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. . To cover the whole globe, the two LAM grids are organized like the two parts of a tennis or baseball but with to rectangular parts.
Each LAM grid covers 270°x90° plus an overlap of at least 3 grid points, which gets specified by and can (or rather should) be enlarged by an overlap respecting the same ratio between X and Y (3 to 1) by using 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 baseballThe 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.The minimum overlap should be 3 to 8 times the grid resolution (delta Y degrees).
If a Grd_overlap (defined by degrees) is given, then each grid is expanded as such:
(270° + (6*Grd_overlap)) x (90°+ (2*Grd_overlap))
For example: if Grd_overlap=2° then, each LAM grid covers 282°x94°.
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.
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Example for Yin-Yang grid centered over Africa at : Grd_xlat1 = 24.5 , Grd_xlon1 = 22.
An overlap option is provided 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. Note that the overlap is 3 times larger on the left and right border than at the top and bottom of each of the two LAM grids! This is done because the solver needs the grids to be 3 times larger in x- than in y-direction to keep the aspect ratio of each LAM grid at 3:1 (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 above would look like this:
