Turbulence modelling in Titan's zonal wind collapse

International audience1. Context The atmosphere of Titan is interesting by many aspects: it has the thickest atmosphere for a moon in the solar system, an atmosphere in superrotation in the stratosphere, an hemispheric asymmetry of temperature and an haze feedback of haze distribution on circulation between many others. There is another feature by which the atmosphere of Titan is unique, a strong decrease of the zonal wind between 60 and 100 km known as the "zonal wind collapse" (Fig-ure 1). The first measurement of this feature performed by ground-based radio-telescopes recording the Doppler Wind Experiment measurements of the carrier frequency during the Huygens descent [1]. The wind measured above 120 km was approximately of 100 m s −1. Then, below, the wind decreased to about few meters per seconds around 70 km before increasing again to 40 m s −1 at 60 km. 2. Our methodology 2.1 Principle Global Circulation Models (GCM) are powerful tools to study atmospheric circulations and have been employed to study the different planets of the solar system as well as Titan [2, 3, 4]. Although the different models are able to reproduce a realistic atmospheric circulation with superrotation, they fail to reproduce the observed zonal wind collapse characterized by a decrease towards only a few meters per second. We propose here to study for the first time this wind structure using turbulence-resolving model [5]. 2.2 Model description In order to investigate this peculiar wind feature we use the WRF compressible and non-hydrostatic dy-namical core to perform large-eddy simulation (LES) [6]. The timescale of the resolved turbulence is significantly smaller than the radiative timescale, comparable to one Titan year at this altitude [7], so no radiative Figure 1: Huygens temperature (K) and zonal wind profile (m s −1) between 50 and 100 km. processes are taken into account. The model is initialized using pressure, temperature and wind vertical profile as measured by the Huygens probe and shown in Figure 1. The atmospheric and planetary constants (gravity, heat capacity ...) within the model are set to Titan values. The horizontal grid spacing is 20 m spread over a 2 km-wide domain and the vertical grid features 300 levels from 60 to 90 km altitudes. 3. Wave generation Figure 2 displays the vertical wind (top) the associated vertical Eliassen-Palm flux (bottom) ρu w with ρ the density of the atmosphere and u and w the mean perturbation to the mean (domain-averaged) value of the zonal wind u and vertical wind w. The strong decrease of the zonal wind between 65 and 60 km causes a Kelvin-Helmholtz instability that leads to the generation of gravity waves. These waves propagates both towards the ground and towards the upper atmosphere. The dissipation of the wave engenders a momentum transfer to the flow and impacts the zonal wind

Mesoscale modeling of the atmposhere of Venus : convection and gravity wave

Les observations par les missions Venus Express et Akatsuki ont apporté une vision sans précédent de la turbulence de l’atmosphère de Vénus. L’extension verticale de la couche convective présente au coeur des nuages ainsi que sa variabilité avec la latitude et l’heure locale ont été étudiées, des ondes de gravité de petite échelle ont été observées au-dessus et en dessous de la convection. Les mesures par radio-occultation au sommet des nuages, vers 70 km d’altitude, ont rapporté une atmosphère stable, cependant des cellules ont été observées à ces altitudes aux basses latitudes à midi. Récemment, des ondes stationnaires de grande échelle en forme d’arc de cercle ont été observées au-dessus des plus grands reliefs dans les tropiques. Malgré toutes ces observations des questions demeurent. Pour répondre à ces questions nous avons décidé d’utiliser le modèle WRF pour pouvoir résoudre la turbulence de petite échelle. Le transfert radiatif du modèle de circulation générale (GCM) de Vénus du LMD a été couplé à ce modèle pour être le plus réaliste possible. Avec les simulations aux grands tourbillons, l’activité convective dans l’atmosphère de Vénus a pu être étudiée. Avec le mode mesoscale, les ondes de montagnes ont été étudiées et les plus grands reliefs des tropiques engendrent des ondes de montagnes de grande échelle similaires en amplitude et en extension latitudinale avec les observations.The observations made by the mission Venus Express and Akatsuki gave unprecedented insight of the turbulence of the atmosphere of Venus. The vertical extension of the cloud convective layer as well as the variability with latitude and local time has been studied, small-scale gravity waves have been observed both above and below this convection layer. Despite a stable atmosphere, cellular features have been observed at the top of the cloud at low latitude at noon. Recently large stationnary bow-shape waves have been measured above the main topographic features at low latitude. Despite these observations, some questions remain. In order to address these questions we used the WRF dynamical core to be able to resolve smallscale turbulence. With Large-Eddy Simulations (LES), simulations were performed to resolve the convective activity of the could layer and the induced gravity waves. With the mesoscale mode, high-resolution topography produces stationary bow-shape waves with amplitude and latitudinal extension consistent with observations

Modélisation petite échelle de l'atmosphère de Vénus : convection et onde de gravité

