The National Centers for Environmental Prediction’s (NOAA/NCEP/EMC) Non-hydrostatic Multiscale Model on the B grid (NMMB; Janjic, 2005; Janjic and Black, 2007; Janjic et al., 2011) is an evolution of the operational WRF-NMME model (Janjic et al., 2001; Janjic, 2003) that has been reformulated from the Arakawa E grid to the Arakawa B grid and extended from meso- to global scales. Its unified non-hydrostatic dynamical core supports regional and global simulations and includes generalized hybrid vertical coordinate and improved tracer advection schemes. NMMB became operational at NCEP as the regional North American Mesoscale (NAM) model in October 2011.
The model dynamics preserves a number of important properties of differential operators and conserves a variety of basic and derived first order and quadratic quantities. The conservation of energy and enstrophy in case of the nondivergent flow improves the accuracy of nonlinear dynamics. The formulation of nonhydrostatic dynamics avoids the overspecification of the vertical velocity. The global version is run on the latitude-longitude grid, and the regional version uses a rotated latitude-longitude grid in order to reduce the variation of the grid size. On the global scales, polar filter selectively slows down the waves that would otherwise be unstable. The physical package was developed from the WRF NMM's physics. The NCEP's Global Forecasting System (GFS) physics is also available. The regional version is run operationally as the main deterministic North American short-range forecasting model and in a number of other applications. The global NMMB has also been run over the last few years experimentally in order to assess its capabilities and develop it further. Generally, the performance of the global NMMB in medium range weather forecasting has been comparable to that of other major medium range forecasting systems, and its computational efficiency satisfies and exceeds the current and projected operational requirements. The recent developmental efforts have been focused on the improvement of interaction between clouds and radiation. With the new approach, the clouds are represented directly by the optical properties of their microphysics species, and not by the bulk cloud properties derived from the cloud cover estimated using empirical formulas.
REMS is designed to provide the components of the Earth System (sub-models) to interact through different feedback mechanisms. The sub-models are, in most cases, callable routines in NMMB, i.e. one time step of the atmospheric driver is followed by time steps of the sub-model components. Whenever possible and feasible, two-way interaction rather than one-way forcing between modelling components of the system will be developed.
