WhatIs: The ngroup parameter
The "ngroup" parameter (which one would set in the gcm.def file and has otherwise a default value of 3) is a parameter concerning a specific "filtering" near the poles (which helps stabilizing the model). In practice it is applied to the divergence of air transport (pbaru/v) and of tracers, by grouping meshes in longitudes two by two, assuming a constant divergence in grouped meshes.
Longitudinal grouping of meshes is such that it increases as one reaches the poles, doubling at each new latitude (i.e. groups of 2,...,2**ngroup). To put it a different way, starting from the pole, groups of 2**ngroup are made (i.e. 8 if ngroup==3 or 16 if ngroup==4, which also implies that the number of grid points along longitudes must be a multiple of 2**ngroup), then at the next latitude band groups of 2**(ngroup-1) are used (i.e. 4 if ngrou==3 or 8 if ngroup==4), followed at the next latitude band groups of 2**(ngroup-2), etc. until no grouping remains to be done.
Recommended values of ngroup
A general rule of thumb :
- for horizontal grids of the order of up to 180x90, ngroup=3 is fine
- for horizontal grids of the order of 256x256 or 360x180, ngroup=4 seems better (stabilizing)
- for horizontal grids of 720x360, ngroup=5 is likely better
30/10/2021
