Venus atmospheric circulation: Known and unknown

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006je002814, 2007 Venus atmospheric circulation: Known and unknown Sanjay S. Limaye 1 Received 18 August 2006; revised 26 December 2006; accepted 15 February 2007; published 25 April [1] After a pause of more than two decades, Venus atmosphere is being explored again. Since April 2006, European Space Agency s Venus Express has been acquiring data, exploiting the near-infrared windows that allow us to peer into the deep night-side atmosphere. In June 2007, NASA s MESSENGER mission will fly past Venus on its way to Mercury, collecting useful data for a few weeks. Instruments and experiments on Venus Express are expected to provide a more comprehensive set of atmospheric observations over three and perhaps more Venus days. Venus Express observations are already providing clues about the processes that maintain the rapid circulation of Venus atmosphere. The early observations show not only that the global circulation is organized into two hemispheric vortices centered over respective poles, but also that the vortex organization extends deep into the atmosphere. The limited Galileo NIMS near-infrared observations in 1989 had revealed the deep circulation in the equatorial regions, but because of the spacecraft, flyby trajectory could not observe the high latitudes to elucidate the polar circulation and its organization. The long hiatus in systematic Venus observations has provided an opportunity to perform some new analysis of the previous Pioneer Venus observations. This paper presents a synopsis of the circulation measurements at high latitudes and an analysis of the solar thermal tides seen in the cloud motions and suggests some limitations of previous estimates of transport of angular momentum by eddies. Citation: Limaye, S. S. (2007), Venus atmospheric circulation: Known and unknown, J. Geophys. Res., 112,, doi: /2006je Introduction [2] Much has been learned about the atmospheric circulation of Venus since ultraviolet images revealed the 4-Day cloud-level circulation [Boyer and Guerin, 1969]. Entry probes, flyby and orbiting spacecraft, ground-based imaging, and spectroscopic Doppler observations from Earth have been instrumental in shaping our current understanding. Atmospheric entry probes yielded vertical profiles of the zonal (Venera 4 through 14 and VeGa 1 and VeGa 2 landers as well as the Pioneer Large and Small Probes [Kerzhanovich and Limaye, 1985]) and meridional winds (Pioneer Large and Small Probes [Counselman et al., 1980]) at selected locations and times during 1969 and In addition, the VeGa 1 and VeGa 2 balloons provided vertical and horizontal winds for periods of approximately 48 hours each [Sagdeyev et al., 1992] during Tracking of cloud features in ultraviolet images of Venus provided spatial coverage of winds through cloud motions during 3.5 days of coverage in February 1974, from the Mariner 10 flyby [Limaye and Suomi, 1981]; over periods as long as four months from 1979 to 1985, from Pioneer Venus [Limaye et 1 Space Science and Engineering Center, University of Wisconsin- Madison, Madison, Wisconsin, USA. Copyright 2007 by the American Geophysical Union /07/2006JE al., 1988; Rossow et al., 1990]; and from Galileo flyby observations during February The Galileo images, acquired over a two day period, provide another snapshot view of the cloud-level circulation from 410 and 865 nm reflected light images [Belton et al., 1991; Toigo et al., 1994]. Meridional profiles of the cloud-level zonal and meridional flow have also been obtained from 2.3 m measurements from the Galileo NIMS [Carlson et al., 1991] observations, as well as from ground-based telescope observations [Crisp et al., 1991; Limaye et al., 2006]. Yet some key aspects of the circulation of Venus atmosphere remain a puzzle and an observational challenge. Due to inadequate temporal and spatial coverage, the observations available to date have not been sufficient to understand the circulation mechanisms that control it. [3] With the launch of the Venus Express spacecraft, there is a renewed interest in investigating the atmospheric circulation of Venus. At the same time there are new and continuing efforts to model the atmospheric circulation numerically by as many as five teams: Yamamoto and Takahashi [2004] in Japan; Lee et al. [2005] in the UK; Dowling and Herrnstein [2006] and Hollingsworth et al. [2007] in the US; and Lebonnois et al. [2005] in France. [4] The attempts to understand the atmospheric circulation of Venus and other planets are analogous to our study of weather via observations routinely made of the Earth s atmosphere by the operational weather services around the 1of16

2 world. These include surface and radiosonde observations as well as those from weather satellites in geosynchronous and polar orbits. The key difference is that for Venus and other planets, we have limited observational coverage in space and time. For Earth, the observations are acquired systematically each day at fixed times and are interpreted by generating two-dimensional fields at multiple vertical levels through objective analysis. Aspects of the atmospheric circulation that provide a practical understanding of its working are obtained through zonal (longitudinal) and temporal averages, following the approach described by Lorenz [1967]. In this manner, Rossow et al. [1990] analyzed the circulation determined from Pioneer Venus imaging data, collected from 1979 to 1985, to document the transient and stationary waves in the circulation. [5] It is clear that without a fleet of satellites and other instrumentation, it is not practical to acquire such routine observations of the atmospheres of other planets. The sole exception is Mars, which now has had four orbiting satellites in polar orbit (MGS, Mars Surveyor, Mars Express and the Mars Reconnaissance Orbiter) at the same time. In the absence of systematic observations of atmospheric properties such as temperature, pressure, wind, trace species abundances and cloud properties, assumptions and simplifications have to be employed to reach some understanding. For example, angular momentum transports have been computed from cloud-level winds on Venus, yet a basic shortcoming of these results is often ignored; the observations do not provide a zonal or a longitudinal average. This is because the cloud motions were determined from images acquired in reflected sunlight and hence no systematic circulation measurements