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Venus upper atmosphere revealed by a GCM: II. Model validation with temperature and density measurements

G. Gilli, T. Navarro, S. Lebonnois, D. Quirino, V. Silva, A. Stolzenbach, F. Lefèvre, G. Schubert

Abstract
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 ~ 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.

Keywords
Venus GCM; Upper atmosphere; Variability; Transition region; Astrophysics - Earth and Planetary Astrophysics; Physics - Atmospheric and Oceanic Physics

Icarus
Volume 366, Number 1144, Page 24
2021 September

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia