From global to meso scales with a unified model

The Nonhydrostatic Mesoscale Model (NMM) WRF core has been developed building on NWP experience. Namely, the nonhydrostatic dynamics were formulated relaxing the hydrostatic approximation in a successful regional hydrostatic NWP formulation. The nonhydrostatic extension is introduced through add–on module that can be turned on or off. The extra computational cost of the nonhydrostatic dynamics is low, or nonexistent if the nonhydrostatic extension is switched off at coarser resolutions, which makes this approach attractive for models designed for a wide range of horizontal resolutions, and in particular for unified global and regional forecasting systems.

In order to explore the capabilities of the formulation on larger spatial and temporal scales, a global version of the model is being developed. In the initial global version “rigid wall” polar boundary conditions are specified and polar filtering is used. Another approach to the problem of spherical geometry is also being considered.

Despite the complexity of the finite differencing, the computational efficiency of the global model estimated on the basis of the performance of the existing serial code is competitive with computational efficiency of semi-Lagrangian models. The accuracy of extended experimental forecasts with currently used modest horizontal resolution of about 120 km is also encouraging. The high computational efficiency of the model promises the possibility of application of nonhydrostatic dynamics on the global scale when the single digit resolutions become affordable.