid-latitudes, but become unphysical in polar regions where the values obtained above the clouds are often less than or close to zero. Using a global atmospheric model, we show that the main source of errors that appear in the polar regions when retrieving the zonal thermal winds is most likely due to uncertainties in the zonal wind intensity in the choice of the lower boundary condition.Here we suggest a new and robust method to better estimate the lower boundary condition for high latitudes, thereby improving the retrieved zonal thermal winds throughout the high latitudes middle atmosphere. This new method is applied to temperature fields derived from Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data on board the Venus Express spacecraft. We obtain a zonal thermal wind field that is in better agreement with other, more direct methods based on either retrieving the zonal winds from cloud tracking or from direct measurements of the meridional slope of pressure surfaces.     

Thermal infrared constraints on ammonia ice particles as candidates for clouds in the atmosphere of Saturn

It is possible for large particles of NH₃ ice to explain two phenomena associated with observations of thermal infrared emission from the atmosphere of Saturn: (1) the depression of thermal brightness near the equator, which is coincident with a visibly bright zone-like region, and (2) some disagreements between infrared and radio occultation results. Particles of NH₃ ice can provide the requisite opacity to explain the contrast between the equatorial region and the brighter area near 15°S for Pioneer Saturn Infrared Radiometer 45-μm channel data. NH₃ ice particle clouds can also reconcile the 45-μm brightness of both regions (near the equator and near 15°S) with the mean temperatures structure of the Voyager 2 radio occultation results. A cloud model with ice particles distributed in equal ratio with gas particles up to the 100-mbar pressure level best fits the equatorial data; a thinner cloud or one which does not extend higher than the 400-mbar limit of the convective region best matches data for the 15°S region. At 20 μm, however, the radio occultation temperature structure predicts brightnesses which are lower than those observed for both regions, and it could indicate the possibility that another source of opacity which is latitudinally variable exists in the stratosphere.     

Molecular line mapping of the giant molecular cloud associated with RCW 106 – II. Column density and dynamical state of the clumps

We present a fully sampled C¹⁸O (1–0) map towards the southern giant molecular cloud (GMC) associated with the H  ii region RCW 106, and use it in combination with previous ¹³CO (1–0) mapping to estimate the gas column density as a function of position and velocity. We find localized regions of significant ¹³CO optical depth in the northern part of the cloud, with several of the high-opacity clouds in this region likely associated with a limb-brightened shell around the H  ii region G333.6−0.2. Optical depth corrections broaden the distribution of column densities in the cloud, yielding a lognormal distribution as predicted by simulations of turbulence. Decomposing the ¹³CO and C¹⁸O data cubes into clumps, we find relatively weak correlations between size and linewidth, and a more sensitive dependence of luminosity on size than would be predicted by a constant average column density. The clump mass spectrum has a slope near −1.7, consistent with previous studies. The most massive clumps appear to have gravitational binding energies well in excess of virial equilibrium; we discuss possible explanations, which include magnetic support and neglect of time-varying surface terms in the virial theorem. Unlike molecular clouds as a whole, the clumps within the RCW 106 GMC, while elongated, appear to show random orientations with respect to the Galactic plane.     

Photometry of Venus: II. Theoretical brightness distribution over the disk

Theoretical brightness distributions over the Venus disk have been computed for homogeneous and multilayered cloud models. With homogeneous models, the relative brightness does not depend very much on the optical properties of the cloud, except near the limb or for small phase angles. For multilayered cloud structures, the relative brightness is nearly fixed by the structures and the relative brightness is nearly fixed by the scattering function of the uppermost cloud; the vertical distribution of the absorption is unimportant if the spherical albedo of the planet is given. If the horizontal inhomogeneities currently seen on Venus are due to a layered structure, with an optical thickness of the upper cloud varying from point to point, large simplifications seem possible, and measured contrasts at various wavelengths should permit a test of such a model.     

Temporal variability of ultraviolet cloud features in the Venus stratosphere

Statistics on the temporal variability of uv cloud features on Venus during 66 days of nominal mission imaging by the Pioneer Venus Orbiter Cloud Photopolarimeter reveal at least five types of systematic variability on large scales: (1) a low-latitude global-scale wave of period 3.94 ± 0.1 days corresponding to longitudinal motion of the dark equatorial band and propagating westward relative to the mean flow; (2) a midlatitude wave of period 5.20 ± 0.2 days corresponding to wavenumber 1 oscillations of the latitude of the bright polar bands and propagating eastward relative to the mean flow; (3) ～2- to 3-week fluctuations in the slope of longitudinal cloud brightness power spectra at intermediate wavenumbers manifested by variations in the intensity of large bow-shaped features; (4) ～2-month variations in polar region brightness consistent with polar brightening episodes observed from Earth; and (5) a monotonic decrease in the disk-integrated brightness of Venus during the nominal mission which may be either a true time variation or a solar-locked longitudinal dependence of brightness. Small-scale features appear to correlate with large-scale albedo patterns. Specifically, cellular features exist primarily where large-scale dark material is present, while the orientation of streak features with respect to latitude circles oscillates with the same ～4-day period as the large-scale features at low latitudes. The wide range of time scales present in the data suggests the complexity of Venus stratospheric dynamics. Extended observations over many years may be becessary to define the general circulation.     

