When running GEM on a global grid the grid used is a Yin-Yang grid. The Yin-Yang grid is constructed of two slightly overlapping LAM grids, covering each a little more than half the globe. Each LAM grid covers 270°x90° plus an overlap of at least 3 grid points, which gets specified by the parameter 'Grd_overlap' (in degrees). To cover the whole globe, the two LAM grids are organized like the two parts of a tennis or baseball.
When written in an RPN file the two LAM grid are stored as one grid, with one LAM on top of the other - see image below.
If need be the Yin-Yang grid can be split into the two LAM grids with the command: yydecode.Abs
And the two LAM grids can get put back together with the command: yyencode.Abs
For more information have a look at the following publication:
Qaddouri, A., & Lee, V. (2011). The Canadian Global Environmental Multiscale model on the Yin‐Yang grid system. Quarterly Journal of the Royal Meteorological Society. Royal Meteorological Society (Great Britain), 137(660), 1913–1926. https://doi.org/10.1002/qj.873
Example for Yin-Yang grid centered over Africa at : Grd_xlat1 = 24.5 , Grd_xlon1 = 22.
Note that the overlap is 3 times larger on the left and right border than at the top and bottom! This is done because the solver needs the grids to be 3 times larger in x- than in y-direction (minus 1 point). An overlap is needed because the two LAM grids will give different answers in the overlap region and the larger the overlap region the easier it is for the solver to find the common solution.
When projected on a regular latlon grid the two LAM grids would look like this:
