The Quasi-Elastic Dynamical Core of the Global Environmental Multiscale Model with a Height-Based Terrain-Following Vertical Coordinate

The governing equations of Environment and Climate Change Canada’s (ECCC’s) GEM (Global Environmental Multiscale) model are based on the elastic Euler (EE) system. Its solution includes vertically propagating sound waves that are physically present in the atmosphere, but are meteorologically insignificant. However, as they propagate at the speed of sound, their presence requires the model to utilize very short time-step lengths to maintain computational stability. The ability to function with large time-step lengths is essential for any operational NWP (numerical weather prediction) model for computational efficiency. More importantly, large time steps ensure the delivery of various forecast products in a timely manner, which is of critical importance for any operational NWP centre. At present, ECCC deploys two separate dynamical cores to circumvent this problem: one is based on the hydrostatic approximation to the EE system which eliminates the problematic soundwaves for coarse-resolution global and regional NWP systems, and the other employs a fully-nonhydrostatic core for high-resolution forecasting.

In order to address the challenges imposed by vertically propagating soundwaves, a quasi-elastic version of ECCC’s new height-based GEM-H dynamical core has recently been developed. The quasi-elastic approximation elegantly combines the concepts of the hydrostatic and the anelastic approximations thereby eliminating the problematic soundwaves from the EE system at all scales and for all scenarios. Various numerical experiments have been carried out to evaluate the effectiveness of the quasi-elastic dynamical core including two-dimensional idealized test cases as well as three-dimensional NWP systems covering both hydrostatic and nonhydrostatic scenarios. The concept behind the quasi-elastic approximation and its impact on NWP performance will be presented at the conference.