Yes, this is possible. The trick is to set up a domain that is still 3D, but which is only 1 grid point wide in the z direction.

Clearly, this is not possible with cell-centred refinement, as refinement always splits cells into two. However, as vertex-centred refinement is the default, this might not be relevant here.

I also assume that you have a true 2+1 code that doesn't require any ghost zones in the z direction. You would then explicitly set the number of ghost zones and boundary zones to be 0 in the z direction.

The "usual" problem I encounter when setting up such a domain is that it's slightly inconvenient to specify the coordinates and grid spacings in the z direction. I prefer to define the domain extent via CoordBase, so I assume you're doing the same.

Usually, you would specify zmin, zmax, and ncells_z to define the domain. However, if you specify zmin=0, zmax=0, and then set ncells_z=0 (0 cells correspond to 1 point), then Carpet calculates dz = (zmax - zmin) / ncells_z, which is undefined since you divide zero by zero. So instead of specifying zmin, zmax, ncells_z, you have to specify zmin, zmax, and dz. Carpet then calculates ncells_k = (zmax - zmin) / dz, which is fine. In short, setting CoordBase::spacing = "numcells" will fail, and you have to use CoordBase::spacing = "gridspacing" instead.

That's about it. Of course, it's easy to make off-by-one errors in the parameter file, so looking at 1D ASCII output to see whether the coordinates and domain sizes you specified are correct is important.

Unfortunately I do not have a sample parameter file.

-erik