The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared ...spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (>10,000) and large spectral coverage (0.7 to 17 μm—the visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7–1.6 μm spectral range with a resolving power of ∼20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2–4.4 μm range. MIR achieves a resolving power of >50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7–17 μm with apodized resolution varying from 0.2 to 1.3 cm
−1
. TIRVIM is primarily dedicated to profiling temperature from the surface up to ∼60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described.
•The largest data set of cloud tracked winds – about 0.5 million vectors – from the VMC/Venus Express imaging.•Characterization of the mean circulation at the Venus cloud tops.•Orbit-to-orbit changes ...and diurnal variations of the mean flow.•Long-term trend: acceleration of the mean flow from 2006 to 2012.•Periodicities in the cloud top wind field.
Six years of continuous monitoring of Venus by European Space Agency’s Venus Express orbiter provides an opportunity to study dynamics of the atmosphere our neighbor planet. Venus Monitoring Camera (VMC) on-board the orbiter has acquired the longest and the most complete so far set of ultra violet images of Venus. These images enable a study the cloud level circulation by tracking motion of the cloud features. The highly elliptical polar orbit of Venus Express provides optimal conditions for observations of the Southern hemisphere at varying spatial resolution. Out of the 2300 orbits of Venus Express over which the images used in the study cover about 10 Venus years. Out of these, we tracked cloud features in images obtained in 127 orbits by a manual cloud tracking technique and by a digital correlation method in 576 orbits. Total number of wind vectors derived in this work is 45,600 for the manual tracking and 391,600 for the digital method. This allowed us to determine the mean circulation, its long-term and diurnal trends, orbit-to-orbit variations and periodicities. We also present the first results of tracking features in the VMC near-IR images. In low latitudes the mean zonal wind at cloud tops (67±2km following: Rossow, W.B., Del Genio, A.T., Eichler, T. 1990. J. Atmos. Sci. 47, 2053–2084) is about 90m/s with a maximum of about 100m/s at 40–50°S. Poleward of 50°S the average zonal wind speed decreases with latitude. The corresponding atmospheric rotation period at cloud tops has a maximum of about 5days at equator, decreases to approximately 3days in middle latitudes and stays almost constant poleward from 50°S. The mean poleward meridional wind slowly increases from zero value at the equator to about 10m/s at 50°S and then decreases to zero at the pole. The error of an individual measurement is 7.5–30m/s. Wind speeds of 70–80m/s were derived from near-IR images at low latitudes. The VMC observations indicate a long term trend for the zonal wind speed at low latitudes to increase from 85m/s in the beginning of the mission to 110m/s by the middle of 2012. VMC UV observations also showed significant short term variations of the mean flow. The velocity difference between consecutive orbits in the region of mid-latitude jet could reach 30m/s that likely indicates vacillation of the mean flow between jet-like regime and quasi-solid body rotation at mid-latitudes. Fourier analysis revealed periodicities in the zonal circulation at low latitudes. Within the equatorial region, up to 35°S, the zonal wind show an oscillation with a period of 4.1–5days (4.83days on average) that is close to the super-rotation period at the equator. The wave amplitude is 4–17m/s and decreases with latitude, a feature of the Kelvin wave. The VMC observations showed a clear diurnal signature. A minimum in the zonal speed was found close to the noon (11–14h) and maxima in the morning (8–9h) and in the evening (16–17h). The meridional component peaks in the early afternoon (13–15h) at around 50°S latitude. The minimum of the meridional component is located at low latitudes in the morning (8–11h). The horizontal divergence of the mean cloud motions associated with the diurnal pattern suggests upwelling motions in the morning at low latitudes and downwelling flow in the afternoon in the cold collar region.
The paper is devoted to the investigation of Venus mesosphere circulation at 90–110 km altitudes, where tracking of the O2(a1Δg) 1.27 μm nightglow is practically the only method of studying the ...circulation. The images of the nightglow were obtained by VIRTIS‐M on Venus Express over the course of more than 2 years. The resulting global mean velocity vector field covers the nightside between latitudes 75°S–20°N and local time 19–5 h. The main observed mode of circulation is two opposite flows from terminators to midnight; however, the wind speed in the eastward direction from the morning side exceeds the westward (evening) by 20–30 m/s, and the streams “meet” at 22.5 ± 0.5 h. The influence of underlying topography was suggested in some cases: Above mountain regions, flows behave as if they encounter an “obstacle” and “wrap around” highlands. Instances of circular motion were discovered, encompassing areas of 1,500–4,000 km.
