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1.
We use a simplified terrestrial general circulation model as a nonlinear process model to investigate factors that influence the extent of equatorial superrotation in statically stable atmospheres on slowly rotating planets such as Titan and Venus. The possibility of multiple equilibria is tested by running the same model to equilibrium from vastly different initial conditions. The final state is effectively independent of initial state, reinforcing the impression that equatorial superrotation is inevitable on slowly rotating planets with stable radiative equilibrium structures. Of particular interest is the fact that at Titan rotation, the model equilibrates with strong prograde winds even when initialized with strong retrograde winds. This suggests that reliable remote sensing inferences of latitudinal temperature gradients on Titan can unambiguously be interpreted as evidence for superrotation. We also demonstrate for the first time that significant equatorial superrotation can be produced at Venus' rotation rate in such models, given sufficient numerical precision. The strength of superrotating zonal winds increases with rotation rate in the slowly rotating regime when other parameters are held fixed. However, the efficiency of superrotation relative to the angular momentum of an atmosphere corotating with the solid planet increases with decreasing rotation rate instead, because the Hadley cell strengthens and expands poleward. This allows for the formation of stronger high latitude jets, which ultimately serve as the source for equatorial superrotation via barotropic instability. Estimates of relevant parameter settings for Triton and Pluto tentatively imply that their atmospheres may marginally be in the superrotating regime, but only if temperature decreases with height near the surface. 相似文献
2.
Xun Zhu 《Planetary and Space Science》2006,54(8):761-773
This paper extends Leovy's theory on Venus’ equatorial superrotation by analytically examining additional terms in the mean zonal momentum equation that stably balances the momentum source of pumping by thermal tides. The general analytical solution is applied to the atmospheres of both Venus and Saturn's moon Titan. The main results are: (i) Venus’ equatorial superrotation of 118 m s−1 results primarily from a balance between the momentum source of pumping by thermal tides and the momentum sink of meridional advection of wind shear by horizontal branches of the Hadley circulation; (ii) no solution is found for Titan's stratospheric equatorial superrotation centered at the 1-hPa level; (iii) however, if the main solar radiation absorption layer in Titan's stratosphere is lifted from 1 hPa (∼185 km) to 0.1 hPa (∼288 km), an equatorial superrotation of ∼110 m s−1 centered at 0.1-hPa could be maintained. Titan's equatorial superrotation results mainly from a balance between the momentum source of tidal pumping and the momentum sink of frictional drag. 相似文献
3.
Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 m s−1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 m s−1 Titan-day−1 to maintain peak prograde winds in excess of 100 m s−1 at the 1 mbar level. 相似文献
4.
In this work we study in a general view slow rotating planets as Venus or Titan which present superrotating winds in their atmospheres. We are interested in understanding what mechanisms are candidates to be sources of net angular momentum to generate this kind of dynamics. In particular, in the case of Venus, in its atmosphere around an altitude of 100 km relative to the surface, there exists winds that perform a full rotation around the planet in four terrestrial days, whereas the venusian day is equivalent to 243 terrestrial ones. This phenomenon called superrotation is known since many decades. However, its origin and behaviour is not completely understood. In this article we analise and ponderate the importance of different effects to generate this dynamics. 相似文献
5.