of the night hemisphere over extended periods were obtainable from the previous missions to Venus, until Venus Express. Nevertheless, bulk characteristics of Venus atmospheric circulation are now fairly well known. Some details of spatial structure and temporal variation, however, are not well established, due either to the difficulty of the measurements or a lack of data. [6] Understanding the superrotation of the atmosphere of Venus is the goal of the current numerical modeling efforts. Since the available observations can and are being used to diagnose the ability of such models to reproduce Venus circulation, it is worthwhile to consider the limitations of observations and the published results on the atmospheric circulation. Schubert [1983], Gierasch et al. [1997], and Schubert et al. [2007] have discussed theoretical aspects of Venus circulation at length. This paper focuses on some observational aspects of the circulation that have not been emphasized before but may be significant, including high-latitude measurements [Limaye, 1988a]. While the detection of the solar thermal tidal structure in the winds has been reported previously [Limaye and Suomi, 1981; Limaye, 1988b; Rossow et al., 1990], not much analysis was presented. This paper presents some further analysis and inferences regarding the eddy momentum transports enabled by the tidal model to the cloud motions. The forthcoming observations from Venus Express, particularly of the polar regions, have also prompted another look at the older measurements that have been largely unpublished before and are presented here. 2. Known Circulation 2.1. Vortex Organization of the Hemispheric Circulation [7] Vortex organization of Venus atmosphere was first suggested by Suomi from interpretations of the Mariner 10 images [Suomi, 1975]. It was shown that the observed circulation meets the requirements for an atmospheric vortex: radial, inward flow and a tangential speed profile that shows a peak between the outer periphery and the center from cloud motions (as measured from Mariner 10 images). That the atmosphere of Venus is organized into two giant hemispheric vortices centered over each pole was shown by Suomi and Limaye [1978]. Besides the visual symmetry of the spiral bands, there are qualitative similarities between the radial profile of the tangential component of wind in a terrestrial hurricane and the meridional profile of the zonal wind on Venus. Both show a peak jet, and the inward flow at the lower levels of a hurricane is similar to the average poleward flow measured at the cloud level of Venus [Limaye et al., 1988]. This vortex circulation is driven by the thermal contrast between the equator and poles via a thermally direct Hadley Cell. This was revealed in the solenoidal circulation [Limaye, 1985] by the misalignment of the pressure and density surfaces determined from the Pioneer Venus Radio Occultation temperature profiles. [8] Figure 1 shows a comparison of three vortices: (1) the hemispheric vortex circulation on Venus discerned from Mariner 10 images (left) superposed with the dipole structure seen in the infrared observations from Pioneer Venus Orbiter [Taylor et al., 1979; 1980]; (2) the day and nightside view of Venus from the VIRTIS instrument on Venus Express in April 2006; and (3) a tropical cyclone (right). The VIRTIS image shows day-side cloud cover organization in ultraviolet ( km above surface) and the night-side view at near-infrared wavelengths, revealing the organization at the 53 km level in the atmosphere of Venus. At the core, the swirling S shaped dipole structure, seen first in thermal infrared OIR observations from Pioneer Venus of the northern hemisphere, is evident. This is very similar to the features seen in the eye of many tropical cyclones [Kossin et al., 2002]. [9] While the spiral arms of a hurricane arise through gravity waves, the spiral arms of the Venus vortex are likely produced by advection of ultraviolet contrasts. Suomi [1975] first suggested these are generated by localized convection in equatorial latitudes, caused by the zonal and the gradual poleward meridional drift. Schinder et al. [1990] and Smith et al. [1992] explored the role of gravity waves in the atmosphere of Venus and showed that the streak model can indeed produce features similar to those seen in ultraviolet images of Venus, at least within 50 of the equator. These results suggest that the ultraviolet patterns seen in the Venus images could be interpreted as streaks, i.e., the locations of parcels that have passed over a common point, but the source region and the evolution of contrasts along their trajectory are not well understood. [10] Venus is the only planet besides Earth where such a hemispheric vortex is seen. (Recent analysis images of the 2of16

3 Figure 1. (left) A composite view of vortex circulation on Venus as determined from Mariner 10 images taken in 1974 [after Suomi and Limaye, 1978] of Venus in the (south) polar stereographic projection. The image is overlaid with the northern infrared dipole structure observed by the VORTEX instrument on the Pioneer Venus [Taylor et al., 1980]. The combined view (with the flipped VORTEX image) is meant for illustrative purposes only, and the azimuth angle of the polar dipole is arbitrary given the time difference between the two observations. (middle) An image taken by VIRTIS on Venus Express showing the day side (ultraviolet) and night side (near IR) views of the southern hemisphere of Venus ( esamultimedia.esa.int/images/venusexpress/voi_1_12_04_2006_b.tif). Image courtesy of European Space Agency. (right) Hurricane Fran, imaged by the MODIS instrument. The striking similarities between the three vortices may offer clues to understanding the global circulation on Venus. high southern latitudes of Saturn from the Cassini Orbiter revealed a vortex of a latitudinal limited extent situated over the pole.) The gas giants, with their rapid rotation, show zonal banding almost all the way to the poles. The vortex circulation on Venus has been noted since Mariner 10 s 1974 observations and has most recently been seen from Venus Express, indicating that the global circulation has a long-term stability. Analogous to terrestrial hurricanes, the Venus vortex is sustained by an elevated heat source. On the day side, heating is primarily by the deposition of solar energy in the elevated cloud deck, while both sides are heated by emitted surface radiation from below. Unlike a hurricane, which loses its driving