Plain Language Summary
Recent developments in the studies of Venus atmosphere reveal an intriguing phenomenon of stationary gravity waves, which emerge from the surface, interacting with the cloud layer up to ~70 km. In this paper, analysis of the oxygen nightglow on the nightside of Venus using data from VIRTIS instrument (Venus Express spacecraft) delivers clues that the atmosphere at 90–110 km altitude can as well be influenced by such mechanisms. The horizontal motion, obtained from tracking the displacements of the bright features of the nightglow, appears to hold disturbances, the positions of which in some cases coincide with highlands directly below or shifted by several degrees. The nightglow itself, known to manifest an extremely irregular behavior, sometimes repeats the shapes of the mountain ranges below. As another major result, the mean horizontal circulation, calculated for the nightside southern hemisphere, does represent neither superrotation, nor subsolar‐to‐antisolar circulation, nor a superposition of the two. Both the zonal and the meridional components of the motion have different magnitudes and direction before and after midnight. The results of this research further our knowledge on the upper atmosphere of Venus and pose a challenge for the global circulation models.
Key Points
The horizontal wind velocity in Venus atmosphere at 90–110 km was obtained from the O2(a1Δg) nightglow tracking
Average zonal component from the morning side exceeds its opposite from the evening side by 20–30 m/s; they “meet” at 22–23 h local time
The influence of the underlying topography on the wind direction was suggested in some cases
We present more than 250,000 wind vectors derived from the visible (513 nm) images captured by the Venus Monitoring Camera (VMC) onboard ESA's Venus Express orbiter in the Southern hemisphere from 01 ...July 2007 to 29 January 2013. From comparison to the wind velocity derived from tracking of the descent probes, these measurements correspond to 60 ± 3 km altitude, being between two levels 70 ± 2 km and 55 ± 2 km, probed by VMC in ultraviolet (UV) (365 nm) and NIR (965 nm) channels, respectively. The mean zonal wind suggests retrograde circulation with mean zonal wind speed decreasing from 76.5 to 61.5 m/s at 30°–65°S. In low latitudes, 10–20°S, it increased to 82 m/s over the course of the mission. The mean zonal flow depends on local solar time and latitude and is affected by the large‐scale topography. The meridional winds indicated equatorward flow of up to 7 m/s in the middle and low cloud opposite to that derived from simultaneous UV observations at the cloud top.
Plain Language Summary
Dynamics of the Venus atmosphere is dominated by a strong zonal retrograde circulation called “superrotation.” The physical mechanisms maintaining this unique regime are poorly understood due to insufficient observational data. For about eight years, the Venus Monitoring Camera onboard ESA's Venus Express orbiter monitored motions of the cloud features in three spectral ranges: ultraviolet (UV) (365 nm), visible (513 nm), and near‐infrared (915 nm). These wavelengths probed different altitudes: 70 ± 2 km, 60 ± 3 km, and 55 ± 2 km correspondingly, thus providing wind field tomography. In this paper, we present more than 250,000 wind vectors derived from the visible images. The results suggest decrease of the retrograde mean zonal wind speed from 76.5 to 61.5 m/s at 30°–65°S, and its increase up to 82 m/s at 10–20°S and show pronounced variations with local solar time, latitude, and surface topography. Interestingly, the meridional winds indicate equatorward flow of up to 7 m/s in the deep cloud opposite to that previously derived from UV images at the cloud top.
Key Points
More than 250,000 wind vectors at 57–63 km altitude were derived from the images taken by the Venus Monitoring Camera /Venus Express visible (513 nm) channel
Mean zonal wind speed accelerated by ∼18.5 m/s at 30 ± 5°S over 8 Venusian years
Zonal wind speed decreased from 85 m/s at the equator to 35 m/s at 80°S, meridional wind of up to 7 m/s velocity is directed equatorward
► Venus mesosphere was studied by the radio-occultation experiment VeRa/Venus Express. ► Zonal winds were derived by VeRa pressure profiles using the cyclostrophic balance. ► The retrieved zonal wind ...shows a max speed of 140
±
15
m/s at ∼70
km altitude. ► Stability of the atmosphere with respect to convection and turbulence was analysed. ► The atmosphere is adiabatic between 45 and 60
km altitude, it is highly stable at cloud top.
