30 research outputs found

    Importance of Phase Changes in Titan's Lower Atmosphere. Tools for the Study of Nucleation

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    39 pagesThe uncertainty about possible supersaturation of methane, condensation of volatile species and the existence of clouds in Titan's lower atmosphere affects our understanding of photochemistry, the nature of the surface and the atmospheric thermal structure. Indeed, photochemistry depends on the depth of penetration of energetic photons, affected by methane abundance. Radar and infrared observations of bright surface regions have been explained by rain washing of highlands. As for the thermal profile, it is sensitive to CH4-N2 gas opacity, cloud opacity and could be influenced by latent heat exchange. A rudimentary model with no methane supersaturation and gas transport by eddy diffusion indicates a methane latent heat release of 0.2 W m-2 between 20 and 30 km altitude for a surface mole fraction of 4.4 % and an eddy diffusion coefficient of 0.2 m2 s-1. Description of nucleation seems to be one of the first improvements which should be included in a model of phase changes. The suspicion of difficult methane nucleation comes from analysis of Voyager IRIS spectra. Moreover, species are expected to condense to the solid phase, which excludes very efficient nucleation and condensation processes associated with the presence of a liquid phase, such as deliquescence. The classical theory of heterogeneous nucleation, despite its deficiencies, is employed in atmospheric models, owing to its general nature and relative simplicity. Yet, it requires physical quantities for which experimental values do not exist. We show how surface free enthalpies of solids and contact angles may be linked to other material properties which are within reach of laboratory experiments, mainly ultraviolet absorption spectra of solid phases. We find that a value of 10^{-9} s-1 - 10^{-7} s-1 for the 'critical nucleation rate' (per nucleus) is adapted to the case of Titan, though we question the ability of the critical rate concept to make predictions for the condensation altitudes. A possible consequence of difficult methane nucleation is periodic evolution of the lower atmosphere, on a time scale of the order of 10^2 years

    A stochastic parameterization of the gravity waves due to convection and impact on the equatorial stratosphere

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    Model that includes a well resolved stratosphere. It is based on a stochastic approach, where an ensemble of monochromatic waves is built up, by launching few waves at each time step, and by cumulating the effect of these waves via an AR1 relation between the GWs tendency at a given time step and that at the next time step. Some properties of each waves are chosen randomly, like their wavenumbers and frequencies and with fixed probability distribution. Their amplitude nevertheless, is directly related to the precipitation, translating it in an heating rate and using linear theory to predict the amplitude of the waves such a heating can produce