force upon landfall when it loses its source of moisture, Venus vortex is securely centered over the pole, unable to leave its energy source. Given the physical conditions on Venus, the long term stability is not surprising; the planet has uniform cloud cover, no axial tilt, no hydrologic cycle, no land-ocean differences and a thick atmosphere with a long radiative time constant. Yet, the observations indicate that the deep atmosphere is turbulent and the ultraviolet features show long and short-term variations. The question is what causes the circulation to organize itself as a vortex and how the angular momentum balance is preserved over a long timescale Zonal and Meridional Flow Cloud-Tracked Winds: Ultraviolet Cloud Level [11] The cloud-level circulation (65 km altitude) was first measured from ultraviolet images taken from Earthbased telescopes [Boyer and Guerin, 1969] and has been followed by measurements from images taken from Mariner 10 [Limaye and Suomi, 1981], Pioneer Venus [Limaye et al., 1982, 1988; Rossow et al., 1990] and Galileo [Belton et al., 1991; Toigo et al., 1994]. The Earth-based images yielded only a single value for the average speed of the global circulation (3.995 days [Boyer and Guerin, 1969]) from the Y shaped feature, whereas the higher spatial resolution of the spacecraft images yielded a latitudinal profile of the zonal and meridional components as well as some indication of the longitudinal structure. [12] These measurements, generally made using perspective view images obtained from spacecraft (Mariner 10 and Galileo) or using latitude-longitude maps of re-projected images at 0.25 /pixel scale (Pioneer Venus), did not yield any vectors in high latitudes. Limited measurements in high latitudes were eventually obtained from Pioneer Venus images mapped in polar projections, but were not extensively analyzed or published until now. These measurements are discussed below in the context of the global visual tracking measurements of feature motions for the 1980 and 1982 imaging periods High-Latitude Circulation Determined From Pioneer Venus OCPP Images [13] Since the rectilinear latitude-longitude cloud-tracking format used in previous studies [Limaye et al., 1982, 1988; Rossow et al., 1990] inhibits feature identification at high latitudes due to geometric distortion, an effort was made to track clouds in polar stereographic projections of the images. It was hoped this would help make manual tracking easier. Additionally, the images were also brightness normalized using the Minnaert function to account for scattering geometry variations. Following the same procedure as used before [Limaye et al., 1988], new measurements were made mostly in polar latitudes. The number of targets available for tracking is much smaller poleward of about 45 degrees due to the dramatic change in the cloud cover appearance, so the standard error at high latitudes is somewhat larger than at lower latitudes. The high eccentricity orbit of Pioneer Venus and the imaging technique did not enable cloud tracking from images of high northern 3of16

4 Figure 2a. Day-side visual cloud tracking results for 1980 and 1982, with new measurements extending coverage to high latitudes as much as possible using polar stereographic projections of the original data. The solid lines show the profiles of constant angular velocity profiles, one corresponding to a speed of 140 ms 1 at the equator and the other one corresponding to 85 m/s at the equator. Vectors were binned into 2 wide latitude bins. The error bars indicate standard error (root mean square deviation/n 1 = 2 )in each latitude bin. The sense of the flow is from east to west, in the same sense as the underlying solid surface. latitudes, so polar latitude measurements are possible for the southern hemisphere only Zonal Flow [14] Figures 2a and 2b show the profiles of the cloud tracking results for images from 1980 and 1982 Pioneer Venus observing periods images averaged in latitude bins over all available (day-side) longitudes. These are comparable to those previously reported by Rossow et al. [1990], but extend the range to high latitudes not tracked digitally. Also shown are two dashed lines for solid body rotation, with equatorial speeds of 130 and 90 ms 1. It appears that beyond 50 S latitude the flow is closer to solid body rotation, but not exactly so. The red (1982) and blue (1980) lines represent running two-point averages. The small differences between the rigid body rotation and fine structure not yet resolved may be significant enough closer to the poles to examine whether the necessary barotropic instability condition is met, namely that b 0 U yy must change sign within the local jet (b 0 represents the meridonal derivative of the Coriolis parameter, and U yy is the second meridional derivative of the average zonal flow). This appears to be the case between in both the hemispheres. [15] The results shown in Figures 2a and 2b are similar to those presented from digital tracking which produces more vectors than the visual technique at low latitudes (40 ), but the difference at higher latitudes are larger than respective sampling errors. At higher latitudes the dramatic change in the ultraviolet contrast morphology reduces the efficiency of digital tracking Meridional Flow [16] Figure 3 shows the corresponding day-side average meridional flow. The flow appears to get weaker closer to the pole after peaking between about 50 and 60 S. The profile of zonal average horizontal divergence computed from this day-side average profile (after applying a 3-point running mean) is shown in Figure 4, which displays the convergence zone beginning around 45 latitude in both hemispheres. The peaks and valleys in this profile are not meaningful due to the error in the average flow. Similar results were reported from the 1982 images [Limaye et al., 1988]. The digital tracking results reported by Rossow et al. [1990] also show similar divergence trends for all periods, but the measurement errors in those results are significant enough to preclude any comments about the convergence/ divergence boundary and changes over time Horizontal Divergence [17] The convergence zones seen poleward of 45 latitude arise from a convergence of meridians and the (day side) average poleward flow. The suggestion that this should lead to downwelling is consistent with the inference reached about the structure of the Kelvin wave by Del Genio and Rossow [1990] that bright regions in the ultraviolet are caused by downwelling, as the brightness of Venus in the ultraviolet is much greater at higher poleward of 45 [Limaye and Suomi, 1977]. [18] The recent VIRTIS images from Venus Express show that in southern polar latitudes on Venus emitted radiation from the deep atmosphere escapes through the cloud layer and shows banded or striped structure as might be related to the large scale subsidence bands suggested from the horizontal divergence profiles, but more analysis is warranted. [19] It can be inferred that the mean poleward flow and the convergence of the meridians forces a mass convergence poleward of about 45, which also marks the boundary 4of16

