= Using AGRIF for embedded zooms in NEMO

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== Overview

AGRIF (Adaptive Grid Refinement In Fortran) is a library that allows the seamless refinement of rectangular regions in NEMO. Activating AGRIF requires the cpp key "key_agrif" to be defined at compilation time. Although this is transparent to users, the way the code is processed during compilation is different from the standard case: a preprocessing stage (the so called "conv" program) translates the actual code so that saved arrays may be be switched in memory space from one domain to an other. 

An additional text file "AGRIF_FixedGrids.in" is also required at run time. This is where the grid hierarchy is defined. An example of such a file, here taken from the VORTEX test case, is given below: 
{{{
1
19 38 19 38 3 3 3
0
}}}
The first line indicates the number of zooms (1). The second line contains the starting and ending indexes in both directions on the root grid (19 38 19 38) followed by the space and time refinement factors (3 3 3). The last line is the number of child grid nested in the refined region (0). A more complex example with telescoping grids can be found in the AGRIF_DEMO reference configuration directory (NEMO 4.0 only). 

Note that rectangular regions must be defined so that they are connected to a single parent grid. Hence, defining for instance overlapping grids with the same refinement ratio will not work properly, boundary data exchange and update being only performed between root and child grids.

Knowing the refinement area you may choose, a pre-processing tool (NESTING directory in tools) helps to create needed input files (mesh file, restart, climatological and forcing files). The key here is to ensure volume matching near the child grid interface, a step done by invoking the Agrif_create_bathy.exe program. You may use the namelists provided in the NESTING directory as a guide. These correspond to the namelists used to create AGRIF_DEMO inputs. 

AGRIF is continuously maintained so that it can be activated with all NEMO components (OPA, sea-ice, TOP). Depending on NEMO version it is nevertheless not the case so that some options may not be compatible with the use of online grid refinement. Check out the tables below to know the status according to the NEMO release you may use.
 
== Available in the 3_6 revision of NEMO 
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=== AGRIF demonstrator in reference configuration

* The [wiki:Users/ReferenceConfigurations/ORCA2_LIM3_PISCES global reference configuration includes a demonstrator for using AGRIF in NEMO] 


== Available in the forthcoming 4.0
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In addition to the reference configurations in the 3_6 release, the [wiki:Users/Announcement_4.0.beta_release 4.0 beta release is now available] and includes new configurations and test cases with AGRIF, see below:
=== New reference configurations with AGRIF 

* AGRIF_DEMO ''to be described''

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=== New tests cases demonstrating some major features of AGRIF with NEMO:
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== AGRIF in NEMO documentation
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* general
* How to set up a new configuration
* ...