2013 .

E. Marcq and S. Lebonnois. Simulations of the latitudinal variability of CO-like and OCS-like passive tracers below the clouds of Venus using the Laboratoire de Météorologie Dynamique GCM. Journal of Geophysical Research (Planets), 118:1983-1990, 2013. [ bib | DOI | PDF version | ADS link ]

The lower atmosphere of Venus below the clouds is a transitional region between the relatively calm lowermost scale height and the superrotating atmosphere in the cloud region and above. Any observational constraint is then welcome to help in the development of general circulation models of Venus, a difficult task considering the thickness of its atmosphere. Starting from a state-of-the-art 3-D Venus General Circulation Model (GCM), we have included passive tracers in order to investigate the latitudinal variability of two minor gaseous species, carbonyl sulfide (OCS) and carbon monoxide (CO), whose vertical profiles and mixing ratios are known to vary with latitude between 30 and 40km. The relaxation to chemical equilibrium is crudely parametrized through a vertically uniform time scale τ. A satisfactory agreement with available observations is obtained with 108CO5108 s and 107OCS108 s. These results, in addition to validating the general circulation below the clouds, are also helpful in characterizing the chemical kinetics of Venus' atmosphere. This complements the much more sophisticated chemical models which focus more on thermodynamical equilibrium.

B. Gans, Z. Peng, N. Carrasco, D. Gauyacq, S. Lebonnois, and P. Pernot. Impact of a new wavelength-dependent representation of methane photolysis branching ratios on the modeling of Titan's atmospheric photochemistry. Icarus, 223:330-343, 2013. [ bib | DOI | PDF version | ADS link ]

A new wavelength-dependent model for CH4 photolysis branching ratios is proposed, based on the values measured recently by Gans et al. (Gans, B. et al. [2011]. Phys. Chem. Chem. Phys. 13, 8140-8152). We quantify the impact of this representation on the predictions of a photochemical model of Titan's atmosphere, on their precision, and compare to earlier representations. Although the observed effects on the mole fraction of the species are small (never larger than 50%), it is possible to draw some recommendations for further studies: (i) the Ly-α branching ratios of Wang et al. (Wang, J.H. et al. [2000]. J. Chem. Phys. 113, 4146-4152) used in recent models overestimate the CH2:CH3 ratio, a factor to which a lot of species are sensitive; (ii) the description of out-of-Ly-α branching ratios by the ”100% CH3” scenario has to be avoided, as it can bias significantly the mole fractions of some important species (C3H8); and (iii) complementary experimental data in the 130-140 nm range would be useful to constrain the models in the Ly-α deprived 500-700 km altitude range.