A new approach has been applied in developing the NCEP Nonhydrostatic Meso Model (NMM) within the WRF effort. Namely, instead of extending the cloud models to synoptic scales, the hydrostatic approximation is relaxed in an NWP model. In this way, the validity of the model dynamics is extended to nonhydrostatic motions, the number of prognostic equations is kept the same, and the preferred features of the hydrostatic formulation are preserved. This approach does not imply any additional approximation.

In the model, “isotropic” horizontal finite differencing is employed that conserves a variety of basic and derived dynamical and quadratic quantities. Among these, the conservation of energy and enstrophy improves the accuracy of the nonlinear dynamics of the model. In the vertical, the hybrid pressure-sigma coordinate has been chosen as the primary option. The forward-backward scheme is used for horizontally propagating fast waves, and an implicit scheme is used for vertically propagating sound waves. The Adams-Bashforth scheme is applied for non-split horizontal advection of the basic dynamical variables and for the Coriolis force. Despite the complexity of the spatial differencing, the model is computationally very efficient, primarily due to the design of the time stepping scheme and the choice of the horizontal grid.

Since recently, the NMM has been run operationally at NCEP. In the high-resolution NWP applications, the model has been highly competitive with mature hydrostatic NWP models and with other nonhydrostatic models. However, sensitivity tests reveal that the performance of the model can be further improved by reformulating/retuning of parts of model physics, and by more careful specification of initial and boundary conditions. Examples of results of the sensitivity tests will be presented.