Weakly forced atmospheric GCMs: Lessons from model

Even when forced with mostly identical physical parametrizations, general circulation models (GCMs) of Venus produce significant dispersion in the sim-ulated zonal wind fields and meridional circula-tions. Horizontal resolution, lower boundary layer parametrization and initial state are among the most sensitive aspects, and consistent trends are not ob-tained 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 forc-ing is critical to understand the meridional circulation, and therefore the dynamical cycle of angular momen-tum in this sensitive atmosphere. The role of numerical aspects is also obvious in the case of Titan, another weakly forced atmosphere in su-perrotation. Modeling Titan’s stratospheric superrota-tion has proved difficult, and recent GCM successes highlight that our ability to model these processes cor-rectly is highly sensitive to numerical aspects, espe-cially horizontal dissipation. These studies show us that modelers need to keep humble and aware of the difficulty to assess robust be-haviour with only one GCM. Intercomparison of dif-ferent GCMs is a useful way to identify robust inter-pretations from model-dependent aspects. 1