Further development of a model for a broad range of spatial and temporal scales

The Nonhydrostatic Multiscale Model (NMMB) designed for a broad range of spatial and temporal scales has been under development at the National Centers for Environmental Prediction (NCEP) as a part of the new National Environmental Modeling System (NEMS). The model follows the general modeling philosophy of the NCEP's WRF NMM grid-point regional dynamical core. The model uses the regular latitude-longitude grid for the global domain, and a rotated latitude-longitude grid in regional applications. The nonhydrostatic component of the model dynamics is introduced through an add–on module that can be turned on or off depending on resolution. The 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 the nonlinear dynamics of the model on all scales. “Across the pole” polar boundary conditions are specified in the global limit and the polar filter selectively slows down the wave components of the basic dynamical variables that would otherwise propagate faster in the zonal direction than the fastest wave propagating in the meridional direction.

Several model upgrades have been recently introduced. As a compromise between requirements for computational affordability and accuracy, a fast Eulerian conservative and positive definite scheme has been developed for model tracers. Conservative monotonization is applied in order to control over-steepening within the conservative and positive definite tracer advection scheme. Encouraging results have been obtained concerning the tracer mass conservation and shape preservation, as well as computational efficiency. As an effort for unification among NEMS models that was expected to improve data assimilation, several algorithms for generating general hybrid pressure-sigma coordinates have been examined. The requirements imposed were that the transition to the full pressure coordinate be below the tropopause level, and that the tropopause be well resolved. A coordinate formulation has been selected that allows that the transition point be as low as 300 hPa in the global domain without compromising vertical resolution away from high topography. The top of the model atmosphere has been raised to 0 hPa.