5 Figure 2b. The period of rotation corresponding to the average zonal flow shown in Figure 2a. The lines are polynomial fits to the binned measurements. The scatter of the bin averaged vectors is pronounced poleward of 55 latitude in both hemispheres due to low sampling of different longitudes. The measurements are suggestive of the notion that the peak rotation period is likely about three days near 60, and increases poleward. between the bright and less bright regions in reflected ultraviolet light. Applying the assumption that the ultraviolet bright regions mark downwelling and ultraviolet dark regions correspond to upwelling in the Venus atmosphere [Del Genio and Rossow, 1990; Esposito and Travis, 1982], we can then infer that the convergence poleward of 45 latitude creates a subsidence zone centered over each pole. This is similar to the situation in terrestrial hurricanes, Figure 3. The meridional component of motion corresponding to the measurements shown in Figures 2a and 2b. The lower sampling at high latitudes results in larger errors, but the average day-side flow remains poleward, likely decreasing in magnitude poleward of about 60 latitude. The error bars indicate standard error (RMS deviation/ p N). 5of16

6 Figure 4. Horizontal divergence from the smoothed day-side average meridional flow for the 1980 manually tracked cloud motions. The boundary between divergence and convergence is around 45 latitude in both the hemispheres; the fine structure seen in the latitudinal profile is due to measurement errors. How far the convergence zone extends in the vicinity of the pole likely indicates the extent of the Hadley circulation. except for the vast difference in physical scale between the hemispheric vortex on Venus and a tropical cyclone Circulation in the Deep Atmosphere: Tracking of Features in Near-Infrared Images [20] Below the visible clouds, there is a paucity of observations of atmospheric flow for determining the horizontal structure of the circulation. Measurements of the deep atmosphere are available from entry probes and the VeGa balloons over a period of many years at different locations. Cloud-tracked measurements for the deep atmosphere are available only on the night side from the nearinfrared images and provide circulation information at a particular wavelength dependent vertical level. On day-side, the reflected solar component makes it difficult to detect the features at near-infrared wavelengths. After the discovery of the windows in the near infrared by Allen and Crawford [1984], circulation of the lower atmosphere was measured from both Earth-based [Crisp et al., 1991] and Galileo NIMS images of Venus obtained at 2.3 m [Carlson et al., 1991]. The radiation at this wavelength is believed to represent the atmosphere at an altitude of 53 km, and thus for the first time, the night-side circulation could be measured in the deep atmosphere. The spatial resolution of the near-infrared images of Venus night side obtained from Earth-based telescopes has been improving, and more measurements have been obtained [Limaye et al., 2006]. However, spatial and temporal coverage, particularly in solar longitude, has been limited. The winds determined from the near-infrared images are consistent with data from Venera and Pioneer entry probes as well as VeGa1 and VeGa 2 balloons at the 53 km altitude. The VIRTIS observations of the night-side of Venus from Venus Express, returning data from Venus since May 2006, are expected to make significant contributions via measurements of the deep circulation on the night side at multiple levels. [21] It is worth noting that the Galileo NIMS measurements reported by Carlson et al. [1991] of the night side circulation at the 53 km level showed the observed nightside mean meridional flow that was directed poleward. This is consistent with the notion that the mean meridional (Hadley) circulation and the vortex organization both extend at least down to the 53 km level and perhaps even deeper Known Circulation: Thermal Support for the Midlatitude Jets [22] Leovy [1973] first pointed out that in view of the slow rotation of the planet, the atmospheric flow was likely to follow the cyclostrophic balance, in which the pressure gradient force is balanced by the centripetal force, rather than the Coriolis force. A number of observations have contributed to the spatial and vertical thermal structure of Venus atmosphere, including radio occultations from Pioneer Venus Orbiter (from 1978 to 1983), Venera orbiters and the Magellan orbiter, as well as passive infrared spectral measurements by the VORTEX experiment on the Pioneer Venus Orbiter [Taylor et al., 1979], the Fourier Spectrometer on the Venera 15 and 16 orbiters [Schaefer et al., 1990] and from Galileo Orbiter Venus flyby [Roos-Serote et al., 1995]. The Venus Express mission will also yield radio occultation profiles of temperature in the Venus atmosphere over time. Such measurements allow inferences about the atmospheric circulation under the assumption of cyclostrophic balance if the temporal extent of the data is ignored. [23] The longitudinal and local solar time coverage of Pioneer Venus Orbiter (and other missions) radio occultation temperature profiles is sparse and aliased in local time and latitude due to the constraints of the spacecraft-venus- Earth geometry; however, the latitudinal coverage provided a good estimate of the variation of temperature with pressure and altitude [Kliore and Patel, 1982]. Assuming that the temporal variations are small and can be ignored, 6of16