The global distribution of water vapor in the middle atmosphere of Venus

The mean, solar-fixed horizontal and vertical distribution of water vapor in and above the Venusian cloud layer is presented. This is derived from far-infrared measurements made by the Orbiter Infrared Radiometer (OIR) instrument of the Pioneer Venus mission in the rotation band of water vapor at 45 μm, and from the mean solar-fixed temperature field and cloud structure retrieved from temperature soundings by the same instrument in five spectral channels. The water vapor retrieval scheme is discussed together with the calculation of water vapor transmission functions and their experimental verification. The sensitivity of the results to measurement errors and cloud microphysical properties is also considered. Mean water vapor column abundances above cloud unit optical depth at 11.5 μm are found to be greatest at equatorial latitudes in the early afternoon, reaching 50 ± 20 precipitable microns (100 ppm), and fall to less than 3 ± 2 precipitable microns (6 ppm) on the nightside of the planet. On the nightside mixing ratios fall monotonically with altitude, whereas dayside mixing ratios frequently increase with altitude near cloud unit optical depth. These results are broadly consistent with those of earlier Earth-based measurements.     

Observational definition of the Venus mesopause: vertical structure, diurnal variation, and temporal instability

Submillimeter line observations of CO in the Venus middle atmosphere (mesosphere) were observed with the James Clerk Maxwell Telescope (JCMT, Mauna Kea) about the May 2000, February 2002 superior and July 1999, March 2001 inferior conjunctions of Venus. Combined ¹²CO and ¹³CO isotope spectral line measurements at 345 and 330 gHz frequencies, respectively, provided enhanced sensitivity and vertical coverage for simultaneous retrievals of atmospheric temperatures and CO mixing ratios over the altitude region 75-105 km with vertical resolution 4-5 km. Supporting millimeter ¹²CO spectral line observations with the Kitt Peak 12-m telescope (Steward Observatories) provide enhanced temporal coverage and CO mixing sensitivity. Implementation of CO/temperature profile retrievals for the 2000, 2002 dayside (superior conjunction) and 1999, 2001 nightside (inferior conjunction) periods yields a first-time definition of the vertical structure and diurnal variation of a low-to-mid-latitude mesopause within the Venus atmosphere. At the times of these 1999-2002 observations, the Venus mesopause was located at a slightly lower level in the nightside (0.5 mbar, ∼87 km) versus the dayside (0.2 mbar, ∼91 km) atmosphere. Average diurnal variation of Venus mesospheric temperatures appears to be ≤ 5 K at and below the mesopause. Diurnal variation of Venus thermospheric temperatures increases abruptly just above the mesopause, reaching 50 K by the 0.01-mbar pressure level (∼102 km). Atmospheric temperatures above and below the Venus mesopause exhibited global-scale (≥4000 km horizontal) variations of large amplitude (7-15 K) on surprisingly short timescales (daily to monthly) during the 2001 nightside and 2002 dayside observing periods. Venus dayside mesospheric temperatures observed during the 2002 superior conjunction were also 10-15 K warmer than observed during the 2000 superior conjunction. A characteristic timescale for these global temperature variations is not defined, but their magnitude is comparable to previous determinations of secular variability in nightside mesospheric temperatures (Clancy and Muhleman, 1991).     

Zonal winds near Venus' cloud top level: A model study of the interaction between the zonal mean circulation and the semidiurnal tide

A primitive equation wave-mean flow interaction model, designed by J. R. Holton and used originally to study Earth's middle atmosphere, has been adapted to Venus in order to clarify our understanding of the interaction between the semidiurnal tide and the thermally driven mean meridional circulation near the cloud top level. With or without the tide the model produces midlatitude jets whose structure is insensitive to vertical shear of the background angular velocity above and below the cloud top level, but it is sensitive to background angular velocity at the cloud top level. When this background angular velocity is close to that of Venus, the latitudes and speeds of these jets are similar to the latitudes and speeds of jets at the Venus cloud top level as inferred from observed temperatures and the cyclostrophic balance condition. In agreement with the hypothesis of Fels and Lindzen, the model tide accelerates the equatorial zonal wind near the cloud top level and decelerates it at higher levels. The tidal vertical wavelength, maximum amplitude, dissipative decay length, and zonal flow accelerations are sensitive functions of background angular velocity, in agreement with elementary gravity wave theory. In the equatorial cloud top region, tidal acceleration is comparable in magnitude to the decelerative effects of vertical advection and the model's Rayleigh friction damping. For sufficiently rapid initial zonal flow near the cloud top level, the area-weighted global mean cloud top level zonal wind increases with time over a 50-day model run as a result of tidal acceleration. Agreement between the model tide and the observed tide, or the tide determined in the more detailed calculations of Pechmann and Ingersoll, is best when the background angular velocity at the jet level is about 30% larger than that observed.