Emillour le 5 novembre 2021 à 16:56

2021-11-05T16:56:26Z

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2021-11-05T16:55:36Z

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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

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@@ Ligne 2 : / Ligne 2 : @@
 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.
+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 ngroup==3 or 8 if ngroup==4), followed at the next latitude band groups of 2**(ngroup-2), etc. until no grouping needs to be done.
 == Recommended values of ngroup ==

WhatIs: The ngroup parameter - Historique des versions

Emillour le 5 novembre 2021 à 16:56

Emillour : Page créée avec « 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 (wh... »