Forecast Experiments with a Global Finite-Difference Semi-Lagrangian Model

Abstract

A series of 10-day forecast experiments has been carried out to investigate the sensitivity of a global semi-Lagrangian model to the value of the uncentering parameter ?, the magnitude of the time step for the dynamics, and the numerical treatment of the physical parameterizations (semi-Lagrangian versus Eulerian). The model has been run at a resolution of 2° latitude by 2.5° longitude with 20 vertical levels. Results from the experiments with values of ? ranging from 0 to 0.4 show that ? = 0.2 gives the best overall forecasts. The experiments with the time step for the dynamics varying from 15 to 60 min indicate that the forecasts are sensitive to the time step for the dynamics, even when the time steps for the physical parameterizations are held constant. The forecasts with the 60-min time step for the dynamics show the best overall objective skill scores. The two versions of the semi-Lagrangian model, one with Eulerian physics and the other with semi-Lagrangian physics, give similar forecast skill scores. The semi-Lagrangian model is also compared with a corresponding Eulerian model. It is found that the forecasts from the two models have similar quality, even though the time step for the dynamics in the semi-Lagrangian model is 16 times as long as that in the Eulerian model and the physical parameterizations have been developed and tuned for the Eulerian model.