IMPORTANT: Updated nesting Matlab script grid_connections.m

[arango]

The Matlab script used to compute nested grid connectivity (grid_connections.m) was updated, so it is more robust in telescoping nesting applications.

Currently, in ROMS a telescoping grid is a refined grid (refine_factor > 0) containing a finer grid inside. Notice that under this definition the coarser grid (ng=1, refine_factor=0) is not considered a telescoping grid. The last cascading grid is not considered a telescoping grid since it does not contains a finer grid inside. The telescoping definition is a technical one, and it facilitates the algorithm design.

A tricky algorithm is added at the top of these function to reject indirect connectivity in telescoping refinement. Previously, only one coincident connection between grids was allowed, as shown below:

Here, we have a three nested grid application. Grid-2 is a 1:3 refinement from Grid-1 and Grid-3 is a 1:3 refinement from Grid-2. The Grid-2 is the only telescoping grid in this application according to the above definition. Notice that Grid-3 is inside both Grid-2 (directly) and Grid-1 (indirectly).

The panels show the location of the donor grid (blue plus) and receiver grid (red circle) at PSI-points. Notice that in the top panel, the Grid-2 perimeter lies in the same lines as the donor Grid-1, and some of the points are coincident, as it should be. The same occurs in the middle panel between Grid-2 and Grid-3.

However, we did not allow before to have coincident grid points between the indirect connections of Grid-1 and Grid-3. We were using that property to reject indirect connections between nested grids.

I was aware of this restriction but I forgot to go back and code a robust logic in grid_connections.m to allow coincident grids points in indirect connections for telescoping applications. The restriction is removed with this update/

I know, it is a complicated subject to discuss with words. It needs to be shown geometrically.

Many thanks to Marc Mestres and John Wilkin for ​bringing this problem to my attention.