Weakly forced atmospheric GCMs : Lessons from model comparisons

Even when forced with mostly identical physical parametrizations, general circulation models (GCMs) of Venus produce significant dispersion in the simulated zonal wind fields and meridional circulations. Horizontal resolution, lower boundary layer parametrization and initial state are among the most sensitive aspects, and consistent trends are not obtained between models (or even by the same model at varying resolution). When comparing simplified temperature forcing with realistic radiative transfer in the LMD Venus GCM, it is also clear that the description of this forcing is critical to understand the meridional circulation, and therefore the dynamical cycle of angular momentum in this sensitive atmosphere. The role of numerical aspects is also obvious in the case of Titan, another weakly forced atmosphere in superrotation. Modeling Titan’s stratospheric superrotation has proved difficult, and recent GCM successes highlight that our ability to model these processes correctly is highly sensitive to numerical aspects, especially horizontal dissipation. These studies show us that modelers need to keep humble and aware of the difficulty to assess robust behaviour with only one GCM. Intercomparison of different GCMs is a useful way to identify robust interpretations from model-dependent aspects.