Core _io.concatenate() may fail due to case when one of the cubes i…

…s scalar - this fixes that (#1715)

* fix concatenation when component becomes scalar cube

* add test case/change test case

* reinstate logger debug message

esmvalcore/preprocessor/_io.py

@@ -134,7 +134,6 @@ def load(file, callback=None, ignore_warnings=None):
     ------
     ValueError
         Cubes are empty.
-
     """
     logger.debug("Loading:\n%s", file)
     if ignore_warnings is None:
@@ -501,6 +500,9 @@ def _concatenate_overlapping_cubes(cubes):
                                     data_start_1.month, data_start_1.day,
                                     start_overlap.year, start_overlap.month,
                                     start_overlap.day)
+            # convert c1_delta scalar cube to vector cube, if needed
+            if c1_delta.data.shape == ():
+                c1_delta = iris.util.new_axis(c1_delta, scalar_coord="time")
             cubes = iris.cube.CubeList([c1_delta, cubes[1]])
             logger.debug("Attempting concatenatenation of %s with %s",
                          c1_delta, cubes[1])

tests/integration/preprocessor/_io/test_concatenate.py

@@ -14,7 +14,6 @@
 )
 from iris.coords import AuxCoord, DimCoord
 from iris.cube import Cube, CubeList
-from iris.exceptions import ConcatenateError
 
 from esmvalcore.preprocessor import _io
 
@@ -375,8 +374,10 @@ def test_concatenate_with_iris_exception(self):
                      var_name='sample',
                      dim_coords_and_dims=((time_coord_2, 0), ))
         cubes_single_ovlp = [cube2, cube1]
-        with self.assertRaises(ConcatenateError):
-            _io.concatenate(cubes_single_ovlp)
+        cubess = _io.concatenate(cubes_single_ovlp)
+        # this tests the scalar to vector cube conversion too
+        time_points = cubess.coord("time").core_points()
+        np.testing.assert_array_equal(time_points, [1., 1.5, 5., 7.])
 
     def test_concatenate_no_time_coords(self):
         """Test a more generic case."""