    Modélisation des changements de phase dans la basse atmosphère de Titan

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    Doctorat en astrophysique et techniques spatiales -- Thèse soutenue devant le jury composé de : M. Fulchignoni (président), M. Cabane et C. McKay (rapporteurs), D. Gautier, F. Pacard, C. Régnaut, P. Bruston, F. RaulinI -- Importance of phase changes and nucleation in Titan's lower Atmosphere -- We have pointed out the interactions between phase changes in the lower atmosphere and other processes which occur on Titan. Phase change processes can affect photo-chemistry, the nature of Titan's exposed surface and hence its spectral properties, and the atmospheric thermal profile. We have critically reviewed clues that suggest that nucleation is difficult in Titan's atmosphere and we have tried to make clear why the description of nucleation is a priority in modeling phase changes. The results of the analysis of IRIS spectra and the low temperatures that imply solid rather than liquid phases are the main arguments for nucleation modeling. In the frame of the classical theory of nucleation, we have proposed a definition and a value of the critical nucleation rate which are adapted to the study of Titan's atmosphere. From this critical rate, we have computed the contact angle of methane on aerosols that is needed if the inhibition of methane nucleation is the reason for the super-saturation suggested by IRIS observations. -- II -- Estimation of contact angles and surface free enthalpies of solids -- Those parameters are essential for the description of nucleation. As experimental data for the solid phases of the species we are interested in are unavailable, we have selected and pieced together estimation methods for contact angles and surface free enthalpies of solids, from the existing literature. The surface free enthalpy of a solid may be linked to the surface tension of the liquid and the latent heats of vaporization and sublimation. However, a few variations on this idea have been proposed. In order to choose the adequate correlation for solid hydrocarbons and nitriles, the solid surface free enthalpy for a few selected reference species may also be estimated from the Hamaker constant. The Hamaker constant is itself linked to the dielectric permittivity, hence to the absorption spectra of the solid phase. Thus, we have opened up a way between ongoing laboratory investigations on the spectra of solid hydrocarbons and nitriles, and the computations of surface free enthalpies and contact angles. -- III -- Building models for the description of phase changes in Titan's lower atmosphere -- Essential phenomena to be described in a model of phase changes, beside condensation and evaporation, are nucleation, aerosol settling and gas transport by eddy diffusion. Nucleation is described with the classical theory. The contact angle is not assumed to be zero, it is a free parameter. We have managed to bring together these phenomena in the equations, addressing such problems as the volume of the aerosols just after nucleation and the treatment of aerosols with size smaller than the Kelvin equilibrium size. As we expect non-stationary evolution for some values of the contact angle, we have made the model time-dependent. Therefore, the model (numbered 3) consists of a set of partial differential equations. In order to check the validity of the model itself (the way nucleation is built into the equations) and the validity of the numerical resolution to come, we have considered a simpler model (numbered 2) with instantaneous nucleation, and we have demonstrated that the solutions of model 3 should tend toward a solution of model 2 when the contact angle and the surface free enthalpy of the condensed phase tend toward zero. -- IV -- We have studied two more simple models. One model (numbered 1) assumes negligible super-saturation of all species. This very easy-to-use model provides insight into the influence of such parameters as the eddy diffusion coefficient, the mass flux of tholins falling from the upper atmosphere, the radius of tholin particles, the gas mole fractions at the surface. In particular, for plausible values of those parameters (as found in the literature), by studying the self-consistency of the model, we have shown that methane super-saturation would rather be due to nucleation inhibition or coagulation than the dynamics of gas transport and condensation (after nucleation). Thus, small super-saturation is really not a hypothesis, and model 1 gives correct results in the event of easy nucleation and unimportant coagulation. Then the evaporation-condensation cycle of methane in the troposphere involves the evaporation of typically a few centimeters, up to fifty centimeters, of methane per year, and the concomitant exchange of latent heat may be on the order of 0.1 W m-2, up to 3 W m-2. Condensation of acetylene starts at about 61 km, and condensation of ethane at 51 km. At the tropopause, the radius of aerosols reaches approximately 3 times the radius of tholin nuclei. At 20 km, the radius has increased to typically 200 times the radius of tholin nuclei due to methane condensation. Finally, a model is specifically set up to demonstrate the potential periodic behavior associated to some values of the contact angle. As opposed to the other models mentioned above, it does not aim at providing values for properties of Titan's atmosphere, and gives a qualitative information. Periodicity is expected when the abundance from the instantaneous nucleation model is distinctly lower than the critical abundance (obtained from the critical nucleation rate), and the critical abundance itself is distinctly lower than the abundance without condensation. The period then diminishes with the critical abundance.I -- Importance des changements de phase et de la nucléation dans la basse atmosphère de Titan -- Nous soulignons les interactions entre les changements de phase dans la basse atmosphère et d'autres phénomènes qui se produisent sur Titan. Les processus de changements de phase peuvent affecter la photochimie, la nature de la surface exposée de Titan et donc ses propriétés optiques, et le profil de température de l'atmosphère. Nous passons en revue les indices d'une nucléation difficile dans l'atmosphère de Titan et nous essayons de montrer que la description de la nucléation est une priorité dans la modélisation des changements de phase. Les résultats de l'analyse des spectres d'IRIS et les basses températures qui impliquent des phases solides et non liquides fournissent les principaux arguments en faveur de la modélisation de la nucléation. Dans le cadre de la théorie classique de la nucléation, nous proposons une définition et une valeur du taux de nucléation critique adaptées à l'étude de l'atmosphère de Titan. A partir de ce taux critique, nous calculons l'angle de contact du méthane sur les aérosols nécessaire pour que l'inhibition de la nucléation du méthane puisse expliquer la sursaturation suggérée par les observations d'IRIS. -- II -- Estimation des angles de contact et des enthalpies libres surfaciques des phases solides -- Ces paramètres sont essentiels pour la description de la nucléation. Comme les données expérimentales pour les phases solides des espèces qui nous intéressent ne sont pas disponibles, nous relevons dans la littérature et mettons bout à bout des méthodes pour l'estimation des angles de contact et des enthalpies libres surfaciques des solides. L'enthalpie libre surfacique s d'un solide peut être reliée à la tension superficielle du liquide et aux chaleurs latentes de vaporisation et de sublimation. Cependant, quelques variations sur ce thème ont été proposées. Afin de choisir la corrélation adéquate pour les hydrocarbures et les nitriles solides, s peut être estimé pour quelques espèces de référence à partir de la constante d'Hamaker. La constante d'Hamaker est elle-même liée à la permittivité diélectrique, donc au spectre d'absorption de la phase solide. Ainsi, nous établissons un pont entre les études de laboratoire en cours sur les spectres des hydrocarbures et des nitriles solides, et le calcul des enthalpies libres surfaciques et des angles de contact. -- III -- Élaboration de modèles pour la description des changements de phase dans la basse atmosphère de Titan -- Les phénomènes essentiels qui doivent être décrits dans un modèle de changements de phase, outre la condensation et l'évaporation, sont la nucléation, la sédimentation des aérosols et le transport de gaz par diffusion turbulente. La nucléation est décrite à l'aide de la théorie classique. L'angle de contact n'est pas supposé nul, c'est un paramètre libre. Nous parvenons à mettre tous ces phénomènes ensemble en équation, en gérant des problèmes tels que le volume des aérosols juste après la nucléation et le traitement des aérosols de taille inférieure au rayon d'équilibre associé à l'effet Kelvin. Comme nous nous attendons à une évolution non stationnaire pour certaines valeurs de l'angle de contact, nous laissons le modèle dépendre du temps. Par conséquent, le modèle (numéroté 3) est représenté par un système d'équation aux dérivées partielles. Afin de vérifier la validité du modèle lui-même (la manière dont la nucléation est mise en équations) et la validité de la résolution numérique à venir, nous considérons un modèle (numéroté 2) plus simple avec nucléation instantanée, et nous démontrons que les solutions du modèle 3 doivent tendre vers une solution du modèle 2 lorsque l'angle de contact et l'enthalpie libre surfacique de la phase condensée tendent vers zéro. -- IV -- Nous étudions deux autres modèles simples. L'un des modèles (numéroté 1) suppose une sursaturation négligeable pour toutes les espèces. Ce modèle très facile à mettre en oeuvre permet de comprendre l'influence de certains paramètres comme le coefficient de diffusion turbulente, le flux de masse de tholins arrivant de la haute atmosphère, le rayon des particules de tholins et les fractions molaires en phase gazeuse à la surface. En particulier, pour des valeurs plausibles de ces paramètres (trouvées dans la littérature), en étudiant l'auto-cohérence du modèle, nous montrons que la sursaturation éventuelle du méthane serait due à l'inhibition de la nucléation ou à la coagulation plutôt qu'à la dynamique du transport gazeux et de la condensation (après la nucléation). Ainsi, la faible sursaturation n'est pas réellement une hypothèse et le modèle 1 donne des résultats corrects si la nucléation est facile et la coagulation peu importante. Dans ce cas, le cycle d'évaporation et de condensation du méthane dans la troposphère entraîne l'évaporation de typiquement quelques centimètres, et jusqu'à 50 cm, de méthane par an. L'échange de chaleur latente concomitant peut être de l'ordre de 0,1 W.m-2, et jusqu'à 3 W.m-2. La condensation de l'acétylène commence à environ 61 km, celle de l'éthane à 51 km. A la tropopause, le rayon des aérosols atteint approximativement 3 fois le rayon des noyaux de tholins. A 20 km, suite à la condensation du méthane, le rayon a augmenté jusqu'à une valeur qui est typiquement 200 fois supérieure au rayon des noyaux de tholins. Enfin, nous construisons un modèle spécialement pour illustrer le comportement périodique potentiel associé à certaines valeurs de l'angle de contact. Par opposition aux autres modèles, celui-ci ne prétend pas fournir des valeurs pour les propriétés de l'atmosphère de Titan, mais seulement une information qualitative. La périodicité est attendue quand l'abondance donnée par le modèle avec nucléation instantanée est nettement plus faible que l'abondance critique (déduite du taux de nucléation critique), elle-même nettement plus faible que l'abondance en l'absence de condensation. La période diminue alors avec l'abondance critique

