A Comparison of Climate Simulations from a Semi-Lagrangian and an Eulerian GCM

Abstract

Two parallel 5-year climate simulations have been carried out to assess the effect of changing from an Eulerian to a semi-Lagrangian formulation of a general circulation model's dynamical core with the physical parameterizations unchanged. It has been found that the change in formulation leads to significant differences in the simulated climates, both for fields determined mainly by the dynamics, such as sea level pressure, and for those determined mainly by the physics, such as precipitation. The differences result both directly from the changes in the dynamics and indirectly from the interactions of the dynamics with the physics. Compared to the simulation with the Eulerian model, the principal improvement with the semi-Lagrangian model is a significant reduction in, or even elimination of, the cold bias in the polar upper troposphere and lower stratosphere in both hemispheres. This improvement is evident in both the winter and summer seasons. It results from the more efficient poleward heat transport in the semi-Lagrangian model. The effect on other simulated fields can give results either closer to or farther from the corresponding analyses and observations. The physical parameterizations used in the semi-Lagrangian model have been developed and tuned for the Eulerian model. To optimize the performance of the semi-Lagrangian model, it will be necessary to tune the physical parameterizations explicitly for this model.