Younes Gabsi Jean Loup Bertaux Alain Hauchecorne Jérôme Schmitt Stéphane Guibert 《Planetary and Space Science》2008,56(11):1454-1466
We present a new method of measuring the Venus winds by Doppler velocimetry on the full visible spectrum of solar light scattered by the clouds. In January 2003, we carried out observations to measure the winds of Venus, using the EMILIE high-resolution, cross-dispersed spectrograph and its associated calibrating instrument the Absolute Astronomical Accelerometer (AAA), at Observatoire de Haute-Provence, France. The motivation of this type of measurements is that it measures the actual velocity of cloud particles, while the other method (track of cloud features) may be sensitive to the deformation of the clouds. During observations, Venus was near maximum western elongation, at a phase angle near 90°. The EMILIE-AAA system allows us to measure accurately the Doppler shift induced in the reflected solar spectrum by the radial component of the motion of the clouds of Venus. We present the measurements and compare them with a forward simulation of a solid super-rotation of the atmosphere of Venus. Taking into account the Doppler shift relative to the Sun and that relative to the Earth, the theoretical total Doppler shift induced in the solar spectra is easily computed as a function of the velocity of the reflecting target. A first forward simulation is computed, with a wind model considering a purely horizontal and zonal wind. The magnitude of the wind is assumed to depend on cos(latitude), as for a solid-body rotation. The comparison with the measurements at various points on the illuminated semi-disc allowed us to determine an equatorial velocity of 66, 75, 91 and 85 m/s on 4 consecutive mornings, consistent with previous ultraviolet cloud tracking wind measurements, showing that wave propagation is not a major factor in the apparent motion of the cloud marks. Further, we discuss the effect of the finite angular size of the Sun and its rapid equatorial rotation (that we call the Young effect). It mainly affects measurements taken near the terminator, where the largest discrepancies are found. These discrepancies are alleviated when the Young effect is taken into account in the model but then the retrieved Venus equatorial velocity is reduced to only 48±3 m/s. This is well below classical ultraviolet markings velocities, but the altitude at which the visible photons are scattered (66 km) that we use is 5 km below the UV markings, confirming the vertical gradient of the horizontal winds shown by previous in-situ measurements. 相似文献
6.
The global circulation of the Venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts: orbiters, balloons and probes. Among them, the Galileo spacecraft monitored the Venus atmosphere in a flyby in February 1990 in its route toward Jupiter. Since the flyby was almost equatorial, published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the SSI camera [Belton, M.J.S., and 20 colleagues, 1991. Science 253, 1531-1536] stop at latitudes 50° north and south. In this paper we present new results on Venus winds based on a reanalysis of an extended set of images obtained at two wavelengths, 418 nm (violet) and 986 nm (near infrared), that sense different altitude levels in the upper cloud. Our main result is that we have been able to extend the zonal wind profile up to the polar latitudes: 70° N and 70° S at 418 nm and 70° N at 986 nm. Binned and smoothed profiles are given in tabular form. We show that the zonal winds drop in their velocity poleward of latitudes 45° N and 50° S where an intense meridional wind shear develops at the two cloud levels. Our data confirm the magnitude of this shear, retrieved previously from radio occultation data, but disagrees with it in the latitudinal location of the sheared region. The new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption. The meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region. 相似文献
7.
We review two models describing the Venus climate system: the carbonate and pyrite models. It has been argued carbonate and pyrite are potentially important minerals controlling the climate of Venus, though existence of either minerals has not been confirmed. Although it used to be proposed that carbonation reaction might explain the Venus’ atmospheric CO2 abundance, it is unlikely Venus’ surface is reactive enough to control the Venus’ massive CO2 atmosphere. Venus’ surface carbonate is also able to affect the climate through the reaction with atmospheric SO2 to form anhydrite. Under the carbonate model the climate state is not in equilibrium and would be unstable due to the reaction between carbonate and SO2. On the other hand, pyrite-magnetite reaction is proposed to explain the Venus’ atmospheric SO2 abundance. Under pyrite-magnetite reaction, however, the climate would be stabilized such that the existing climate state is maintained over a geological timescale, while some observational facts such as atmospheric abundance of SO2 and surface temperature could also be reasonably explained. 相似文献
8.
We present new wind measurements in Venus’ lower mesosphere from visible spectroscopy during the 2007 worldwide coordinated ground campaign in support of ESA's Venus Express mission. These observations consisted of high-resolution spectra of Fraunhofer lines in the entire visible range (0.37-1.05 μm) to measure the winds near 68 km using the Doppler shift of solar radiation scattered by clouds toward the observer's direction. The observations included various points of the dayside hemisphere at a phase angle of ∼109°. We took advantage of two symmetrical elongations in July and September 2007 at Canada-France-Hawaii's 3.6-m telescope. Kinematical fits to the Doppler winds provide a mean equatorial velocity of (104±10) m s−1 for the zonal retrograde flow. This velocity agrees quite well with the mean value obtained by tracking the UV markings from several spacecraft. 相似文献
9.