The dynamics of Venus’ mesosphere (60–100
km altitude) was investigated using data acquired by the radio-occultation experiment VeRa on board Venus Express. VeRa provides vertical profiles of density, temperature and pressure between 40 and 90
km of altitude with a vertical resolution of few hundred meters of both the Northern and Southern hemisphere. Pressure and temperature vertical profiles were used to derive zonal winds by applying an approximation of the Navier–Stokes equation, the cyclostrophic balance, which applies well on slowly rotating planets with fast zonal winds, like Venus and Titan. The main features of the retrieved winds are a midlatitude jet with a maximum speed up to 140
±
15
m
s
−1 which extends between 20°S and 50°S latitude at 70
km altitude and a decrease of wind speed with increasing height above the jet. Cyclostrophic winds show satisfactory agreement with the cloud-tracked winds derived from the Venus Monitoring Camera (VMC/VEx) UV images, although a disagreement is observed at the equator and near the pole due to the breakdown of the cyclostrophic approximation. Knowledge of both temperature and wind fields allowed us to study the stability of the atmosphere with respect to convection and turbulence. The Richardson number
Ri was evaluated from zonal field of measured temperatures and thermal winds. The atmosphere is characterised by a low value of Richardson number from ∼45
km up to ∼60
km altitude at all latitudes that corresponds to the lower and middle cloud layer indicating an almost adiabatic atmosphere. A high value of Richardson number was found in the region of the midlatitude jet indicating a highly stable atmosphere. The necessary condition for barotropic instability was verified: it is satisfied on the poleward side of the midlatitude jet, indicating the possible presence of wave instability.
Based on the analysis of UV images (at 365 nm) of Venus cloud top (altitude 67 ± 2 km) collected with Venus Monitoring Camera on board Venus Express (VEX), it is found that the zonal wind speed south ...of the equator (from 5°S to 15°S) shows a conspicuous variation (from −101 to −83 m/s) with geographic longitude of Venus, correlated with the underlying relief of Aphrodite Terra. We interpret this pattern as the result of stationary gravity waves produced at ground level by the uplift of air when the horizontal wind encounters a mountain slope. These waves can propagate up to the cloud top level, break there, and transfer their momentum to the zonal flow. Such upward propagation of gravity waves and influence on the wind speed vertical profile was shown to play an important role in the middle atmosphere of the Earth by Lindzen (1981) but is not reproduced in the current GCM of Venus atmosphere from LMD. (Laboratoire de Météorologie Dynamique)
In the equatorial regions, the UV albedo at 365 nm varies also with longitude. We argue that this variation may be simply explained by the divergence of the horizontal wind field. In the longitude region (from 60° to −10°) where the horizontal wind speed is increasing in magnitude (stretch), it triggers air upwelling which brings the UV absorber at cloud top level and decreases the albedo and vice versa when the wind is decreasing in magnitude (compression). This picture is fully consistent with the classical view of Venus meridional circulation, with upwelling at equator revealed by horizontal air motions away from equator: the longitude effect is only an additional but important modulation of this effect. This interpretation is comforted by a recent map of cloud top H2O, showing that near the equator the lower UV albedo longitude region is correlated with increased H2O. We argue that H2O enhancement is the sign of upwelling, suggesting that the UV absorber is also brought to cloud top by upwelling.
Key Points
At equator the Venus zonal wind at cloud top level is discovered to vary with geographic longitude
Stationary gravity waves generated by winds on orography at ground may decelerate the winds higher
The UV albedo geographic map is anticorrelated to H2O map suggesting upwelling of UV absorber
Winds derived by a digital tracking technique from ultraviolet (365 nm) images captured by the Venus Monitoring Camera (VMC) onboard the Venus Express spacecraft from 2006 to 2013 were used to study ...the atmospheric circulation at cloud top level (70 ± 2 km). This data set allows variations of the wind speed with both latitude and longitude to be studied and establishes their correlation with surface topography as well as local time dependence. Both zonal and meridional wind components show some correlation with topography. The minimum zonal wind speed was found at noon above Ovda Regio (10°S, 93°E), the highest region of Aphrodite Terra, one of the largest highlands in the equatorial region. The area of slow zonal wind extends to at least 30°S and shifts in the direction of superrotation in the afternoon and with increasing latitude (poleward). The observed deceleration of cloud top wind was recently attributed to the interaction of the gravity (mountain) waves generated by Aphrodite Terra with the atmospheric circulation. The present study was performed for different local time over the mountainous longitudes. The deceleration pattern in the zonal wind field is mainly conserved within a few hours around noon. Systematic longitude shift is observed in the afternoon in the direction of the evening terminator. Another area of perturbation of both zonal and meridional wind components is observed in the equatorial region around LT = 13–14 hr and may be explained by the solar tide.
Plain Language Summary
Venus is completely covered with a thick cloud layer with its top at about 70 km. Surprisingly, recent observations show that the cloud level circulation is affected by the surface topography. In this paper we analyzed wind velocities derived from tracking of cloud features in the UV images acquired by the Venus Monitoring Camera onboard the European Space Agency's Venus Express orbiter during its operations from 2006 to 2013. The zonal wind at the cloud top decelerates by about 20% above the highest part of Aphrodite Terra and reaches its minimum at local noon. The zonal wind deceleration is explained by interaction of gravity waves generated by the surface relief with the atmospheric circulation. An additional deceleration occurs in the afternoon in the equatorial region and is probably caused by solar heating of the clouds. The combination of both effects results in a vast area of slow wind during the daytime.
