Merge pull request #789 from MetOffice/convert_time_to_dimcoord

Convert time AuxCoord to DimCoord for LFRic data

src/CSET/operators/read.py

@@ -23,6 +23,7 @@
 import iris
 import iris.coords
 import iris.cube
+import iris.util
 import numpy as np
 
 
@@ -144,6 +145,13 @@ def read_cubes(
     cubes = cubes.merge()
     cubes = cubes.concatenate()
 
+    # Ensure dimension coordinates are bounded.
+    for cube in cubes:
+        for dim_coord in cube.coords(dim_coords=True):
+            # Iris can't guess the bounds of a scalar coordinate.
+            if not dim_coord.has_bounds() and dim_coord.shape[0] > 1:
+                dim_coord.guess_bounds()
+
     logging.debug("Loaded cubes: %s", cubes)
     if len(cubes) == 0:
         warnings.warn(
@@ -185,6 +193,7 @@ def callback(cube: iris.cube.Cube, field, filename: str):
         else:
             _deterministic_callback(cube, field, filename)
         _lfric_normalise_callback(cube, field, filename)
+        _lfric_time_coord_fix_callback(cube, field, filename)
 
     return callback
 
@@ -252,6 +261,21 @@ def _lfric_normalise_callback(cube: iris.cube.Cube, field, filename):
         cube.attributes["um_stash_source"] = str(sorted(ast.literal_eval(stash_list)))
 
 
+def _lfric_time_coord_fix_callback(cube: iris.cube.Cube, field, filename):
+    """Ensure the time coordinate is a DimCoord rather than an AuxCoord.
+
+    The coordinate is converted and replaced if not. SLAMed LFRic data has this
+    issue, though the coordinate satisfies all the properties for a DimCoord.
+    Scalar time values are left as AuxCoords.
+    """
+    if cube.coords("time"):
+        time_coord = cube.coord("time")
+        if not isinstance(time_coord, iris.coords.DimCoord) and cube.coord_dims(
+            time_coord
+        ):
+            iris.util.promote_aux_coord_to_dim_coord(cube, time_coord)
+
+
 def _check_input_files(input_path: Path | str, filename_pattern: str) -> Iterable[Path]:
     """Get an iterable of files to load, and check that they all exist.
 

tests/operators/test_read.py

@@ -14,6 +14,8 @@
 
 """Reading operator tests."""
 
+import iris
+import iris.coords
 import iris.cube
 import pytest
 
@@ -192,3 +194,31 @@ def test_lfric_normalise_callback_sort_stash(cube):
     actual = cube.attributes["um_stash_source"]
     expected = "['m01s00i025', 'm01s03i025']"
     assert actual == expected
+
+
+def test_lfric_time_coord_fix_callback():
+    """Correctly convert time from AuxCoord to DimCoord."""
+    time_coord = iris.coords.AuxCoord([0, 1, 2], standard_name="time")
+    cube = iris.cube.Cube([0, 0, 0], aux_coords_and_dims=[(time_coord, 0)])
+    read._lfric_time_coord_fix_callback(cube, None, None)
+    assert isinstance(cube.coord("time"), iris.coords.DimCoord)
+    assert cube.coord_dims("time") == (0,)
+
+
+def test_lfric_time_coord_fix_callback_scalar_time():
+    """Correctly convert time from AuxCoord to DimCoord for scalar time."""
+    length_coord = iris.coords.DimCoord([0, 1, 2], var_name="length")
+    time_coord = iris.coords.AuxCoord([0], standard_name="time")
+    cube = iris.cube.Cube([0, 0, 0], aux_coords_and_dims=[(length_coord, 0)])
+    cube.add_aux_coord(time_coord)
+    read._lfric_time_coord_fix_callback(cube, None, None)
+    assert isinstance(cube.coord("time"), iris.coords.AuxCoord)
+    assert cube.coord_dims("time") == ()
+
+
+def test_lfric_time_coord_fix_callback_no_time():
+    """Don't do anything if no time coordinate present."""
+    length_coord = iris.coords.DimCoord([0, 1, 2], var_name="length")
+    cube = iris.cube.Cube([0, 0, 0], aux_coords_and_dims=[(length_coord, 0)])
+    read._lfric_time_coord_fix_callback(cube, None, None)
+    assert len(cube.coords("time")) == 0