7 these ensemble measurements can then be used to determine the pressure surface topography. From this structure, assuming cyclostrophic balance, the balanced zonal flow was determined directly without the assumption of the zonal flow at a boundary. These data also show that the density and pressure surfaces intersect, indicating the meridional overturning of the atmosphere [Limaye, 1985]. [24] Newman et al. [1984] used the same data with the thermal wind equation (which requires an assumption of the zonal flow at some level) to arrive at a similar zonal flow structure. Both the thermal wind and the direct estimate of the balanced zonal flow indicate the presence of a midlatitude jet. Radio occultation-derived thermal profiles from Venera orbiters have also yielded similar circulation profiles. In addition, the Fourier Transform Spectrometer observations from Venera 15 and 16 orbiters [Schaefer et al., 1990] and limited Galileo NIMS [Roos-Serote et al., 1995] have also produced similar results on the thermal structure and winds using the thermal wind equation and an initial lower boundary zonal flow profile. [25] The zonal wind results obtained from the direct application of the cyclostrophic balance approach and the thermal wind assumption applied to the Pioneer Venus thermal structure data both indicate a zonal jet in midlatitude with a magnitude of 110 ms 1 [Newman et al., 1984; Limaye, 1985]. Similar jet magnitudes have been reported from Venera radio occultation and Fourier Spectrometer as well Galileo NIMS derived thermal structure results, close to the level of the ultraviolet contrast features. Such a welldefined jet was not seen in the cloud-tracked winds from the OCPP tracking results between 1979 and There are several possibilities as to why: [26] 1. The zonal circulation on Venus is variable and the midlatitude jet may or may not exist at any given time. [27] 2. The midlatitude jet is present at a certain vertical altitude, but the tracers used to measure the atmospheric circulation are at a different level. [28] 3. The spatial and temporal resolution of the data was not adequate to discern the midlatitude jet. [29] The first possibility requires that the kinetic energy of the jet region of the atmosphere can change by 20 percent or more over time (Rossow et al. [1990] results for the equatorial day-side average zonal speed range from 88.3 to 97.1 ms 1 ). Such a change in the average zonal flow requires an accompanying change in the energy transfer processes that drive and maintain the circulation. Del Genio and Rossow [1990] have argued that the Kelvin wave activity may control the circulation and cause the circulation to vary on such timescales, although no energetics arguments were made. On a planet with uniform cloud cover, no oceans, and no significant hydrologic cycle, such a change in the driving mechanism is inexplicable but not implausible. The absorption of solar energy ultimately causes the rapid rotation of the Venus atmosphere, but a change of this magnitude is not easy to understand or reconcile with what we think we know about clouds of Venus and the energy deposition in the atmosphere. [30] The last two appear more plausible, but it is also quite possible that the thermal structure of the cloud layer on Venus is responsive to the solar ultraviolet flux over a solar cycle and changes accordingly. In this case, changes in the circulation would result from contribution by the ultraviolet absorber to the solar energy deposition. Indeed, it has been shown that the global scale ultraviolet contrast changes on Venus over a timescale of years from ground-based observations [Dollfus, 1968, 1975]. But because of a lack of systematic ground-based observations in recent years, it is not apparent whether such changes have continued to occur. The data coverages from space missions, except for Pioneer Orbiter, have not been of a sufficient duration to draw any inferences regarding such changes, but no detailed efforts have been made to link the changes in circulation with changes in ultraviolet clouds. 3. Inferred Circulation: Solar Thermal Tides [31] It has been suggested that solar thermal tides play a role in the maintenance of the circulation of Venus atmosphere [Newman and Leovy, 1992]. However, obtaining precise, detailed information about the dynamical structure in the deep atmosphere has been difficult. Unlike the thermal structure, the information about the spatial structure of the circulation has been available only on the day side from cloud tracking until recently. [32] Cloud motion measurements on the day side of Venus from Mariner 10 were obtained over essentially the same phase angle. The coverage corresponds to less than 8 hours of local time. Although solar longitude dependence was observed [Limaye and Suomi, 1981], the few measurements did not enable a reliable determination of the solar thermal tides in the zonal or the meridional components. [33] Cloud motions have been determined from the Pioneer Venus OCPP images (reflected ultraviolet sunlight) acquired in several imaging periods between 1979 and 1985, some lasting as long as two months. Cloud features are generally trackable when the spacecraft zenith angle is less than 60 degrees. The duration of each OCPP imaging period provides enough local coverage of the cloud motions that it is possible to determine the tidal amplitudes and phases with some confidence, even lacking any night-side observations. Given the spatial and temporal variability of the motions, the tidal structure can be discerned only when the motions are averaged over time, thereby making it difficult to evaluate temporal changes or transient waves with great confidence. [34] In order to determine the amplitudes of the solar thermal tides, the measured cloud motions were first binned in longitude relative to the sub-solar longitude, i.e., in local solar time. A two-component tidal model was fit to the data, allowing determination of a true zonal mean value, diurnal and semidiurnal amplitudes and corresponding phases for both the zonal component and the meridional component. uðq; lþ ¼ U 0 þ U 1 Sinðl þ F 1 ÞþU 2 Sinð2l þ F 2 Þ ð1þ where U 0 is the zonal mean value, q and l are the latitude and longitude, U 1 and U 2 are the diurnal and semidiurnal amplitudes and F 1 and F 2 are the diurnal and semidiurnal phase angles, and are expected to vary with latitude. [35] Similarly for the meridional component: vðq; lþ ¼ V 0 ðþþ q V 1 ðþsin q ðl þ Y 1 Þþ V 2 ðþsin q ð2l þ Y 2 Þ ð2þ 7of16

