Gradient between vertical levels

improver/cli/gradient_between_vertical_levels.py

@@ -16,15 +16,16 @@ def process(*cubes: cli.inputcubelist):
 
     Input cubes can be provided at height or pressure levels. If the cubes are provided
     at a pressure level, the height above sea level is extracted from height_of_pressure_levels
-    cube. If the cubes are provided at height levels this is assumed to be a height above ground
-    level and the height above sea level is calculated by adding the height of the orography to
-    the "height" coordinate of the cube. It is possible for one cube to be defined at height
-    levels and the other at pressure levels.
+    cube. If exactly one of the cubes are provided at height levels this is assumed to be a
+    height above ground level and the height above sea level is calculated by adding the height
+    of the orography to the "height" coordinate of the cube. If both cubes have height coordinates
+    no additional cubes are required.
+    It is possible for one cube to be defined at height levels and the other at pressure levels.
 
     Args:
         cubes (iris.cube.CubeList):
             Contains two cubes of a diagnostic at two different vertical levels. The cubes must
-            either have a height coordinate or a pressure coordinate. If either cube is defined at
+            either have a height coordinate or a pressure coordinate. If only one cube is defined at
             height levels, an orography cube must also be provided. If either cube is defined at
             pressure levels, a geopotential_height cube must also be provided.
 

improver/utilities/gradient_between_vertical_levels.py

@@ -53,8 +53,9 @@ def gradient_over_vertical_levels(
 
         If the cubes are provided at height levels this is assumed to be a height above ground level
         and the height above sea level is calculated by adding the height of the orography to the
-        height coordinate.If the cubes are provided at pressure levels, the height above sea level
-        is extracted from a geopotential_height cube.
+        height coordinate. If the cubes are provided at pressure levels, the height above sea level
+        is extracted from a geopotential_height cube.If both cubes have a
+        height coordinate then no additional cubes are required.
 
         Args:
             cubes:
@@ -67,15 +68,15 @@ def gradient_over_vertical_levels(
                 the height above sea level at the pressure level of the input cubes.
             orography:
                 Optional cube containing the orography height above sea level. This cube is required
-                if any input cube is defined at height levels and is used to convert the height
-                above ground level to height above sea level.
+                if exactly one input cube is defined at height levels and is used to convert the
+                height above ground level to height above sea level.
 
         Returns:
             A cube containing the gradient between the cubes between two vertical levels.
 
         Raises:
-            ValueError: If either input cube is defined at height levels and no orography cube is
-                        provided.
+            ValueError: If exactly one input cube is defined at height levels and no orography cube
+                        is provided.
             ValueError: If either input cube is defined at pressure levels and no
                         geopotential_height cube is provided.
         """
@@ -98,6 +99,8 @@ def gradient_over_vertical_levels(
             else:
                 if orography:
                     height_ASL = orography + cube_height
+                elif not (orography or geopotential_height):
+                    height_ASL = cube_height
                 else:
                     raise ValueError(
                         """No orography cube provided but one of the input cubes has height
@@ -121,10 +124,11 @@ def process(self, *cubes: CubeList) -> Cube:
                 cubes:
                     A cubelist of two cubes containing a diagnostic at two vertical levels.
                     The cubes must contain either a height or pressure scalar coordinate.
-                    If either of the cubes contain a height scalar coordinate this is assumed
+                    If exactly one of the cubes contain a height scalar coordinate this is assumed
                     to be a height above ground level and a cube with the name "surface_altitude"
                     must also be provided. If either cube contains a pressure scalar coordinate
-                    a cube with the name "geopotential_height" must be provided.
+                    a cube with the name "geopotential_height" must be provided.If both cubes are
+                    on height levels then no additional cubes are required.
 
             Returns:
                 A cube containing the gradient between two vertical levels. The cube will be

improver_tests/utilities/test_GradientBetweenVerticalLevels.py

@@ -68,28 +68,24 @@ def test_height_and_pressure(
     """Test that the plugin produces the expected result with cubes defined either
     both on height or pressure levels. Also check the plugin produces the expected
     result with one cube defined on height levels and the other on pressure levels."""
+    cubes = [orography, height_of_pressure_levels]
     if height_or_pressure == "height_both":
         temperature_at_850hPa.add_aux_coord(
             iris.coords.AuxCoord(101.5, long_name="height", units="m")
         )
         temperature_at_850hPa.remove_coord("pressure")
+        cubes = []
     elif height_or_pressure == "pressure_both":
         temperature_at_screen_level.add_aux_coord(
             iris.coords.AuxCoord(100000, long_name="pressure", units="Pa")
         )
         temperature_at_screen_level.remove_coord("height")
+        cubes = [height_of_pressure_levels]
+
+    cubes.append([temperature_at_850hPa, temperature_at_screen_level])
 
     expected = [[0.03, 0.01], [-0.01, -0.03]]
-    result = GradientBetweenVerticalLevels()(
-        iris.cube.CubeList(
-            [
-                temperature_at_850hPa,
-                temperature_at_screen_level,
-                height_of_pressure_levels,
-                orography,
-            ]
-        )
-    )
+    result = GradientBetweenVerticalLevels()(iris.cube.CubeList(cubes))
     np.testing.assert_array_almost_equal(result.data, expected)
     assert result.name() == "gradient_of_air_temperature"
     assert result.units == "K/m"