# Reduced grid

From tm5

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.

## Characterization

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:

~/TM5/tools/misc/redgrid_series.py -h

You can try:

~/TM5/tools/misc/redgrid_series.py 6 4

or

~/TM5/tools/misc/redgrid_series.py 1 1