Models of Venus atmosphere

In the context of an International Space Science Institute(ISSI) working group, we have conducted a project to compare the most recent General Circulation Models (GCMs) of the Venus atmospheric circulation. A common configuration has been decided, with simple physical parametrization for the solar forcing and the boundary layer scheme. Six models have been used in this intercomparison project. The nominal simulation was run for more than 200 Venus days, and additional sensitivity runs have been done by several models to test the trends visible in these models when parameters are varied: topography, upper and lower boundary conditions, horizontal and vertical resolution, initial conditions. The results show that even in very similar modelling conditions, the wind speeds obtained with the different GCMs are widely different. Superrotation is obtained, but the shape (with or without marked high-latitude jets) and amplitude of the maximum zonal wind jet is different from one model to the other, from 15 to 50 m/s. Minor sensitivity is seen in several models to the upper boundary conditions, the topography or the vertical grid. Horizontal resolution and lower boundary conditions induce variations that are significant, affecting the amplitude and shape of the region of maximum zonal wind. Two models were started from an atmosphere already in superrotation. The simulations did not converge back to the nominal simulations, maintaining maximum zonal winds over 70 m/s (and even 100 m/s) without marked high-latitude jets. This study shows how sensitive GCMs are to the weak forcing of Venus atmosphere, and how difficult it is to draw precise conclusions on the circulation obtained with a single model, as well as on its sensitivity to some parameters