8 Figure 5a. Day-side average zonal component from digital (Digital-80) and visual tracking (hui Visual) and the tidal model derived U 0 and V 0 (top part of the figure). The bottom part of the figure shows the diurnal (U 1 ) and semidiurnal (U 2 ) amplitudes of the solar thermal. Error bars for the tidal amplitudes are from the tidal model fit. The 1980 tidal mean component U 0 reveals midlatitude jets not seen in the day-side average profile. where v 0 is the zonal mean value, q and l are the latitude and longitude, V 1 and V 2 are the diurnal and semidiurnal amplitudes for that latitude (q) and Y 1 and Y 2 are the phase angles for the diurnal and semidiurnal tides and are expected to vary with latitude q. When sufficient measurements are available to bin the vectors in latitude and solar longitude, the zonal average values U 0 and V 0, the tidal amplitudes U 1,U 2,V 1, and V 2, as well as the respective diurnal and diurnal phases can be computed by a least squares fit that also yields corresponding errors from the covariances. [36] The benefit of the tidal model fit is that it yields an estimate of the true zonal values by essentially extrapolating the model to the night side. This may be useful in estimating the eddy momentum transports differently by perhaps more realistically estimating the eddies than the previously-used day-side mean values, as was done due to lack of any night side observations. It is worth noting here that even with the likely availability of the VIRTIS observations of cloud motions on the night side, such an approach may be warranted as the day-side measurements from reflected ultraviolet images and on the night-side from emitted near-infrared observations refer to different altitude levels in the Venus atmosphere. [37] Limaye [1988b, 1990] and Rossow et al. [1990] have previously reported limited results on the tidal model fits. Additional analysis of the tidal model applied to the 1980 and 1982 visual tracking vectors are presented here. The vectors were binned in 4 4 latitude and solar longitude bins. The statistics for the 1980 results are comparable to those for 1982 reported by Limaye et al. [1988], although the 1980 measurements were made by three different individuals. Tidal model coefficients were determined for each latitude band, and excellent fits were obtained within 40 latitude band about the equator, the low sampling in local solar time being the major issue at higher latitudes. The tacit assumption is that the measurements refer to the same vertical level. In the absence of any other information, and on the basis of the thermal structure inferences [Limaye, 1985; Newman et al., 1984] and the arguments provided by Rossow et al. [1990] and Del Genio et al. [1990], this is a reasonable assumption Tides in the Zonal Component [38] Figures 5a and 5b show results for the 1980 and 1982 imaging seasons, respectively. They display the zonal mean component (U 0 ) and the diurnal (U 1 ) and the semidiurnal amplitudes (U 2 ) as derived from the visual tracking OCPP measurements. For comparison, the (day-side) latitude bin averaged digital tracking results [Rossow et al., 1990] and visual tracking [Limaye et al., 1988; this work] are also shown. The day-side average profiles of the zonal flow from digital and visual tracking results are practically the same except near the extremes of the latitudinal coverage of digital results presented by Rossow et al. The error bars displayed for U 0 are formal estimates from the least squares regression. Errors in the zonal average zonal component and the meridional components derived from the tidal model are larger beyond 45 latitude, due to fewer vectors. [39] The interesting feature of the 1982 profile of U 0 is that weak midlatitude jets are seen in the 1980 results, supporting the radio occultation results. The amplitude of the diurnal tide is seen as larger than that of the semidiurnal 8of16

9 Figure 5b. Profiles of the tidal amplitudes and day-side average zonal component from 1982 visual tracking results. The standard error determined from the number of samples and the root mean square deviation in a latitude bin is shown for the visual tracking results. Rossow et al. [1990] estimate the error for digital tracking as between 3 and 5 ms. component except between S, where they are almost the same for the 1980 results. For the 1982 results, the diurnal amplitude is smaller than that of the semidiurnal component only at low northern latitudes and near S. [40] The day-side 1982 visual tracking results at high latitudes are similar to those for 1980, although the errors are larger due to a paucity of trackable features. The 1980 day-side average digital and visual tracking results are practically the same at all latitudes. However, the differences between U 0 determined from the tidal model and the day-side averages are significant. [41] The 1982 tidal fit results for U 0 differ somewhat from those for the 1980 imaging period in that the day-side average zonal component profile shows jets near 45 latitude in both hemispheres, but the tidal model derived average zonal component does not show similar jets. The absence of the jet in the 1982 profile of U 0 is inconsistent with the thermal structure support for the jet and may be indicative of temporal variability in the Venus atmosphere either in the thermal data (making the extension of the cyclostrophic/thermal wind derived jet) for 1982 period, or cloud level variability or tidal activity. [42] Schofield and Taylor [1983] derived the solar thermal tidal structure from the Pioneer Orbiter Infrared Radiometer (OIR) observations. Subsequently, Pechman and Ingersoll [1984] interpreted these results in the context of an analytical model, convolving the low vertical resolution of the thermal observations with the model results that showed that the semidiurnal amplitude was dominant in the tides (at least in the thermal data). Newman and Leovy [1992] report similar amplitude difference for the tides in the winds. This is contrary to the tidal structure seen in the cloud motions for both 1980 and Tides in the Meridional Component [43] The profiles for the 1980 and 1982 visual tracking results of thermal tidal zonal average meridional component (V 0 ) are shown in Figures 6a and 6b, respectively. The 1980 digital and visual day-side average meridional component profiles are also shown in Figure 6a. For 1982, only the visual tracking profile is shown for comparison with the tidal mean V 0, as well as diurnal and semidiurnal amplitudes. Similar to the zonal component, the profiles of V 0 determined from the tidal model are somewhat different from the day-side average mean profiles, suggesting that the night side structure is somewhat different. The differences between V 0 and the day-side average values are larger than the respective errors except near the extremes of latitudinal coverage due to larger errors in the tidal fits. [44] The tidal fit indicates the same poleward flow in respective hemispheres (V 0 ) and similar to the zonal component, the semidiurnal tide amplitude is weaker than the diurnal tide. The errors in V 0 from the tidal model are somewhat larger than those for the zonal component due to the meridional flow itself being generally almost an order of magnitude weaker than the zonal flow, and improvement in measuring the meridional component more accurately is desirable. [45] Within the respective errors, the 1980 digital and visual meridional component profiles are the same within 30 of the equator, but differ slightly in magnitude at higher latitudes. The tidal model derived zonal average meridional component is weaker than the day-side average in the northern hemisphere but comparable to the day-side average 9of16

