Sensitivity of the NCEP regional spectral model to domain size and nestingstrategy

This paper evaluates the performance of the National Centers for Environmen tal Prediction (NCEP) Regional Spectral Model (RSM) based on the sensitivit ies of different model domain sizes and horizontal resolutions. The perturb ation method and the spectral computation in the NCEP RSM construct the nes ting strategy as a "domain nesting'' in physical space as well as a "spectr al nesting'' in spectral space, instead of the conventional "lateral bounda ry nesting'' as used in most regional models. The NCEP RSM has the same mod el structure, dynamics, and physics as its outer coarse-resolution global m odel, and it also has a terrain blending along the lateral boundary at the initial time. Both together result in a smooth lateral boundary behavior th rough one-way nesting. An optimal lateral boundary relaxation reduces the i nfluence of lateral boundary error and generates more areas with small-scal e features. The treatment of numerical stabilities, such as a semi-implicit time scheme, time filter, and horizontal diffusion, are applied in perturb ation without recomputing or disturbing the large-scale waves. The combinat ion of the aforementioned methods is the uniqueness of the NCEP RSM, which demonstrates the capabilities to conserve the large-scale waves, resolve th e mesoscale features, and minimize the lateral boundary errors. A case of winter cyclogenesis with propagation of the synoptic-scale distur bances through the lateral boundaries was selected to investigate the sensi tivities of the NCEP RSM based on different nesting strategies. The results from the experiment over a quarter-sphere domain with similar resolutions between RSM and T126 global model demonstrated that the domain nesting was a success, because the lateral boundary error and perturbation were negligi bly small. The experiments in a 48-km resolution with different sizes of th e model domain had mixed results. The continental domain had the best perfo rmance but inclined to generate erroneous large-scale waves that degraded i ts performance after 60 h. The results of the regional and subregional doma ins were proximity to their base field, T126, in terms of root-mean-square differences. They had similar mesoscale features in a 48-km horizontal reso lution regardless of the different model domain sizes. The results from the experiments with nesting in different coarse grids over the radar-range do main imply that it can use either a T126 or subregional domain as its base field for similar performances. Nevertheless, more mesoscale features were obtained by the experiment with the base field at higher resolution. The re sults from the experiments, with 30-day integration, reveal that the perfor mance of the experiment in the subregional domain was much closer to its ba se field than that in the continental domain. It indicates that the predict ability of the global model is the predictability of the NCEP RSM in the re gional domain; however, the regional domain could generate higher-resolutio n features than its base field. This successful long-range integration with the regional domain is because the lateral boundary errors are relatively small and the large-scale waves are preserved through the domain and spectr al nesting.