The NCEP Mesoscale Spectral Model: A Revised Version of the Nonhydrostatic Regional Spectral Model

Abstract

This paper illustrates a modified nonhydrostatic version of the National Centers for Environmental Prediction regional spectral model (RSM). This nonhydrostatic version of the RSM can simulate atmospheric motions of all scales, especially mesoscale. For simplicity, it is referred to in this paper as the mesoscale spectral model (MSM). The preliminary results of the previous version of the MSM have been published on the year of 1992, with coarse resolution in three dimensions and no model physics. A fine-resolution two-dimensional version has been tested on classical problems and published on the year of 1994. Instead of an externally determined hydrostatic coordinate as originally designed in 1992 MSM, the internally evolved hydrostatic coordinate as used in the RSM is implemented. This modification makes the MSM closer to the hydrostatic version in model structure and dynamics. Besides the hydrostatic perturbation, related to the external hydrostatic state as perturbation nesting, the nonhydrostatic perturbation related to the internally evolved hydrostatic state is introduced. The same model physics used in the RSM are used in the MSM without the hydrostatic assumption. The major numerical techniques used in the hydrostatic version are used in the MSM as well. They are spectral computation, fourth-order horizontal diffusion, time filter, and semi-implicit adjustment for perturbation. The hydrostatic state, interpolated from the hydrostatic global model, is used as the initial condition without initialization or data assimilation, and it can be integrated up to several days with reasonable predictions. Extended tests of thermal bubbles and mountain waves in very fine resolutions by this revised MSM showed its behavior to be the same as, but not superior to, those of the previous version. These results are compatible to other model results in the literature. Cases using real data with full model physics as used in the RSM show that the revised MSM has reasonable results and 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 can be successfully nested into the hydrostatic global model at 10- to 20-fold differences in horizontal resolution with a small domain due to the well-behaved perturbation nesting over flat plains, coastal oceans, and steep mountains.