added in a vertical rule for surface fields

docs/src/whatsnew/latest.rst

@@ -41,6 +41,9 @@ This document explains the changes made to Iris for this release
 #. `@rcomer`_ enabled partial collapse of multi-dimensional string coordinates,
    fixing :issue:`3653`. (:pull:`5955`)
 
+#. `@ESadek-MO`_ has updated :mod:`iris.fileformats.pp_save_rules` to set
+   `pp.lbelv` of surface fields to 9999. (:issue:`3280`, :pull:`5734`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/fileformats/pp.py

@@ -2156,7 +2156,7 @@ def _load_cubes_variable_loader(
     return result
 
 
-def save(cube, target, append=False, field_coords=None):
+def save(cube, target, append=False, field_coords=None, label_surface_fields=False):
     """Use the PP saving rules (and any user rules) to save a cube to a PP file.
 
     Parameters
@@ -2185,11 +2185,11 @@ def save(cube, target, append=False, field_coords=None):
     of cubes to be saved to a PP file.
 
     """
-    fields = as_fields(cube, field_coords)
+    fields = as_fields(cube, field_coords, label_surface_fields=label_surface_fields)
     save_fields(fields, target, append=append)
 
 
-def save_pairs_from_cube(cube, field_coords=None):
+def save_pairs_from_cube(cube, field_coords=None, label_surface_fields=False):
     """Use the PP saving rules to generate (2D cube, PP field) pairs from a cube.
 
     Parameters
@@ -2301,12 +2301,12 @@ def save_pairs_from_cube(cube, field_coords=None):
 
         # Run the PP save rules on the slice2D, to fill the PPField,
         # recording the rules that were used
-        pp_field = verify(slice2D, pp_field)
+        pp_field = verify(slice2D, pp_field, label_surface_fields=label_surface_fields)
 
         yield (slice2D, pp_field)
 
 
-def as_fields(cube, field_coords=None):
+def as_fields(cube, field_coords=None, label_surface_fields=False):
     """Use the PP saving rules to convert a cube to an iterable of PP fields.
 
     Use the PP saving rules (and any user rules) to convert a cube to
@@ -2322,7 +2322,12 @@ def as_fields(cube, field_coords=None):
         If None, the final two  dimensions are chosen for slicing.
 
     """
-    return (field for _, field in save_pairs_from_cube(cube, field_coords=field_coords))
+    return (
+        field
+        for _, field in save_pairs_from_cube(
+            cube, field_coords=field_coords, label_surface_fields=label_surface_fields
+        )
+    )
 
 
 def save_fields(fields, target, append: bool = False):

lib/iris/fileformats/pp_save_rules.py

@@ -663,7 +663,7 @@ def _lbproc_rules(cube, pp):
     return pp
 
 
-def _vertical_rules(cube, pp):
+def _vertical_rules(cube, pp, label_surface_fields=False):
     """Rule for setting vertical levels for the PP field.
 
     Parameters
@@ -773,6 +773,22 @@ def _vertical_rules(cube, pp):
         pp.brsvd[0] = depth_coord.bounds[0, 0]
         pp.brlev = depth_coord.bounds[0, 1]
 
+    # Surface field.
+    if (
+        height_coord is None
+        and depth_coord is None
+        and pressure_coord is None
+        and soil_mln_coord is None
+        and apt_coord is None
+        and air_pres_coord is None
+        and level_height_coord is None
+        and mln_coord is None
+        and sigma_coord is None
+        and label_surface_fields
+    ):
+        pp.lbvc = 129
+        pp.lblev = 9999
+
     # Single potential-temperature level.
     if (
         apt_coord is not None
@@ -883,7 +899,7 @@ def _all_other_rules(cube, pp):
     return pp
 
 
-def verify(cube, field):
+def verify(cube, field, label_surface_fields=False):
     # Rules functions.
     field = _basic_coord_system_rules(cube, field)
     field = _um_version_rules(cube, field)
@@ -893,7 +909,7 @@ def verify(cube, field):
     field = _grid_and_pole_rules(cube, field)
     field = _non_std_cross_section_rules(cube, field)
     field = _lbproc_rules(cube, field)
-    field = _vertical_rules(cube, field)
+    field = _vertical_rules(cube, field, label_surface_fields=label_surface_fields)
     field = _all_other_rules(cube, field)
 
     return field

lib/iris/tests/test_cube_to_pp.py

@@ -370,6 +370,31 @@ def test_lbvc(self):
             self.assertEqual(field.lblev, lblev)
             self.assertEqual(field.blev, blev)
 
+    def test_surface_field(self):
+        def setup_cube(coord=None):
+            cube = stock.lat_lon_cube()
+            if coord:
+                cube.add_aux_coord(coord)
+            temp_pp_path = iris.util.create_temp_filename(".pp")
+            iris.fileformats.pp.save(
+                cube, target=temp_pp_path, label_surface_fields=True
+            )
+            cube = iris.fileformats.pp.load(temp_pp_path)
+            return cube
+
+        # check surface fields are correctly applied
+        cube = setup_cube()
+        for field in cube:
+            self.assertEqual(field.lbvc, 129)
+            self.assertEqual(field.lblev, 9999)
+
+        # check surface fields aren't incorrectly applied
+        v_coord = iris.coords.DimCoord(standard_name="depth", units="m", points=[-5])
+        cube = setup_cube(v_coord)
+        for field in cube:
+            self.assertNotEqual(field.lbvc, 129)
+            self.assertNotEqual(field.lblev, 9999)
+
 
 def fields_from_cube(cubes):
     """Return an iterator of PP fields generated from saving the given cube(s)