An analysis of ion data from 390 Venus Express, VEX, orbits demonstrates that the flow of solar wind- and ionospheric ions near Venus is characterized by a marked asymmetry. The flow asymmetry of solar wind H+ and ionospheric O+ points steadily in the opposite direction to the planet’s orbital motion, and is most pronounced near the Pole and in the tail/nightside region. The flow asymmetry is consistent with aberration forcing, here defined as lateral forcing induced by the planet’s orbital motion. In addition to solar wind forcing by the radial solar wind expansion, Venus is also subject a lateral/aberration forcing induced by the planet’s orbital motion transverse to the solar wind flow.The ionospheric response to lateral solar wind forcing is analyzed from altitude profiles of the ion density, ion velocity and ion mass-flux. The close connection between decreasing solar wind H+ mass-flux and increasing ionospheric O+ mass-flux, is suggestive of a direct/local solar wind energy and momentum transfer to ionospheric plasma. The bulk O+ ion flow is accelerated to velocities less than 10 km/s inside the dayside/flank Ionopause, and up to 6000 km in the tail. Consequently, the bulk O+ outflow does not escape, but remains near Venus as a fast (km/s) O+ zonal wind in the Venus polar and nightside upper ionosphere. Furthermore, the total O+ mass-flux in the Venus induced magnetosphere, increases steadily downward to a maximum of 2 × 10−14 kg/(m2 s) at ≈400 km altitude, suggesting a downward transport of energy and momentum. The O+, and total mass-flux, decay rapidly below 400 km. With no other plasma mass-flux as replacement, we argue that the reduction of ion mass-flux is caused by ion-neutral drag, a transfer of ion energy and momentum to neutrals, implying that the O+ plasma wind is converted to a neutral (thermosphere) wind at Venus. Incidentally, such a neutral wind would go in the same direction as the Venus atmosphere superrotation. 相似文献
10.
TitanWRF general circulation model simulations performed without sub-grid-scale horizontal diffusion of momentum produce roughly the observed amount of superrotation in Titan’s stratosphere. We compare these results to Cassini-Huygens measurements of Titan’s winds and temperatures, and predict temperature and winds at future seasons. We use angular momentum and transformed Eulerian mean diagnostics to show that equatorial superrotation is generated during episodic angular momentum ‘transfer events’ during model spin-up, and maintained by similar (yet shorter) events once the model has reached steady state. We then use wave and barotropic instability analysis to suggest that these transfer events are produced by barotropic waves, generated at low latitudes then propagating poleward through a critical layer, thus accelerating low latitudes while decelerating the mid-to-high latitude jet in the late fall through early spring hemisphere. Finally, we identify the dominant waves responsible for the transfers of angular momentum close to northern winter solstice during spin-up and at steady state. Problems with our simulations include peak latitudinal temperature gradients and zonal winds occurring ∼60 km lower than observed by Cassini CIRS, and no reduction in zonal wind speed around 80 km, as was observed by Huygens. While the latter may have been due to transient effects (e.g. gravity waves), the former suggests that our low (∼420 km) model top is adversely affecting the circulation near the jet peak, and/or that we require active haze transport in order to correctly model heating rates and thus the circulation. Future work will include running the model with a higher top, and including advection of a haze particle size distribution. 相似文献
11.
T. Tavenner E.F. Young M.A. Bullock J. Murphy S. Coyote 《Planetary and Space Science》2008,56(10):1435-1443
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. 相似文献
12.
