From 5496ebcee49294e0f5993991969dae99762f5f40 Mon Sep 17 00:00:00 2001
From: Ruth Comer <10599679+rcomer@users.noreply.github.com>
Date: Thu, 30 Nov 2023 16:44:15 +0000
Subject: [PATCH] Fix pp save of realization coordinate (#5568)

* Fix pp save of realization coordinate

* Reduce lbrsvd to 4 elements

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

* review actions

* credit reviewer

---------

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>
---
 docs/src/whatsnew/latest.rst                    |  3 +++
 lib/iris/fileformats/pp_save_rules.py           | 14 +++++++++++---
 lib/iris/tests/unit/fileformats/pp/test_save.py | 12 ++++++++++++
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/docs/src/whatsnew/latest.rst b/docs/src/whatsnew/latest.rst
index de69eddd4e..4c6f06e13f 100644
--- a/docs/src/whatsnew/latest.rst
+++ b/docs/src/whatsnew/latest.rst
@@ -76,6 +76,9 @@ This document explains the changes made to Iris for this release
 #. `@acchamber`_ and `@rcomer`_ modified 2D plots so that time axes and their
    ticks have more sensible default labels.  (:issue:`5426`, :pull:`5561`)
 
+#. `@rcomer`_ and `@trexfeathers`_ (reviewer) added handling for realization
+   coordinates when saving pp files (:issue:`4747`, :pull:`5568`)
+
 
 💣 Incompatible Changes
 =======================
diff --git a/lib/iris/fileformats/pp_save_rules.py b/lib/iris/fileformats/pp_save_rules.py
index 7db21d5f99..9effba3c0a 100644
--- a/lib/iris/fileformats/pp_save_rules.py
+++ b/lib/iris/fileformats/pp_save_rules.py
@@ -614,7 +614,7 @@ def _non_std_cross_section_rules(cube, pp):
 
 def _lbproc_rules(cube, pp):
     """
-    Rules for setting the horizontal grid and pole location of the PP field.
+    Rules for setting the processing code of the PP field.
 
     Note: `pp.lbproc` must be set to 0 before these rules are run.
 
@@ -844,7 +844,10 @@ def _vertical_rules(cube, pp):
 
 def _all_other_rules(cube, pp):
     """
-    Rules for setting the horizontal grid and pole location of the PP field.
+    Fields currently managed by these rules:
+
+    * lbfc (field code)
+    * lbrsvd[3] (ensemble member number)
 
     Args:
         cube: the cube being saved as a series of PP fields.
@@ -859,13 +862,18 @@ def _all_other_rules(cube, pp):
     if check_items in CF_TO_LBFC:
         pp.lbfc = CF_TO_LBFC[check_items]
 
-    # Set STASH code.
+    # Set field code.
     if (
         "STASH" in cube.attributes
         and str(cube.attributes["STASH"]) in STASH_TRANS
     ):
         pp.lbfc = STASH_TRANS[str(cube.attributes["STASH"])].field_code
 
+    # Set ensemble member number.
+    real_coord = scalar_coord(cube, "realization")
+    if real_coord is not None:
+        pp.lbrsvd[3] = real_coord.points[0]
+
     return pp
 
 
diff --git a/lib/iris/tests/unit/fileformats/pp/test_save.py b/lib/iris/tests/unit/fileformats/pp/test_save.py
index fc0535f428..f49d389841 100644
--- a/lib/iris/tests/unit/fileformats/pp/test_save.py
+++ b/lib/iris/tests/unit/fileformats/pp/test_save.py
@@ -43,6 +43,18 @@ def test_grid_and_pole__scalar_dim_longitude(unit, modulus):
     assert field.lbnpt == lon.points.size
 
 
+def test_realization():
+    cube = stock.lat_lon_cube()
+    real_coord = DimCoord(42, standard_name="realization", units=1)
+    cube.add_aux_coord(real_coord)
+    with mock.patch("iris.fileformats.pp.PPField3", autospec=True) as pp_field:
+        pp_field.lbrsvd = list(range(4))
+        verify(cube, pp_field)
+        member_number = pp_field.lbrsvd[3]
+
+    assert member_number == 42
+
+
 def _pp_save_ppfield_values(cube):
     """
     Emulate saving a cube as PP, and capture the resulting PP field values.