P. Rannou, M. Coutelier, E. Rivière, S. Lebonnois, M. Rey, and L. Maltagliati. Convection behind the Humidification of Titan's Stratosphere. Astrophysical Journal, 922(2):239, December 2021. [ bib | DOI | PDF version | ADS link ]
On Titan, methane is responsible for the complex prebiotic chemistry, the global haze, most of the cloud cover, and the rainfall that models the landscape. Its sources are located in liquid reservoirs at and below the surface, and its sink is the photodissociation at high altitude. Titan's present and past climates strongly depend on the connection between the surface sources and the atmosphere upper layers. Despite its importance, very little information is available on this topic. In this work, we reanalyze two solar occultations made by Cassini before the northern spring equinox. We find a layer rich in methane at 165 km and at 70S (mixing ratio 1.62% 0.1%) and a dryer background stratosphere (1.1%-1.2%). In the absence of local production, this reveals an intrusion of methane transported into the stratosphere by convective circulation. On the other hand, methane transport through the tropopause at a global scale appears quite inhibited. Leaking through the tropopause is an important bottleneck of Titan's methane cycle at all timescales. As such, it affects the long-term evolution of Titan's atmosphere and the exchange fluxes with the surface and subsurface reservoirs in a complex way. Global climate models accounting for cloud physics, thermodynamical feedbacks, and convection are needed to understand the methane cycle, and specifically the humidification of the stratosphere, at the present time, and its evolution under changing conditions at a geological timescale.
G. Gilli, T. Navarro, S. Lebonnois, D. Quirino, V. Silva, A. Stolzenbach, F. Lefèvre, and G. Schubert. Venus upper atmosphere revealed by a GCM: II. Model validation with temperature and density measurements. Icarus, 366:114432, September 2021. [ bib | DOI | arXiv | PDF version | ADS link ]
An improved high resolution (96 longitude by 96 latitude points) ground- to-thermosphere version of the Institut Pierre-Simon Laplace (IPSL) Venus General Circulation Model (VGCM), including non- orographic gravity waves (GW) parameterization and fine-tuned non-LTE parameters, is presented here. We focus on the validation of the model built from a collection of data mostly from Venus Express (2006-2014) experiments and coordinated ground-based telescope campaigns, in the upper mesosphere/lower thermosphere of Venus (80-150 km). These simulations result in an overall better agreement with temperature observations above 90 km, compared with previous versions of the VGCM. Density of CO2 and light species, such as CO and O, are also comparable with observations in terms of trend and order of magnitude. Systematic biases in the temperature structure are found between 80 and 100 km approximately (e.g. GCM is 20 to 40 K warmer than measurements) and above 130 km at the terminator (e.g. GCM is up to 50 K colder than observed). Possible candidates for those discrepancies are the uncertainties on the collisional rate coefficients used in the non-LTE parameterization (above 130 km), and assumptions on the CO2 mixing ratio made for stellar/solar occultation retrievals. Diurnal and latitudinal distribution of dynamical tracers (i.e. CO and O) are also analyzed, in a region poorly constrained by wind measurements and characterized by high variability over daily to weekly timescale. Overall, our simulations indicate that a weak westward retrograde wind is present in the mesosphere, up to about 120 km, producing the CO bulge displacement toward 2 h-3 h in the morning, instead of piling up at the anti-solar point, as for an idealized sub-solar to anti- solar circulation. This retrograde imbalance is suggested to be produced by perturbations of a -0.5ex 5 days Kelvin wave impacting the mesosphere up to 110 km (described in a companion paper Navarro et al., 2021), combined with GW westward acceleration in the lower thermosphere, mostly above 110 km. On the whole, these model developments point to the importance of the inclusion of the lower atmosphere, higher resolution and finely tuned parameterizations in GCM of the Venusian upper atmosphere, in order to shed light on existing observations.
T. Navarro, G. Gilli, G. Schubert, S. Lebonnois, F. Lefèvre, and D. Quirino. Venus' upper atmosphere revealed by a GCM: I. Structure and variability of the circulation. Icarus, 366:114400, September 2021. [ bib | DOI | PDF version | ADS link ]
A numerical simulation of the upper atmosphere of Venus is carried out with an improved version of the Institut Pierre-Simon Laplace (IPSL) full-physics Venus General Circulation Model (GCM). This simulation reveals the organization of the atmospheric circulation at an altitude above 80 km in unprecedented detail. Converging flow towards the antisolar point results in supersonic wind speeds and generates a shock-like feature past the terminator at altitudes above 110 km. This shock-like feature greatly decreases nightside thermospheric wind speeds, favoring atmospheric variability on a hourly timescale in the nightside of the thermosphere. A ~5-day period Kelvin wave originating in the cloud deck is found to substantially impact the Venusian upper atmosphere circulation. As the Kelvin wave impacts the nightside, the poleward meridional circulation is enhanced. Consequently, recombined molecular oxygen is periodically ejected to high latitudes, explaining the characteristics of the various observations of oxygen nightglow at 1 . 27 μm . An analysis of the simulated 1 . 27 μm oxygen nightglow shows that it is not necessarily a good tracer of the upper atmospheric dynamics, since contributions from chemical processes and vertical transport often prevail over horizontal transport. Moreover, dayside atomic oxygen abundances also vary periodically as the Kelvin wave momentarily decreases horizontal wind speeds and enhances atomic oxygen abundances, explaining the observations of EUV oxygen dayglow. Despite the nitrogen chemistry not being currently included in the IPSL Venus GCM, the apparent maximum NO nightglow shifted towards the morning terminator might be explained by the simulated structure of winds.
J. E. Silva, P. Machado, J. Peralta, F. Brasil, S. Lebonnois, and M. Lefèvre. Characterising atmospheric gravity waves on the nightside lower clouds of Venus: a systematic analysis. Astronomy Astrophysics, 649:A34, May 2021. [ bib | DOI | arXiv | PDF version | ADS link ]
We present the detection and characterisation of mesoscale waves on the lower clouds of Venus using images from the Visible Infrared Thermal Imaging Spectrometer onboard the European Venus Express space mission and from the 2 μm camera (IR2) instrument onboard the Japanese space mission Akatsuki. We used image navigation and processing techniques based on contrast enhancement and geometrical projections to characterise morphological properties of the detected waves, such as horizontal wavelength and the relative optical thickness drop between crests and troughs. Additionally, we performed phase velocity and trajectory tracking of wave packets. We combined these observations to derive other properties of the waves such as the vertical wavelength of detected packets. Our observations include 13 months of data from August 2007 to October 2008, and the entire available data set of IR2 from January to November 2016. We characterised almost 300 wave packets across more than 5500 images over a broad region of the globe of Venus. Our results show a wide range of properties and are not only consistent with previous observations but also expand upon them, taking advantage of two instruments that target the same cloud layer of Venus across multiple periods. In general, waves observed on the nightside lower cloud are of a larger scale than the gravity waves reported in the upper cloud. This paper is intended to provide a more in-depth view of atmospheric gravity waves on the lower cloud and enable follow-up works on their influence in the general circulation of Venus.