The abundance of carbon monoxide in the Venus’ dayside atmosphere above the clouds was measured by ground-based 2.3 μm spectroscopy for 4 days. The hemispherical distributions found show no significant latitudinal or longitudinal structure. The disc-averaged mixing ratio of 58 ± 17 ppm found at a representative height of 62-67 km is consistent with previous measurements. Such a flat distribution of CO abundance above the clouds seems to be controlled by an efficient horizontal eddy diffusion with a time scale of 30 days or shorter although the CO distribution below the clouds seems to be controlled by the meridional circulation. The pole-ward wind speed of the meridional circulation above the clouds is estimated to be 0.2 m s−1 or less based on the difference between the CO mixing ratios above and below the clouds. 相似文献
13.
Near-infrared observations of the nightside of Venus reveal regions of high brightness temperatures. These regions of high brightness temperatures are caused by the localized evaporation of the middle and lower cloud decks, which are about 50 to 60 km above the surface of the planet. We simulate the Venus condensational middle and lower cloud deck with the University of Colorado/NASA Ames Community Aerosol and Radiation Model for Atmospheres (CARMA). Our simulated clouds have similar characteristics to the observed Venus clouds. Our radiative transfer model reproduces the observed temperature structure and atmospheric stability structure within the middle cloud region. A radiative-dynamical feedback occurs which generates mixing due to increased absorption of upwelling infrared radiation within the lower cloud region, as previously suggested by others. We find that localized variations in temperature structure or in sub-grid scale mixing cannot directly explain the longevity and optical depth of the clouds. However, vertical motions are capable of altering the cloud optical depth by a sufficient magnitude in a short enough timescale to be responsible for the observed clearings. 相似文献
14.
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. 相似文献
15.
M. N. Izakov 《Solar System Research》2011,45(3):240-245
It is shown in the paper that the mechanism of superrotation of the atmosphere of Venus consists of several interrelated processes,
including the transport of angular momentum from the solid planet to the atmosphere at a wind flow over the planet’s surface
relief, the upward transport of momentum by threedimensional turbulent vortices, the inverse energy cascade in a large-scale
quasi-two-dimensional flow, and the horizontal transfer of angular momentum in the Hadley cell. These processes make different
contributions to the superrotation at different altitudes. 相似文献
16.
A condensing cloud parameterization is included in a super-rotating Venus General Circulation Model. A parameterization including condensation, evaporation and sedimentation of mono-modal sulfuric acid cloud particles is described. Saturation vapor pressure of sulfuric acid vapor is used to determine cloud formation through instantaneous condensation and destruction through evaporation, while pressure dependent viscosity of a carbon dioxide atmosphere is used to determine sedimentation rates assuming particles fall at their terminal Stokes velocity. Modifications are described to account for the large range of the Reynolds number seen in the Venus atmosphere.Two GCM experiments initialized with 10 ppm-equivalent of sulfuric acid are integrated for 30 Earth years and the results are discussed with reference to “Y” shaped cloud structures observed on Venus. The GCM is able to produce an analog of the “Y” shaped cloud structure through dynamical processes alone, with contributions from the mean westward wind, the equatorial Kelvin wave, and the mid-latitude/polar Mixed Rossby/Gravity waves. The cloud top height in the GCM decreases from equator to pole and latitudinal gradients of cloud top height are comparable to those observed by Pioneer Venus and Venus Express, and those produced in more complex microphysical models of the sulfur cycle on Venus. Differences between the modeled cloud structures and observations are described and dynamical explanations are suggested for the most prominent differences. 相似文献
17.
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. 相似文献
18.