10 Figure 6a. Thermal tides in the meridional component (1980 OCPP visual measurements). Error bars represent formal errors from the least squares fit of the tidal model fit to the observations. For comparison, the digital tracking results of Rossow et al. [1990] are also shown. in the southern hemisphere. For 1982 the day-side average meridional flow is generally weaker than V 0, but has the same sense. The amplitude of the diurnal tide in the meridional component is generally larger than that of the semidiurnal component. Similar results were obtained for Extrapolated Circulation on the Night Side [46] The tidal model coefficients enable the extrapolation of the zonal and meridional component to the night-side close to the morning and evening terminator regions on the day side, where a lack of trackable features inhibits cloud motion measurements. The model-derived zonal components for 1980 and 1982 are shown in Figures 7a and 7b, respectively, while the meridional component field for 1982 is shown in Figure 8 (1980 results are similar). At low latitudes the extrapolated fields for the zonal component on the day-side show similar characteristics, including the minimum zonal flow just before local noon. At higher latitudes and particularly on the night-side the 1980 and Figure 6b. Thermal tides in the meridional component (1982 OCPP measurements). Error bars represent formal errors from the least squares fit of the tidal model to the observations. 10 of 16

11 Figure 7a. The global zonal component field extrapolated on the night side using the solar tidal model fit to the observed day side solar longitude averaged cloud motions from 1980 OCPP observations results show different strengths for the zonal flow. Whereas the night-side extrapolated zonal component is larger at higher latitudes than on the day side at midlatitudes for the 1980 data, it is just the opposite for The sampling statistics for the 1980 and 1982 visual tracking results are generally the same and the errors similar, so this contrast is striking. Within 40 of the equator the confidence in the tidal model estimate of the zonal average values is relatively high, so it appears that the tidal structure is different enough during 1980 and Whether it is related to changes within the cloud layer of Venus or directly related to the solar cycle is an intriguing question. [47] The meridional profile on the night-side retains the same poleward nature, but the flow appears much stronger, peaking near mid-night, and no equatorward flow is seen on the night-side. Some night-side observations at the same level would bolster these findings or at least help pin down the zonal average magnitude better. The 1980 meridional component field shows characteristics similar to those of [48] The zonal and meridional components, extrapolated to the night side and near the evening and morning terminator, show how the unmeasured or inaccessible longitudes may impact the zonal average. It can be inferred that the night-side structure amplifies the jets in the tidal model zonal component meridional profile for 1980, while in 1982 the night-side structure depletes the jets, completely Figure 7b. The global zonal component field extrapolated on the night side using the solar tidal model fit to the observed day side solar longitude averaged cloud motions from the 1982 OCPP observations. Figure 8. The global meridional component field extrapolated on the night side using the solar tidal model fit to the observed day-side solar longitude averaged cloud motions from the 1982 OCPP observations. On the night side the meridional flow is still generally toward the pole in each hemisphere. removing them from the profile. Similar inferences were made from the modeling results by Newman and Leovy [1992]. 4. Unknown Circulation [49] The lack of observations of circulation on the night side has been a major difficulty in understanding Venus circulation. So far most of the inferences about the global circulation have come from the analysis of the day-side observations, which generally have been restricted to only about 120 wide longitude span. While they provide a good estimate of the true zonal average values, they are a poor substitute and it is worth exploring whether the inferences are affected by the assumption that the day-side average circulation is the same as the day-night average circulation. The transport of angular momentum in Venus atmosphere is a key aspect of the atmospheric circulation. Newman and Leovy [1992] inferred that the thermal tides play a role in horizontal and vertical angular momentum transports. It is therefore worthwhile to compare the transfer of the strengths of the momentum transports by the average and the eddy circulations, as estimated from the day-side observations, and the zonal average estimates obtained from the application of the thermal tidal model to the day-side observations Transport by the Mean Circulation [50] The maintenance of the super rotation of the Venus atmosphere requires transport of momentum toward the equator. One possible process responsible for this is transport by eddies, since the mean circulation transports angular momentum poleward through the Hadley circulation [Leovy, 1973; Gierasch, 1975]. Using the traditional nomenclature, the eddy momentum transport term, hu 0 v 0 i, is computed from the deviations u 0 and v 0 of the measured winds from the calculated mean zonal and meridional components (hui and hvi) and, in the absence of night side measurements, is determined from the day-side vectors only. In the discussion that follows, U 0 and V 0 are equivalent to the zonal average values hui and hvi respectively. For lack of night-side results, the day-side longitudinal average values of the zonal and meridional components were substituted for hui and hvi. 11 of 16

12 Figure 9. Comparison of the meridional momentum transport term huihvi computed from the day-side averages, and those determined from the fit of the solar thermal tide model visually tracked vectors for the 1980 and 1982 seasons. The differences between the thermal tide model estimates, u 0 v 0, are considerable in the low northern latitudes, but the overall trend is the same: an export of retrograde momentum toward higher latitudes. [51] Figure 9 shows the profile of the momentum transport term by the mean circulation, huihvi computed from the day-side digital and visual tracking vectors and from the tidal fit results (i.e., the product U 0 V 0 ) for both 1980 and The difference between the 1980 and 1982 momentum transport term for the mean circulation (for the day side only) is much smaller than the difference between the results for the tidal mean circulation, and likely significant even accounting for errors in the tidal fit (for clarity the error bars for the 1982 tidal results are not shown in Figure 9, but the errors are comparable to or smaller than those for 1980 results) Poleward Momentum Transport by Eddies [52] Estimates of the eddy momentum transport term hu 0 v 0 i from the Mariner 10 and Pioneer Venus OCPP measurements of cloud motions (day-side) indicate poleward transport, but the magnitude is much weaker, almost insignificant compared to the day-side mean circulation contribution. This led to the previous suggestions that the equatorward transport of angular momentum either occurs on the night side at the cloud level, or at other, unmeasured levels of the atmosphere if the eddy transport plays any role in the super rotation of the Venus atmosphere. [53] Limaye and Suomi [1981], Limaye et al. [1982, 1988], and Rossow et al. [1990] were all forced to use the day-side average rather than the true zonal mean values hui and hvi, as no night-side observations were possible. This can impact the eddy amplitudes, as the true zonal mean can be (and probably is) different from the day-side mean values of the zonal and meridional components; thus the hu 0 v 0 i can be different. It is possible to illustrate this using an estimation of the zonal mean, obtained by applying our two component (diurnal and semidiurnal) tidal model as presented below Eddy Momentum Transport Inferred From the Solar Thermal Tidal Fit to Observations [54] If the true zonal average values for the zonal and meridional components differ significantly from the dayside averages, it must be asked whether the previous estimates of eddy momentum transports, hu 0 v 0 i are biased. Unfortunately, the original vectors were measured more than two decades ago and lost due to changes in computer hardware, so it is not possible to use them to estimate the eddy terms relative to the true zonal means rather than the day-side values. However, it is illustrative to examine the eddy momentum transport term from the tidal model, similar in a sense to the analytical modeling by Newman and Leovy [1992] but, in this case, based on actual fits to the data. [55] From equations (1) and (2), the eddy components can be expressed as u 0 ðq; l and v 0 ðq; l Þ ¼ U 0 uðq; lþ ¼ U 1 Sinðl þ F 1 ÞþU 2 Sinð2l þ F 2 Þ ð3þ Þ ¼ V 0 vðq; lþ ¼ V 1 Sinðl þ F 1 ÞþV 2 Sinð2l þ F 2 Þ ð4þ 12 of 16

13 Figure 10. Eddy transport term u 0 v 0 from the tidal model for 1982, wherein the eddies u 0 and v 0 are defined for each 4 latitude 10 solar longitude bin relative to the tidal fit zonal mean values U 0 and V 0. so that for a given latitude and longitude, u 0 v 0 ¼ fu 1 Sinðl þ F 1 ÞþU 2 Sinð2l þ F 2 Þg fv 1 Sinðl þ Y 1 Þþ V 2 Sinð2l þ Y 2 Þg ¼ U 1 V 1 Sinðl þ F 1 ÞSinðl þ Y 1 Þ þ U 2 V 1 Sinð2l þ F 2 ÞSinðl þ Y 1 Þ þ U 1 V 2 Sinðl þ F 1 ÞSinð2l þ Y 2 Þ þ U 2 V 2 Sinð2l þ F 2 ÞSinð2l þ Y 2 Þ ð5þ [56] Using the coefficients determined from the thermal tide model, it is possible to extrapolate the eddy transport field associated with the diurnal and semidiurnal tides alone. The eddies were defined relative to U 0 and V 0 for zonal and meridional components determined from the tidal model in latitude and longitude bins. Figure 10 shows the eddy momentum term u v for each of the bins for For both the periods, the confidence at the latitude extremes is lower due to sparse sampling. [57] The zonal average profiles with latitude of the transport term hu 0 v 0 i from the tidal field for 1980 and 1982 are shown in Figure 11. The magnitudes of the eddy transport term computed from the diurnal and semidiurnal tides combined are somewhat different from the day-side results obtained from the cloud vectors for 1980 and The transport term computed from the tidal field is much weaker than the contribution by the mean meridional circulation for 1982, just as that computed from the dayside vectors directly, but for 1980 the mean transport from the tidal field is much weaker than the day-side vectors. The estimated errors in mean transport from the tidal field for 1980 considerable such that the magnitude of both the mean and eddy transport terms are comparable, different from 1982 results and also in contrast with the transports estimated directly from the day-side vectors only. [58] Differences are seen between the meridional profiles of tidal field derived hu 0 v 0 i for the two seasons, and also from the profiles determined from the original vectors (i.e., day-side averages substituted for the zonal mean averages). The tidal field derived eddy transport term has the opposite sign compared to the mean term at most latitudes for the 1982 results. The shape of the eddy transport profile for 1980 profile is mostly consistent for both the tidal field Figure 11. Latitudinal profiles of the eddy momentum transport term hu 0 v 0 i derived from the synthesized tidal fields for 1980 and 1982 vectors. Magnitudes of these estimates are comparable to those obtained from the day-side vectors, but the direction is different for Since the peak eddy transport is much weaker than the mean transport for 1982, it is not apparent whether there is any real impact on the total transport, but this result indicates that the real momentum transport in the Venus atmosphere at the cloud level may be different from that estimated from the day-side vectors alone. 13 of 16