The reduced grid consists in combining grid box together for the advection across longitudes (x-direction). The main idea is that, on the regular latitude-longitude grid, the boxes are getting smaller and smaller as we get closer to the poles, so it is reasonable to combine more and more boxes.
The definition of a reduced grid is done in the region rc file with four keys:
region.glb600x400.im : 60 ... region.glb600x400.redgrid.nh.n : 1 region.glb600x400.redgrid.nh.comb : 60 region.glb600x400.redgrid.sh.n : 3 region.glb600x400.redgrid.sh.comb : 60 20 10
where nh (sh) refers to the North (South) Hemisphere. The *.n key is the number of rings (zonal band) where reduction is applied. The *.comb key provides the number of boxes to combine in each of the ring. There are several rules to follow:
- All the *.comb numbers must divide *.im the total number of grid box (here, 60), so that all boxes are used.
- Starting from the second ring, each ring's combined box must aligned with those of the previous ring. That is, in the above example for the south pole, 20 divides 60, and 10 divides 20.
- You cannot combine all boxes (a/k/a one-cell ring) more than once per pole: TM5 will stop with an error if you have two consecutive one-cell rows. But you can have consecutive rings with the same number of boxes, as long as they are not one-cell ring. For example from the glb100x100 region file:
region.glb100x100.redgrid.sh.n : 30 region.glb100x100.redgrid.sh.comb : 72 24 24 8 8 8 8 4 4 4 4 4 4 4 4 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
A little script is available to help users define reduced grids. You can find it in the tools directory of TM5:
You can try:
~/TM5/tools/misc/redgrid_series.py 6 4
~/TM5/tools/misc/redgrid_series.py 1 1