    Gravity waves over Antarctica and the Southern Ocean: Consistent momentum fluxes in mesoscale simulations and stratospheric balloon observations

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    International audienceStratospheric balloons from the Vorcore campaign have provided a unique description of the gravity-wave field in the lower stratosphere above Antarctica and the Southern Ocean, during the austral spring of 2005. Mesoscale simulations are carried out to analyze further the gravity-wave field. First, the realism of the simulated waves is assessed by comparison to the observations. A satisfactory overall agreement is found, but different behaviour is noted for orographic waves (overestimation in the simulations relative to the observations) and non-orographic waves (underestimation). Second, the gravity-wave field is analyzed in more detail than was possible from observations alone. It is necessary to distinguish and quantify orographic and non-orographic waves separately. Orographic waves are larger and more intermittent, yet affect only a limited geographical region. Hence, although orographic sources stand out as 'hot spots' for gravity waves, their contribution to momentum fluxes entering the stratosphere is comparable to or smaller than the contribution of non-orographic sources. A diagnostic for intermittency, the Gini coefficient, is proposed. It clearly marks the distinction between orographic and non-orographic sources (intermittencies of ~ 0.8 and ~ 0.5 respectively). Sensitivity to resolution is quite small regarding the spatial structure of the gravity-wave field, but is significant for the amplitudes. The momentum flux values increase by ~ 2 when the horizontal resolution is doubled, and possible biases of both simulations and observations are discussed. Nonetheless, the good agreement between observations and simulations and the complementary information on the biases of each dataset promises that in the future these different estimates of gravity-wave momentum fluxes may converge. © 2012 Royal Meteorological Society

    Simulations of gravity waves over Antarctica and the Southern Ocean : comparison to balloon observations and investigation of nonorographic sources

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    Gravity waves in the lower stratosphere above Antarctica and the Southern Ocean have been characterized from observations by superpressure balloons from the Vorcore campaign (Hertzog et al 2008). Mesoscale simulations with the WRF (Weather Research and Forecast Model) are carried out to analyze further the gravity-wave field, with an emphasis on non-orographic waves

    The afterlives of network-based artworks

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    Towards glocal leadership: taking up the challenge of new local governance in Europe?

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    In recent years local leaders have become simultaneously intensely local, in fostering partnerships with local players, and intensely global, in going abroad to get resources, be they political, cognitive, or financial, to implement large urban projects; they have become ‘glocal’ leaders. At the same time, local stakeholders such as metropolitan or regional authorities, chambers of commerce, firms, or civil society organizations have pursued their own international objectives independently. Not all cities have therefore developed such a thing as a strategy. Nor have they all played the same hand in the international game. From this, two questions immediately arise: (i) why have some cities succeeded in carrying out an international strategy and others not? (ii) why have the successful ones chosen different paths? By focusing on local political leadership patterns in three European case studies, we argue that along with their institutional strengths the personal traits of the mayors are more than crucial to understanding why some cities have strategically played the international game and how they have played it. But this does not tell the whole story. One needs to make a detour via place-specific governance patterns, and more precisely the key local stakeholders to be convinced, persuaded, or cajoled, to properly assess the urban international strategies. With these major findings, we conclude that the ‘new international city era’ phenomenon tends to personalize local power.

    Tropical variability and stratospheric equatorial waves in the IPSLCM5 model

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    International audienceThe atmospheric variability in the equatorial regions is analysed in the Earth System Model pre-industrial simulation done at IPSL in the framework of CMIP5. We find that the model has an interannual variability of about the right amplitude and temporal scale, when compared to the El-Niño Southern Oscillation (ENSO), but that is too confined to the western Pacific. At the intra-seasonal periods, the model variability lacks of large-scale organisation, and only produces one characteristic Madden-Julian Oscillation every 10 winters typically. At shorter time-scales and in the troposphere, the model has Rossby and Kelvin Convectively Coupled Equatorial Waves (CCEWs), but underestimates the Kelvin CCEWs signal on OLR. In the model stratosphere, a composite analysis shows that the Temperature and velocities fluctuations due to the Kelvin waves are quite realistic. In the model nevertheless, the stratospheric waves are less related to the convection than in the observations, suggesting that their forcing by the midlatitudes plays a larger role. Still in the model, the Kelvin waves are not predominantly occurring during the life cycle of the tropospheric Kelvin CCEWs, a behaviour that we find to be dominant in the observations. The composite analysis is also used to illustrate how the waves modify the zonal mean-flow, and to show that the model Kelvin waves are too weak in this respect. This illustrates how a model can have a reasonable Kelvin waves signal on the velocities and temperature, but can at the same time underestimate their amplitude to modify the mean flow. We also use this very long simulation to establish that in the model, the stratospheric equatorial waves are significantly affected by ENSO, hence supporting the idea that the ENSO can have an influence on the Quasi-Biennial Oscillation. © 2011 Springer-Verlag
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