Adding check on decreasing depth

Author: erikvansebille

src/parcels/_core/xgrid.py

@@ -104,9 +104,12 @@ def __init__(self, grid: xgcm.Grid, mesh="flat"):
         if "lat" in ds:
             ds.set_coords("lat")
 
-        if len(set(grid.axes) & {"X", "Y", "Z"}) > 0:  # Only if spatial grid is >0D (see #2054 for further development)
+        if len(set(grid.axes) & {"X", "Y"}) > 0:  # Only if spatial grid is >0D (see #2054 for further development)
             assert_valid_lat_lon(ds["lat"], ds["lon"], grid.axes)
 
+        if "Z" in grid.axes:
+            assert_valid_depth(ds["depth"])
+
         assert_valid_mesh(mesh)
         self._ds = ds
 
@@ -474,6 +477,11 @@ def assert_valid_lat_lon(da_lat, da_lon, axes: _XGCM_AXES):
             )
 
 
+def assert_valid_depth(da_depth):
+    if not np.all(np.diff(da_depth.values) > 0):
+        raise ValueError(f"Depth DataArray {da_depth.name!r} with dims {da_depth.dims} must be strictly increasing.")
+
+
 def _convert_center_pos_to_fpoint(
     *, index: int, bcoord: float, xgcm_position: _XGCM_AXIS_POSITION, f_points_xgcm_position: _XGCM_AXIS_POSITION
 ) -> tuple[int, float]:

tests/test_xgrid.py

@@ -126,6 +126,14 @@ def test_invalid_lon_lat():
         XGrid.from_dataset(ds)
 
 
+def test_invalid_depth():
+    ds = datasets["ds_2d_left"].copy()
+    ds = ds.reindex({"ZG": -ds.ZG})
+
+    with pytest.raises(ValueError, match="Depth DataArray .* must be strictly increasing*"):
+        XGrid.from_dataset(ds)
+
+
 @pytest.mark.parametrize(
     "ds",
     [