Impact of a semi-Lagrangian and an Eulerian dynamical core on climate simulations

Abstract

To assess the impact of dynamical formulation on climate simulations, a semi-Lagrangian and an Eulerian dynamical core have been used for 5-yr climate simulations with the same physical parameterizations. The comparison of the climate simulations is focused on various eddy statistics (the study of time-mean states from the simulations has been published in a previous paper). Significant differences between the two simulations are evident. Generally, the stationary eddy variances are stronger in the semi-Lagrangian simulation while the transient eddy variances are stronger in the Eulerian simulation. Compared to the data assimilated by the Goddard Earth Observing System data assimilation system, the semi-Lagrangian simulation is closer to the assimilation in many aspects than the Eulerian simulation, even though the Eulerian model was used in the data assimilation. The paper shows that rather than corrupting the ability to diagnose model performance with a parallel data assimilation, quantitative rigor can be advanced because the model environment is more controlled. The two dynamical cores have been run for the idealized Held-Suarez tests to help understand the differences found in the climate simulations. The eddy statistics from the Held-Suarez tests are weaker and more diffused in the semi-Lagrangian than the Eulerian core. The transformed Eulerian mean diagnostics reveal that less wave activity propagates from the lower and middle troposphere into the upper troposphere in the semi-Lagrangian core. The residual circulation driven by eddy forcing is weaker in the semi-Lagrangian core than in the Eulerian core. Consequently, the semi-Lagrangian simulation is closer to the radiative equilibrium state than the Eulerian simulation. These diagnostics show that the different treatment of small-scale processes in the model (e.g., diffusion) profoundly impacts the simulation of the general circulation.