A Global Nonhydrostatic Semi-Lagrangian Atmospheric Model with Orography

Abstract

A global nonhydrostatic semi-implicit semi-Lagrangian (SISL) atmospheric model with orography has been developed. The height-based terrain-following σz coordinate of Gal-Chen and Somerville is used to incorporate the orography. A 3D vector form of the SISL formulation is proposed. It is based on the complete Navier?Stokes equations. The model is stable for large time steps of up to 1 h at horizontal/vertical resolution of 2.8125°/1200 m. Isolated bell-shaped mountain profiles and real orography are employed to evaluate the model performance. The sensitivity of the model with orography to the order of accuracy of the uncentering scheme, the reference temperature (T), and size of the time step are similar to that of the model without orography described in Semazzi et al. The authors find that for successful execution of the model, it is important that the orographic height Zs, the reference state mass variable (qs), and T satisfy the hydrostatic balance relationship in the terrain-following σz coordinate system. This formulation effectively controls the generation of unphysical orographic induced computational noise usually encountered in connection with semi-Lagrangian models. The global model used here is based on the complete dynamical Navier?Stokes equations, however, it is run at coarse resolutions, for which nonhydrostatic effects are negligible. In the future, when the computing resources permit, the model will be a valuable vehicle for investigating the role of multiple-scale interaction, including the effects of nonhydrostatic dynamics.