The NMC Nested Regional Spectral Model

A nested primitive equation regional spectral model is developed. The model consists of two components—a low-resolution global spectral model and a high-resolution regional spectral model. The two components have identical vertical structure and physical processes. The global model component is a low-resolution version of the operational National Meteorological Center (NMC) global spectral model and uses spherical harmonics as horizontal basis functions. The regional spectral model component is a primitive equation model on a stereographic projection and uses sine-cosine series as horizontal basis functions. The feature of the regional component is that it predicts deviations from the forecast of the global model component, first proposed by Hoyer. A semi-implicit time scheme, time filtering, initialization, and horizontal diffusion are applied to these deviations in the regional domain. Several sensitivity experiments on "nesting periods," lateral boundary treatments, and different global model base fields were performed. The results indicate that experiments with 3- or 6-h nesting periods had less noise along the lateral boundaries than those with 1-h nesting period. It was also found that blending along the lateral boundaries may not be necessary but that with relaxation the use of T30 or T62 resolution in the global model was sufficient for a regional model with a horizontal resolution of 80 km. The model was tested on real-data cases and was shown to have skill comparable to or better than the other NMC operational regional models. The cases shown in this paper included a 48-h prediction of an East Coast disturbance and the "U.S. storm of the century" in March 1993, all done with a horizontal resolution of 80 km, and a 5-day forecast of a hurricane track done with a horizontal resolution of 40 km.