Simulations of the latitudinal variability of CO-like and OCS-like passive tracers below the clouds of Venus using the Laboratoire de Mété orologie Dynamique GCM

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 10⁸sâ‰τ _COâ‰5·10⁸ s and 10⁷sâ ‰τ_OCSâ‰10⁸ 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. Key Points Latitudinal variations of CO and OCS are well reproduced in a 3D GCM Timescales for chemical relaxation differ for CO and OCS CO is more controlled by dynamics than by chemistry (conversely for OCS)