2012 .

S. Lebonnois, C. Covey, A. Grossman, H. Parish, G. Schubert, R. Walterscheid, P. Lauritzen, and C. Jablonowski. Angular momentum budget in General Circulation Models of superrotating atmospheres: A critical diagnostic. Journal of Geophysical Research (Planets), 117:12004, 2012. [ bib | DOI | PDF version | ADS link ]

To help understand the large disparity in the results of circulation modeling for the atmospheres of Titan and Venus, where the whole atmosphere rotates faster than the surface (superrotation), the atmospheric angular momentum budget is detailed for two General Circulation Models (GCMs). The LMD GCM is tested for both Venus (with simplified and with more realistic physical forcings) and Titan (realistic physical forcings). The Community Atmosphere Model is tested for both Earth and Venus with simplified physical forcings. These analyses demonstrate that errors related to atmospheric angular momentum conservation are significant, especially for Venus when the physical forcings are simplified. Unphysical residuals that have to be balanced by surface friction and mountain torques therefore affect the overall circulation. The presence of topography increases exchanges of angular momentum between surface and atmosphere, reducing the impact of these numerical errors. The behavior of GCM dynamical cores with regard to angular momentum conservation under Venus conditions provides an explanation of why recent GCMs predict dissimilar results despite identical thermal forcing. The present study illustrates the need for careful and detailed analysis of the angular momentum budget for any GCM used to simulate superrotating atmospheres.

R. D. Lorenz, C. E. Newman, T. Tokano, J. L. Mitchell, B. Charnay, S. Lebonnois, and R. K. Achterberg. Formulation of a wind specification for Titan late polar summer exploration. Planetary and Space Science, 70:73-83, 2012. [ bib | DOI | PDF version | ADS link ]

Titan's polar regions, and its hydrocarbon lakes in particular, are of interest for future exploration. The polar conditions have considerable seasonal variation and are distinct from the equatorial environment experienced by Huygens. Thus specific environmental models are required for these regions. This paper, informed by Cassini and groundbased observations and four independent Global Circulation Models (GCMs), summarizes northern summer polar conditions (specifically, regions north of 65degN, during the 2023-2024 period, or solar longitude Ls150o-170deg) and presents a simple analytical formulation of expected, minimum and maximum winds as a function of altitude to aid spacecraft and instrument design for future exploration, with particular reference to the descent dispersions of the Titan Mare Explorer (TiME) mission concept presently under development. We also consider winds on the surface, noting that these (of relevance for impact conditions, for waves, and for wind-driven drift of a floating capsule) are weaker than those in the lowest cell in most GCMs: some previously-reported estimates of 'surface' wind speeds (actually at 90-500 m altitude) should be reduced by 20-35% to refer to the standard 10 m 'anemometer height' applicable for surface phenomena. A Weibull distribution with scale speed C=0.4 m/s and shape parameter k=2.0 embraces the GCM-predicted surface wind speeds.

C. F. Wilson, E. Chassefière, E. Hinglais, K. H. Baines, T. S. Balint, J.-J. Berthelier, J. Blamont, G. Durry, C. S. Ferencz, R. E. Grimm, T. Imamura, J.-L. Josset, F. Leblanc, S. Lebonnois, J. J. Leitner, S. S. Limaye, B. Marty, E. Palomba, S. V. Pogrebenko, S. C. R. Rafkin, D. L. Talboys, R. Wieler, L. V. Zasova, and C. Szopa. The 2010 European Venus Explorer (EVE) mission proposal. Experimental Astronomy, 33:305-335, 2012. [ bib | DOI | PDF version | ADS link ]

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an `M-class' mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55 km, where temperatures and pressures are benign (25degC and 0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.

S. Lebonnois, J. Burgalat, P. Rannou, and B. Charnay. Titan global climate model: A new 3-dimensional version of the IPSL Titan GCM. Icarus, 218:707-722, 2012. [ bib | DOI | PDF version | ADS link ]

We have developed a new 3-dimensional climate model for Titan's atmosphere, using the physics of the IPSL Titan 2-dimensional climate model with the current version of the LMDZ General Circulation Model dynamical core. Microphysics and photochemistry are still computed as zonal averages. This GCM covers altitudes from surface to 500 km altitude, with barotropic waves now being resolved and the diurnal cycle included. The boundary layer scheme has been changed, yielding a strong improvement in the tropospheric zonal wind profile modeled at Huygens descent position and season. The potential temperature profile is fairly consistent with Huygens observations in the lowest 10 km. The latitudinal profile of the near-surface temperature is close to observed values. The minimum of zonal wind observed by the Huygens probe just above the tropopause is also present in these simulations, and its origin is discussed by comparing solar heating and dynamical transport of energy. The stratospheric temperature and wind fields are consistent with our previous works. Compared to observations, the zonal wind peak is too weak (around 120 m/s) and too low (around 200 km). The temperature structures appear to be compressed in altitude, and depart strongly from observations in the upper stratosphere. These discrepancies are correlated, and most probably related to the altitude of the haze production. The model produces a detached haze layer located more than 150 km lower than observed by the Cassini instruments. This low production altitude is due to the current position of the GCM upper boundary. However, the temporal behaviour of the detached haze layer in the model may explain the seasonal differences observed between Cassini and Voyager 1. The waves present in the GCM are analyzed, together with their respective roles in the angular momentum budget. Though the role of the mean meridional circulation in momentum transport is similar to previous work, and the transport by barotropic waves is clearly seen in the stratosphere, a significant part of the transport at high latitudes is done all year long through low-frequency tropospheric waves that may be baroclinic waves.

