to_xgcm_grid_dataset doesn't make z_w_top a Z axis

[ALDepp]

Hi there,

there is a problem with using to_xgcm_grid_dataset when opening a file only partially (e.g. with the function read_dat)

def read_dat(files, variables, pop=False, chunks={'time':-1, 'nlat':100, 'nlon':100, 'z_t':-1}):
    def preprocess(ds):
        return ds[variables].reset_coords(drop=True) # reset coords means they are reset as variables
    ds = xr.open_mfdataset(files, parallel=True, preprocess=preprocess,
                           chunks=chunks,
                           combine='by_coords')
    if pop==True:
        file0 = xr.open_dataset(files[0], chunks=chunks)
        ds.update(file0[['ULONG', 'ULAT', 'TLONG', 'TLAT']])
        file0.close()

    ds
    return ds

My dataset does contain z_w_top:

ds_sort.dims
Frozen(SortedKeysDict({'nlat': 305, 'nlon': 1301, 'time': 73, 'z_t': 62, 'z_w_top': 62, 'z_w_bot': 62}))

but when I convert to a xgcm compatible dataset and grid

metrics = {
    ("X",): ["DXU", "DXT"],  # X distances
    ("Y",): ["DYU", "DYT"],  # Y distances
    ("Z",): ["DZU", "DZT"],  # Z distances
    ("X", "Y"): ["UAREA", "TAREA"],
}

# here we get the xgcm compatible dataset
gridxgcm, dsxgcm = pop_tools.to_xgcm_grid_dataset(
    ds_sort,
    periodic=False,
    metrics=metrics,
    boundary={"X": "extend", "Y": "extend", "Z": "extend"},
)

I get this grid:

<xgcm.Grid>
Y Axis (not periodic, boundary='extend'):
  * center   nlat_t --> right
  * right    nlat_u --> center
X Axis (not periodic, boundary='extend'):
  * center   nlon_t --> right
  * right    nlon_u --> center
Z Axis (not periodic, boundary='extend'):
  * center   z_t --> right
  * right    z_w_bot --> center

While converting a dataset loaded with xr.open_dataset() and converting in the same way results in this grid:

<xgcm.Grid>
Y Axis (not periodic, boundary='extend'):
  * center   nlat_t --> right
  * right    nlat_u --> center
X Axis (not periodic, boundary='extend'):
  * center   nlon_t --> right
  * right    nlon_u --> center
Z Axis (not periodic, boundary='extend'):
  * center   z_t --> left
  * right    z_w_bot --> center
  * left     z_w_top --> center

Why is z_w_top not converted in the upper case?