Double Fourier series dynamical core with hybrid sigma-pressure vertical coordinate

The hybrid sigma-pressure vertical coordinate is implemented into the double Fourier series (DFS) dynamical core of the Global/Regional Integrated Model system (GRIMs). Using traditional verification metrics, the model is quantitatively compared to a model that uses the terrain-following sigma coordinate. The distribution and skill scores for precipitation simulated with the hybrid coordinate are not significantly different from those found using the sigma coordinate. The hybrid coordinate has a positive effect on medium-range forecast skill in terms of geopotential height and temperature, especially in the tropics and upper layers of the atmosphere. Furthermore, the root-mean-squared error for relative humidity is significantly reduced near 100 hPa in the Northern (Southern) Hemisphere for a boreal summer (winter). The effect of the hybrid coordinate is found to be almost the same in the GRIMs-spherical harmonics (SPH) dynamical core. For the GRIMs-DFS dynamical core, the hybrid coordinate is insensitive to the abrupt transition of diffusivity at approximately 100 hPa, where numerical diffusion errors occur with the sigma coordinate. This suggests that the hybrid coordinate is necessary for the unique horizontal diffusion method of the GRIMs-DFS dynamical core.