Incorporation of sgrid conventions

[hetland]

I would like to propose that parcels accept Arakawa C-grids that follow the sgrid conventions. Currently, curvilinear grids in parcels follow a convention that is used by NEMO. Unfortunately, this is not a common convention in other C-grid based models (ROMS and MITGCM, in particular). This would allow a wider variety of models to be easily used, without cumbersome pre-processing.

[erikvansebille]

Hi @hetland, thanks for the question. In fact, from the latest v2.2.1, Parcels supports MITgcm grid indexing, using FieldSet.from_mitgcm(), or by setting Field.gridindexingtype='mitgcm', see also #897.

Is that what you were after?

[hetland]

I did not know about the support for MITgcm indexing. This will be very useful.

This specifically is the part that I was after, but generally I still think considering s-grid support would make it so that a wider range of models could be supported. S-grid conventions would let parcels identify the indexing convention from the Dataset attributes. It would also allow for any type of model output. Basically, the node and face configuration can be specified per dimension, for so that arbitrary (though still structured) grids can be used. This removes the requirement to specify the particular grid configuration for each specific model output type.

[erikvansebille]

Interesting idea, @hetland! But how would you identify the indexing convention from the Dataset attributes? Is there some CF-convention for indexing that we can query from the attributes? Could you give an example?

[hetland]

The S-grid conventions, linked above, describe how this would work: https://sgrid.github.io/sgrid/

A simple example on that page looks like this:

dimensions:
    time = UNLIMITED ;
    inode = 10 ;
    jnode = 20 ;
    icell = 9 ;
    jcell = 19 ;

variables:
    char time(time) ;
        time:standard_name = "time" ;
        time:long_name = "time" ;
        time:units = "seconds since 2015-01-01 00:00:00" ;
    float u(time, jcell, inode) ;
        u:description = "x-velocity" ;
        u:units = "m s-1" ;
        u:grid = "MyGrid" ;
        u:location = "edge1" ;
    float v(time, jnode, icell) ;
        u:description = "y-velocity" ;
        u:units = "m s-1" ;
        u:grid = "MyGrid" ;
        u:location = "edge2" ;
    float c(time, jcell, icell) ;
        c:description = "some concentration" ;
        c:grid = "MyGrid" ;
        c:location = "face" ;
    float node_lat(jnode, inode) ;
        node_lat:standard_name = "latitude" ;
        node_lat:units = "degree_north" ;
    float node_lon(jnode, inode) ;
        node_lon:standard_name = "longitude" ;
        node_lon:units = "degree_east" ;

    int MyGrid ;
        grid:cf_role = grid_topology
        grid:topology_dimension = 2 ;
        grid:node_dimensions = "inode jnode" ;
        grid:face_dimensions = "icell: inode (padding: none) jcell: jnode (padding: none)" ;
        grid:node_coordinates = "node_lon node_lat" ;