NWP Experiments with a Gridpoint Semi-Lagrangian Semi-ImplicitGlobal Model at NCEP

Abstract

A series of 5-day forecasts for both winter and summer initial conditions have been performed using a fairly high resolution version (0.9375° latitude?longitude grid with 28 vertical levels) of a gridpoint semi-Lagrangian global forecast model with second-order semi-implicit off-centering and a time step of 1800 s. Most of the physics included in the model are taken from the NCEP operational MRF model. The forecasts are evaluated by comparing the anomaly correlations computed using the NCEP operational global analyses with those for the operational MRF forecasts. It is found that the skill scores of the semi-Lagrangian model are fairly close to those of the operational MRF model despite the handicap of not having its own analyses forecast cycle. The forecasts are also used to get an estimate of the climate drift of the model. It is found that the drift is similar to but somewhat larger than that for the NCEP MRF model. In the second part of the paper, a spatially averaged Eulerian treatment of orography is incorporated into the semi-Lagrangian model and its performance is compared against its semi-Lagrangian counterpart. Eight 5-day forecasts are made, four with the Eulerian treatment of orography and four with the semi-Lagrangian treatment, for both first-order and second-order off-centering and with time steps of 1800 and 2700 s, respectively. It is found that the spatially averaged Eulerian representation of orography has superior mass conservation but has slightly more noise at upper levels over the Himalayas. It is also found that first-order off-centering with ? = 0.1 gives as good a forecast as the second-order off-centering. Increasing the time step from 1800 to 2700 s is associated with some damping of the fields.