NCEP’s Nonhydrostatic Multiscale Atmospheric Model

Zavisa Janjic


As a part of the NOAA Environmental Modeling System (NEMS), a unified Nonhydrostatic Multiscale Model (NMMB) is being developed at the National Centers for Environmental Prediction (NCEP). The model has been designed for a broad range of spatial and temporal scales extending from LES and meso to global, and from short range weather forecasting to climate studies. The latitude-longitude grid is used for the global domain, and rotated latitude-longitude coordinate is employed for regional applications. A general, pressure-based terrain following vertical coordinate is applied. The model is fully compressible. The nonhydrostatic component of the model is designed in such a way as to avoid overspecification. As a consequence, the number of prognostic equations is reduced by one. “Isotropic” quadratic conservative finite-volume horizontal differencing employed in the model conserves a variety of basic and derived dynamical and quadratic quantities and preserves some important properties of differential operators. Among these, the conservation of energy and enstrophy improves the accuracy of nonlinear dynamics of the model on all scales. A fast Eulerian conservative and positive definite scheme has been developed for model tracers. Conservative polar boundary conditions are specified in the global limit, and the polar filter selectively slows down the wave components of the basic dynamical variables.

In very high-resolution tests the model dynamics successfully reproduced classical two-dimensional nonhydrostatic solutions. In regional short range runs, the model dynamics demonstrated the ability to develop the observed –3 and –5/3 spectral slopes with realistic transition between the two spectral ranges. These properties of the spectra were not induced by computational noise and maintained by numerical filters. In a decaying turbulence case on convective cloud scales, the model dynamics developed the –5/3 spectrum consistent with the 3D turbulence theory. On the meso scales, the NMMB replaced the WRF NMM in operations at NCEP as the North American Model (NAM) and in a number of ensemble and nested high resolution runs. A global forecasting system based on the NMMB has been run in order to test and tune the model and to examine its potential for medium range weather forecasting. The skill of the NMMB medium range forecasts has been comparable to that of other major medium range forecasting systems.

Remaining issues will be discussed and recent developmental work will be reviewed. Results of recent tests will be presented. In particular, the computational efficiency of the model on regional and global scales will be addressed, as well as scalability on massively parallel computers.