The NCEP mesoscale spectral model: A revised version of the nonhydrostaticregional spectral model

This paper illustrates a modified nonhydrostatic version of the National Ce nters for Environmental Prediction regional spectral model (RSM). This nonh ydrostatic version of the RSM can simulate atmospheric motions of all scale s, especially mesoscale. For simplicity, it is referred to in this paper as the mesoscale spectral model (MSM). The preliminary results of the previou s version of the MSM have been published on the year of 1992, with coarse r esolution in three dimensions and no model physics. A fine-resolution two-d imensional version has been tested on classical problems and published on t he year of 1994. Instead of an externally determined hydrostatic coordinate as originally de signed in 1992 MSM, the internally evolved hydrostatic coordinate as used i n the RSM is implemented. This modification makes the MSM closer to the hyd rostatic version in model structure and dynamics. Besides the hydrostatic p erturbation, related to the external hydrostatic state as perturbation nest ing, the nonhydrostatic perturbation related to the internally evolved hydr ostatic state is introduced. The same model physics used in the RSM are use d in the MSM without the hydrostatic assumption. The major numerical techni ques used in the hydrostatic version are used in the MSM as well. They are spectral computation, fourth-order horizontal diffusion, time filter, and s emi-implicit adjustment for perturbation. The hydrostatic state, interpolat ed from the hydrostatic global model, is used as the initial condition with out initialization or data assimilation, and it can be integrated up to sev eral days with reasonable predictions. Extended tests of thermal bubbles and mountain waves in very fine resolutio ns by this revised MSM showed its behavior to be the same as, but not super ior to, those of the previous version. These results are compatible to othe r model results in the literature. Cases using real data with full model ph ysics as used in the RSM show that the revised MSM has reasonable results a nd is superior to the previous version as compared with the RSM in a coarse horizontal grid resolution of about 50 km. Furthermore, it shows that it c an be successfully nested into the hydrostatic global model at 10- to 20-fo ld differences in horizontal resolution with a small domain due to the well -behaved perturbation nesting over flat plains, coastal oceans, and steep m ountains.