Modifies LightningFromCapePrecip plugin to accept 3h precipitation-ra…

…te-max data (metoppv#1568)

* Modifies LightningFromCapePrecip plugin to accept 3h precipitation-rate-max data

- We need this so we can generate data for days 6-8 when only 3h data are available.

* First review

- Improves tests and doc-strings

* Black

improver/lightning.py

@@ -74,7 +74,8 @@ def _get_inputs(cubes: CubeList) -> Tuple[Cube, Cube]:
         """
         Separates CAPE and precipitation rate cubes and checks that the following
         match: forecast_reference_time, spatial coords, time-bound interval and
-        that CAPE time is exactly one hour older than precipitation rate time.
+        that CAPE time is at the lower bound of precipitation rate time.
+        The precipitation rate data must represent a period of 1 or 3 hours.
         """
         cape = cubes.extract(
             iris.Constraint(
@@ -100,14 +101,14 @@ def _get_inputs(cubes: CubeList) -> Tuple[Cube, Cube]:
             raise ValueError(f"No cube named precipitation_rate_max found in {cubes}")
         (cape_time,) = list(cape.coord("time").cells())
         (precip_time,) = list(precip.coord("time").cells())
-        if cape_time.point + timedelta(hours=1) != precip_time.point:
+        if cape_time.point != precip_time.bound[0]:
             raise ValueError(
-                f"CAPE cube time ({cape_time.point}) should be valid one hour earlier "
-                f"than precipitation_rate_max cube time ({precip_time.point})."
+                f"CAPE cube time ({cape_time.point}) should be valid at the "
+                f"precipitation_rate_max cube lower bound ({precip_time.bound[0]})."
             )
-        if np.diff(precip_time.bound) != timedelta(hours=1):
+        if np.diff(precip_time.bound) not in [timedelta(hours=1), timedelta(hours=3)]:
             raise ValueError(
-                f"Precipitation_rate_max cube time window must be one hour, "
+                f"Precipitation_rate_max cube time window must be one or three hours, "
                 f"not {np.diff(precip_time.bound)}."
             )
         if cape.coord("forecast_reference_time") != precip.coord(

improver_tests/lightning/test_LightningFromCapePrecip.py

@@ -129,6 +129,20 @@ def test_basic(cape_cube, precip_cube, expected_cube):
     assert np.allclose(result.data, expected_cube.data)
 
 
+def test_3h_cubes(cape_cube, precip_cube, expected_cube):
+    """Run the plugin again with 3h cubes"""
+    cape_cube.coord("time").points = cape_cube.coord("time").points - 2 * 3600
+    bounds = precip_cube.coord("time").bounds
+    precip_cube.coord("time").bounds = (bounds[0][0] - 2 * 3600, bounds[0][1])
+    precip_cube.rename("precipitation_rate_max-PT03H")
+    expected_cube.coord("time").bounds = (bounds[0][0] - 2 * 3600, bounds[0][1])
+    result = LightningFromCapePrecip()(CubeList([cape_cube, precip_cube]))
+    assert result.xml().splitlines(keepends=True) == expected_cube.xml().splitlines(
+        keepends=True
+    )
+    assert np.allclose(result.data, expected_cube.data)
+
+
 def test_with_model_attribute(cape_cube, precip_cube, expected_cube):
     """Run the plugin with model_id_attr and check the result cube matches the expected_cube"""
     expected_cube.attributes["mosg__model_configuration"] = "gl_ens"
@@ -142,21 +156,18 @@ def test_with_model_attribute(cape_cube, precip_cube, expected_cube):
 
 
 def break_time_point(cape_cube, precip_cube):
-    """Modifies precip_cube time points to be incremented by 1 second and
+    """Modifies cape_cube time points to be incremented by 1 second and
     returns the error message this will trigger"""
-    precip_cube.coord("time").points = precip_cube.coord("time").points + 1
-    return (
-        r"CAPE cube time .* should be valid one hour earlier than "
-        r"precipitation_rate_max cube time .*"
-    )
+    cape_cube.coord("time").points = cape_cube.coord("time").points + 1
+    return r"CAPE cube time .* should be valid at the precipitation_rate_max cube lower bound .*"
 
 
 def break_time_bound(cape_cube, precip_cube):
-    """Modifies lower bound on precip_cube time coord to be incremented by 1 second and
+    """Modifies upper bound on precip_cube time coord to be incremented by 1 second and
     returns the error message this will trigger"""
     bounds = precip_cube.coord("time").bounds
-    precip_cube.coord("time").bounds = (bounds[0][0] + 1, bounds[0][1])
-    return r"Precipitation_rate_max cube time window must be one hour, not .*"
+    precip_cube.coord("time").bounds = (bounds[0][0], bounds[0][1] + 1)
+    return r"Precipitation_rate_max cube time window must be one or three hours, not .*"
 
 
 def break_reference_time(cape_cube, precip_cube):