Incremental implementation of model dynamics for NCEP GFS

The global atmospheric model of NCEP (National Centers for Environmental Prediction) GFS (Global Forecast System) has been used as the base model for several dynamic implementations. The original design of the model structure and spectral computation in horizontal are not changed, and the current operational version model physics are used. However, different discretization in vertical, prognostic variables, and time integration scheme are implemented and tested.

The first phase of implementation is to change vertical discretization from sigma and/or sigma-pressure vertical coordinates into generalized vertical coordinates, which cover the capabilities to combine sigma, pressure, and entropy as coordinates, with fully conservations. With combined coordinates, we can have more freedom to select coordinates for better numerical accuracy. The second phase is to have all gas tracers into the thermodynamic equation, which leads to use enthalpy as thermodynamic variable instead of temperature in the entire model dynamics for further accuracy. The third phase is to implement non-iteration dimensional-split semi-Lagrangian (NDSL) positive-definite advection for all tracers, so the negative tracers from model dynamic due to numerical method can be avoid. The fourth phase is to implement entire system with NDSL to have large time step for computational efficiency. Some results of these four phases will be presented.