The observations made by the mission Venus Express and Akatsuki gave unprecedented insight of the turbulence of the atmosphere of Venus. The vertical extension of the cloud convective layer as well as the variability with latitude and local time has been studied, small-scale gravity waves have been observed both above and below this convection layer. Despite a stable atmosphere, cellular features have been observed at the top of the cloud at low latitude at noon. Recently large stationnary bow-shape waves have been measured above the main topographic features at low latitude. Despite these observations, some questions remain. In order to address these questions we used the WRF dynamical core to be able to resolve smallscale turbulence. With Large-Eddy Simulations (LES), simulations were performed to resolve the convective activity of the could layer and the induced gravity waves. With the mesoscale mode, high-resolution topography produces stationary bow-shape waves with amplitude and latitudinal extension consistent with observations.Les observations par les missions Venus Express et Akatsuki ont apporté une vision sans précédent de la turbulence de l’atmosphère de Vénus. L’extension verticale de la couche convective présente au coeur des nuages ainsi que sa variabilité avec la latitude et l’heure locale ont été étudiées, des ondes de gravité de petite échelle ont été observées au-dessus et en dessous de la convection. Les mesures par radio-occultation au sommet des nuages, vers 70 km d’altitude, ont rapporté une atmosphère stable, cependant des cellules ont été observées à ces altitudes aux basses latitudes à midi. Récemment, des ondes stationnaires de grande échelle en forme d’arc de cercle ont été observées au-dessus des plus grands reliefs dans les tropiques. Malgré toutes ces observations des questions demeurent. Pour répondre à ces questions nous avons décidé d’utiliser le modèle WRF pour pouvoir résoudre la turbulence de petite échelle. Le transfert radiatif du modèle de circulation générale (GCM) de Vénus du LMD a été couplé à ce modèle pour être le plus réaliste possible. Avec les simulations aux grands tourbillons, l’activité convective dans l’atmosphère de Vénus a pu être étudiée. Avec le mode mesoscale, les ondes de montagnes ont été étudiées et les plus grands reliefs des tropiques engendrent des ondes de montagnes de grande échelle similaires en amplitude et en extension latitudinale avec les observations

The impact of turbulent vertical mixing in the Venus clouds on chemical tracers

International audienceVenus' clouds host a convective layer between roughly 50 and 60 km that mixes heat, momentum, and chemical species. Observations and numerical modelling have helped to understand the complexity of this region. However, the impact on chemistry is still not known. Here, we use for the first time a three-dimensional convection-resolving model with passive tracers to mimic SO 2 and H 2 O for two latitudinal cases. The tracers are relaxed towards a vertical profile in agreement with measured values, with a timescale varying over several orders of magnitude. The vertical mixing is quantified, it is strong for a relaxation timescale high in front of the convective timescale, around 4 h. The spatial and temporal variability of the tracer due to the convective activity is estimated, with horizontal structures of several kilometres. At the Equator, the model is resolving a convective layer at the cloud top (70 km) suggested by some observations, the impact of such turbulent activity on chemical species is accounted for the first time. From the resolved convective plumes, a vertical eddy diffusion is estimated, consistent with past estimations from in-situ measurements, but several orders of magnitude higher than values used in 1D chemistry modelling. The results are compared to observations, with some spatial and temporal variability correlation, suggesting an impact of the convective layer on the chemical species

Turbulent vertical mixing of H2O and SO2 in the Venus cloud layer

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Turbulent Vertical Mixing in the Venus Cloud Layer

International audienceVenus hosts a global cloud layer with a convective layer that mixes momentum, heat, and chemical species and generates gravity waves. This vertical mixing is still not understood properly. We proposed to use convection-resolving models to study i

Mesoscale modeling of Venus' bow-shape waves

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Organization of the convection in the Venusian cloud layer

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Convection modeling of pure-steam atmospheres

Condensable species are crucial to shaping planetary climate. A wide range of planetary climate systems involve understanding nondilute condensable substances and their influence on climate dynamics. There has been progress on large-scale dynamical effects and on 1D convection parameterization, but resolved 3D moist convection remains unexplored in nondilute conditions, though it can have a profound impact on temperature/humidity profiles and cloud structure. In this work, we tackle this problem for pure-steam atmospheres using three-dimensional, high-resolution numerical simulations of convection in postrunaway atmospheres. We show that the atmosphere is composed of two characteristic regions, an upper condensing region dominated by gravity waves and a lower noncondensing region characterized by convective overturning cells. Velocities in the condensing region are much smaller than those in the lower, noncondensing region, and the horizontal temperature variation is small. Condensation in the thermal photosphere is largely driven by radiative cooling and tends to be statistically homogeneous. Some condensation also happens deeper, near the boundary of the condensing region, due to triggering by gravity waves and convective penetrations and exhibits random patchiness. This qualitative structure is insensitive to varying model parameters, but quantitative details may differ. Our results confirm theoretical expectations that atmospheres close to the pure-steam limit do not have organized deep convective plumes in the condensing region. The generalized convective parameterization scheme discussed in Ding & Pierrehumbert is appropriate for handling the basic structure of atmospheres near the pure-steam limit but cannot capture gravity waves and their mixing which appear in 3D convection-resolving models