D. Cordier, O. Mousis, J. I. Lunine, S. Lebonnois, P. Rannou, P. Lavvas, L. Q. Lobo, and A. G. M. Ferreira. Titan's lakes chemical composition: Sources of uncertainties and variability. Planetary and Space Science, 61:99-107, 2012. [ bib | DOI | arXiv | PDF version | ADS link ]

Between 2004 and 2007 the instruments of the Cassini spacecraft, orbiting within the Saturn system, discovered dark patches in the polar regions of Titan. These features are interpreted as hydrocarbon lakes and seas with ethane and methane identified as the main compounds. In this context, we have developed a lake-atmosphere equilibrium model allowing the determination of the chemical composition of these liquid areas present on Titan. The model is based on uncertain thermodynamic data and precipitation rates of organic species predicted to be present in the lakes and seas that are subject to spatial and temporal variations. Here we explore and discuss the influence of these uncertainties and variations. The errors and uncertainties relevant to thermodynamic data are simulated via Monte Carlo simulations. Global circulation models (GCM) are also employed in order to investigate the possibility of chemical asymmetry between the south and the north poles, due to differences in precipitation rates. We find that mole fractions of compounds in the liquid phase have a high sensitivity to thermodynamic data used as inputs, in particular molar volumes and enthalpies of vaporization. When we combine all considered uncertainties, the ranges of obtained mole fractions are rather large (up to 8500%) but the distributions of values are narrow. The relative standard deviations remain between 10% and 300% depending on the compound considered. Compared to other sources of uncertainties and variability, deviation caused by surface pressure variations are clearly negligible, remaining of the order of a few percent up to 20%. Moreover, no significant difference is found between the composition of lakes located in north and south poles. Because the theory of regular solutions employed here is sensitive to thermodynamic data and is not suitable for polar molecules such as HCN and CH3CN, our work strongly underlines the need for experimental simulations and the improvement of Titan's atmospheric models.

B. Charnay and S. Lebonnois. Two boundary layers in Titan's lower troposphere inferred from a climate model. Nature Geoscience, 5:106-109, 2012. [ bib | DOI | PDF version | ADS link ]

Saturn's moon Titan has a dense atmosphere, but its thermal structure is poorly known. Conflicting information has been gathered on the nature, extent and evolution of Titan's planetary boundary layer-the layer of the atmosphere that is influenced by the surface-from radio-occultation observations by the Voyager 1 spacecraft and the Cassini orbiter, measurements by the Huygens probe and by dune-spacing analyses. Specifically, initial analyses of the Huygens data suggested a boundary layer of 300m depth with no diurnal evolution, incompatible with alternative estimates of 2-3km (refs , , ). Here we use a three-dimensional general circulation model, albeit not explicitly simulating the methane cycle, to analyse the dynamics leading to the thermal profile of Titan's lowermost atmosphere. In our simulations, a convective boundary layer develops in the course of the day, rising to an altitude of 800m. In addition, a seasonal boundary of 2km depth is produced by the reversal of the Hadley cell at the equinox, with a dramatic impact on atmospheric circulation. We interpret fog that had been discovered at Titan's south pole earlier as boundary layer clouds. We conclude that Titan's troposphere is well structured, featuring two boundary layers that control wind patterns, dune spacing and cloud formation at low altitudes.

A. Migliorini, D. Grassi, L. Montabone, S. Lebonnois, P. Drossart, and G. Piccioni. Investigation of air temperature on the nightside of Venus derived from VIRTIS-H on board Venus-Express. Icarus, 217:640-647, 2012. [ bib | DOI | PDF version | ADS link ]

We present the spatial distribution of air temperature on Venus' night side, as observed by the high spectral resolution channel of VIRTIS (Visible and Infrared Thermal Imaging Spectrometer), or VIRTIS-H, on board the ESA mission Venus Express. The present work extends the investigation of the average thermal fields in the northern hemisphere of Venus, by including the VIRTIS-H data. We show results in the pressure range of 100-4 mbar, which corresponds to the altitude range of 65-80 km. With these new retrievals, we are able to compare the thermal structure of the Venus' mesosphere in both hemispheres. The major thermal features reported in previous investigations, i.e. the cold collar at about 65-70degS latitude, 100 mbar pressure level, and the asymmetry between the evening and morning sides, are confirmed here. By comparing the temperatures retrieved by the VIRTIS spectrometer in the North and South we find that similarities exist between the two hemispheres. Solar thermal tides are clearly visible in the average temperature fields. To interpret the thermal tide signals (otherwise impossible without day site observations), we apply model simulations using the Venus global circulation model Venus GCM (Lebonnois, S., Hourdin, F., Forget, F., Eymet, V., Fournier, R. [2010b]. International Venus Conference, Aussois, 20-26 June 2010) of the Laboratoire de Météorologie Dynamique (LMD). We suggest that the signal detected at about 60-70deg latitude and pressure of 100 mbar is a diurnal component, while those located at equatorial latitudes are semi-diurnal. Other tide-related features are clearly identified in the upper levels of the atmosphere.