The Deterministic Global Environmental Multiscale (GEM) operational model constitutes a very important tool for medium-range numerical weather prediction at the Canadian Meteorological Centre (CMC). Recently we replaced the Latitude--Longitude grid system in GEM by an overset grid system named Yin--Yang. The global forecast on the Yin--Yang grid is performed with a domain decomposition method between two limited-area models (LAM), discretized on the two panels of Yin--Yang grid and using the same time step.

The problem associated with the construction of any global atmospheric model is the numerical representation the the spherical geometry. The Latitude Longitude grid system leads to singularities and convergence of meridians in polar regions. Using today's computer architectures with distributed memory, this may even result in an unbalanced computational load in the context of domain decomposition with message-passing interfaces. The Yin-Yang grid imposes a more uniform grid spacing and allows us a more balanced computational load for scalability purposes. I will present the Canadian GEM model on the new Yin--Yang grid system.