Venus cloud covered atmosphere offers a well-suited framework to study the coupling between the atmospheric dynamics and the structure of the cloud field. Violet images obtained during the Galileo flyby from 12 to 17 February 1990 have been analyzed to retrieve the zonal power spectra of the cloud brightness distribution field between latitudes 70° N and 50° S. The brightness distribution spectra serve as a diagnostic of the eddy kinetic energy spectrum providing indirect information about the distribution of energy along different spatial scales. We composed images covering a full rotation of the atmosphere at the level of the UV contrasted clouds obtaining maps of almost 360° that allowed us to obtain the brightness power spectra from wavenumbers k=1 to 50. A full analysis of the spectrum slope for different latitude bands and ranges of wave numbers is presented. The power spectra follow a classical law k−n with exponent n ranging from −1.7 to −2.9 depending on latitude and the wavenumber range. For the whole planet, the average of this parameter is −2.1 intermediate between those predicted by the classical turbulence theories for three- and two-dimensional motions (n=−5/3 and n=−3). A comparison with previous analysis of Mariner 10 (in 1974) and Pioneer Venus (in 1979) shows significant temporal changes in the cloud global structure and in the turbulence characteristics of the atmosphere. 相似文献
19.
Three-dimensional numerical simulations show that large-scale latent heating resulting from condensation of water vapor can produce multiple zonal jets similar to those on the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). For plausible water abundances (3-5 times solar on Jupiter/Saturn and 30 times solar on Uranus/Neptune), our simulations produce ∼20 zonal jets for Jupiter and Saturn and 3 zonal jets on Uranus and Neptune, similar to the number of jets observed on these planets. Moreover, these Jupiter/Saturn cases produce equatorial superrotation whereas the Uranus/Neptune cases produce equatorial subrotation, consistent with the observed equatorial-jet direction on these planets. Sensitivity tests show that water abundance, planetary rotation rate, and planetary radius are all controlling factors, with water playing the most important role; modest water abundances, large planetary radii, and fast rotation rates favor equatorial superrotation, whereas large water abundances favor equatorial subrotation regardless of the planetary radius and rotation rate. Given the larger radii, faster rotation rates, and probable lower water abundances of Jupiter and Saturn relative to Uranus and Neptune, our simulations therefore provide a possible mechanism for the existence of equatorial superrotation on Jupiter and Saturn and the lack of superrotation on Uranus and Neptune. Nevertheless, Saturn poses a possible difficulty, as our simulations were unable to explain the unusually high speed (∼) of that planet’s superrotating jet. The zonal jets in our simulations exhibit modest violations of the barotropic and Charney-Stern stability criteria. Overall, our simulations, while idealized, support the idea that latent heating plays an important role in generating the jets on the giant planets. 相似文献
20.
Three-dimensional numerical simulations of the atmospheric flow on giant planets using the primitive equations show that shallow thermal forcing confined to pressures near the cloud tops can produce deep zonal winds from the tropopause all the way down to the bottom of the atmosphere. These deep winds can attain speeds comparable to the zonal jet speeds within the shallow, forced layer; they are pumped by Coriolis acceleration acting on a deep meridional circulation driven by the shallow-layer eddies. In the forced layer, the flow reaches an approximate steady state where east-west eddy accelerations balance Coriolis accelerations acting on the meridional flow. Under Jupiter-like conditions, our simulations produce 25 to 30 zonal jets, similar to the number of jets observed on Jupiter and Saturn. The simulated jet widths correspond to the Rhines scale; this suggests that, despite the three-dimensional nature of the dynamics, the baroclinic eddies energize a quasi-two-dimensional inverse cascade modified by the β effect (where β is the gradient of the Coriolis parameter). In agreement with Jupiter, the jets can violate the barotropic and Charney-Stern stability criteria, achieving curvatures ∂2u/∂y2 of the zonal wind u with northward distance y up to 2β. The simulations exhibit a tendency toward neutral stability with respect to Arnol'd's second stability theorem in the upper troposphere, as has been suggested for Jupiter, although deviations from neutrality exist. When the temperature varies strongly with latitude near the equator, our simulations can also reproduce the stable equatorial superrotation with wind speeds greater than . Diagnostics show that barotropic eddies at low latitudes drive the equatorial superrotation. The simulations also broadly explain the distribution of jet-pumping eddies observed on Jupiter and Saturn. While idealized, these simulations therefore capture many aspects of the cloud-level flows on Jupiter and Saturn. 相似文献