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).
When projected on a regular latlon grid the two LAM grids would look like this:
