A COMPARISON OF CLIMATE SIMULATIONS FROM A SEMI-LAGRANGIAN AND AN EULERIAN GCM

Two parallel 5-year climate simulations have been carried out to asses s the effect of changing from an Eulerian to a semi-Langrangian formul ation of a general circulation model's dynamical core with the physica l parameterizations unchanged. It has been found that the change in fo rmulation leads to significant differences in the simulated climates, both for fields determined mainly by the dynamics, such as sea level p ressure, and for those determined mainly by the physics, such as preci pitation. The differences result both directly from the changes in the dynamics and indirectly from the interactions of the dynamics with th e physics. Compared to the simulation with the Eulerian model, the pri ncipal improvement with the semi-Lagrangian model is a significant red uction in, or even elimination of, the cold bias in the polar upper tr oposphere 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 phys ical parameterizations used in the semi-Lagrangian model have been dev eloped and tuned for the Eulerian model. To optimize the performance o f the semi-Lagrangian model, it will be necessary to tune the physical parameterizations explicitly for this mode.