Approximations and sensitivity experiments with a baroclinic semi- Lagrangian spectral model

Abstract

First of all it is confirmed that even in these global runs with full physical parameterizations the sensitivity to the conversion from an Eulerian to a semi-Lagrangian formulation is acceptably small. In time truncation error tests with a T79 horizontal resolution it is found that, on average and with full physics, 30 min is about the upper limit for the time step based on acceptable time truncation errors. Since the Courant-Freidrichs-Lewy limit for the corresponding Eulerian model is about 10 min and the overhead of the semi-Lagrangian calculations is less than 30% per time step, there is a significant gain in efficiency by using the optimized semi-Lagrangian formulation. Results are also presented showing the improvement in the accuracy of the 5-day forecasts obtained by raising the model top and by increasing the horizontal resolution. -from Authors