Key Points
A maximum deceleration of the mean zonal flow is observed at noon above the highest region of Aphrodite Terra, Venus
The mean zonal and meridional flows at cloud top level in the equatorial region are perturbed by a solar tide at 13–14 hr
A dependence of the mean zonal and meridional flows on topography is observed from the equator to at least 30°S
Structure of the Venus atmosphere Zasova, L.V.; Ignatiev, N.; Khatuntsev, I. ...
Planetary and space science,
10/2007, Letnik:
55, Številka:
12
Journal Article
Recenzirano
The structure of the Venus atmosphere is discussed. The data obtained in the 1980s by the last Soviet missions to Venus: orbiters Venera 15, 16 and the entry probes and balloons of Vega 1 and 2 are ...compared with the Venus International Reference Atmosphere (VIRA) model. VIRA is based on the data of the extensive space investigations of Venus in the 1960s and 1970s. The results of the IR Fourier Spectrometry experiment on Venera 15 are reviewed in detail. This instrument is considered as a precursor of the long wavelength channel of the Planetary Fourier Spectrometer on Venus Express.
To date dynamical observations of the Venus clouds have delivered mainly either only short‐term or long‐term averaged results. With the Venus Monitoring Camera (VMC) it finally became possible to ...investigate the global dynamics with a relatively high resolution in space and time on a long‐term basis. Our findings from manual cloud feature wind tracking in VMC UV image sequences so far show that the details of the mesospheric dynamics of Venus appear to be highly variable. Although the general rotation of the atmosphere remained relatively stable since Mariner 10, more than 30 years ago, by now, there are indications of short‐term variations in the general circulation pattern of the Venus atmosphere at cloud top level. In some cases, significant variations in the zonal wind properties occur on a timescale of days. In other cases, we see rather stable conditions over one atmospheric revolution, or longer, at cloud top level. It remains an interesting question whether the irregularly observed midlatitude jets are indeed variable or simply become shielded from view by higher H2SO4 haze layers for varying time intervals. Winds at latitudes higher than 60°S are still difficult to obtain track because of low contrast and scarcity of features but increasing data is being collected. Over all, it was possible to extend latitudinal coverage of the cloud top winds with VMC observations. Thermal tides seem to be present in the data, but final confirmation still depends on synthesis of Visible and Infrared Thermal Imaging Spectrometer and VMC observations on night and dayside. Although poorly resolved, meridional wind speed measurements agree mainly with previous observations and with the presence of a Hadley cell spanning between equatorial region and about 45°S latitude.
The Venus Monitoring Camera (VMC) acquired a set of ultraviolet (UV) images during the Venus Express mission unprecedented in its duration from May 2006 to September 2013. Here we present the results ...of digital tracking of the cloud features in the upper cloud layer at latitudes 25–75°S using images from 257 orbits with the best spatial coverage. The method relies on analysis of correlations between pairs of UV images separated in time. The bulk of data processed allows us to clarify the reasons why the mid-latitude jet is not always present in latitudinal wind profiles. Comparing VMC images with wind velocity fields we found a relationship between cloud morphology at middle latitudes and the circulation. The vector field in middle latitudes depends on the presence of a contrast global streak in the cloud morphology tilted with respect to latitude circles. The angle of the flow deflection (the angle between the wind velocity and latitudinal circles) and the difference of the zonal velocity on the opposite sides of the streak are in direct relationship to the angle between the streak and latitude circles. During such orbits the jet bulge does not appear in the latitudinal profile of the zonal wind component. Otherwise a zonal flow with small changes of the meridional velocity dominates in middle latitudes and manifests itself as a jet bulge. The relationship between the cloud cover morphology and circulation peculiarities can be attributed to the motion of global cloud features, like the Y-feature. We prepared plots of zonal and meridional velocities averaged with respect to the entire observation period. The average zonal velocity has a diurnal maximum at 15:00 local solar time and at 40°S. The meridional velocity reaches its maximum between 13:00 and 16:00 and at 50°S. The velocities obtained by the digital method are in good agreement with results of the visual method in the middle latitudes published earlier by Khatuntsev et al. (2013).
•Digital correlation method of cloud features tracking.•Relationship between the cloud morphology and atmospheric circulation.•Dependence of the cloud top wind speed on local solar time and cloud top altitude.
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