Impact of a Three-Dimensional Reference State in a Global Semi-Implicit Semi-Lagrangian Non-Hydrostatic Atmospheric Model on Yin–Yang Grids

The definition of a reference state close to the realistic atmosphere in an atmospheric model is essential for deriving prognostic deviations and improving numerical accuracy. In this study, a new dynamical framework allowing easy switch between a one-dimensional (1D) and a three-dimensional (3D) time-independent reference state is developed for the semi-implicit semi-Lagrangian solver in a global non-hydrostatic atmospheric model on Yin–Yang grids. The 3D reference state is introduced with consideration of additional horizontal gradient terms, and the corresponding reference-state terms are discretized in high-order accuracy. It is featured with reduced magnitude of deviations, more accurate pressure gradient force as well as alleviated numerical noises. Four idealized benchmark tests and multiple full-physics real-case forecasts are carried out to assess the impact of the 3D and 1D reference states. The 3D reference state shows significant advantages in the simulation of atmospheric transport and wave propagation in the idealized experiments. In the real-case forecasts, batched forecasts from June to August 2021 show a comprehensive improvement of medium-range prediction by using the 3D reference state. The new scheme achieves an enhanced prediction skill for large-scale circulation and extends the effective forecast period by 0.8 days in the Northern Hemisphere.