Enable lazy computation of wind vector rotation

docs/src/whatsnew/latest.rst

@@ -49,6 +49,10 @@ This document explains the changes made to Iris for this release
 #. `@lbdreyer`_ and `@trexfeathers`_ (reviewer)  added :func:`iris.plot.hist`
    and :func:`iris.quickplot.hist`. (:pull:`5189`)
 
+#. `@tinyendian`_ edited :func:`~iris.analysis.cartography.rotate_winds` to
+   enable lazy computation of rotated wind vector components (:issue:`4934`,
+   :pull:`4972`)
+
 
 🐛 Bugs Fixed
 =============
@@ -156,9 +160,11 @@ This document explains the changes made to Iris for this release
 .. _@ed-hawkins: https://github.com/ed-hawkins
 .. _@scottrobinson02: https://github.com/scottrobinson02
 .. _@agriyakhetarpal: https://github.com/agriyakhetarpal
+.. _@tinyendian: https://github.com/tinyendian
+
 
 .. comment
     Whatsnew resources in alphabetical order:
 
 .. _#ShowYourStripes: https://showyourstripes.info/s/globe/
-.. _README.md: https://github.com/SciTools/iris#-----
+.. _README.md: https://github.com/SciTools/iris#-----

lib/iris/analysis/cartography.py

@@ -15,6 +15,7 @@
 import cartopy.crs as ccrs
 import cartopy.img_transform
 import cf_units
+import dask.array as da
 import numpy as np
 import numpy.ma as ma
 
@@ -1206,9 +1207,15 @@ def rotate_winds(u_cube, v_cube, target_cs):
         x = x.transpose()
         y = y.transpose()
 
-    # Create resulting cubes.
-    ut_cube = u_cube.copy()
-    vt_cube = v_cube.copy()
+    # Create resulting cubes - produce lazy output data if at least
+    # one input cube has lazy data
+    lazy_output = u_cube.has_lazy_data() or v_cube.has_lazy_data()
+    if lazy_output:
+        ut_cube = u_cube.copy(data=da.empty_like(u_cube.lazy_data()))
+        vt_cube = v_cube.copy(data=da.empty_like(v_cube.lazy_data()))
+    else:
+        ut_cube = u_cube.copy()
+        vt_cube = v_cube.copy()
     ut_cube.rename("transformed_{}".format(u_cube.name()))
     vt_cube.rename("transformed_{}".format(v_cube.name()))
 
@@ -1236,8 +1243,16 @@ def rotate_winds(u_cube, v_cube, target_cs):
     apply_mask = mask.any()
     if apply_mask:
         # Make masked arrays to accept masking.
-        ut_cube.data = ma.asanyarray(ut_cube.data)
-        vt_cube.data = ma.asanyarray(vt_cube.data)
+        if lazy_output:
+            ut_cube = ut_cube.copy(
+                data=da.ma.masked_array(ut_cube.core_data())
+            )
+            vt_cube = vt_cube.copy(
+                data=da.ma.masked_array(vt_cube.core_data())
+            )
+        else:
+            ut_cube.data = ma.asanyarray(ut_cube.data)
+            vt_cube.data = ma.asanyarray(vt_cube.data)
 
     # Project vectors with u, v components one horiz slice at a time and
     # insert into the resulting cubes.
@@ -1250,16 +1265,20 @@ def rotate_winds(u_cube, v_cube, target_cs):
         for dim in dims:
             index[dim] = slice(None, None)
         index = tuple(index)
-        u = u_cube.data[index]
-        v = v_cube.data[index]
+        u = u_cube.core_data()[index]
+        v = v_cube.core_data()[index]
         ut, vt = _transform_distance_vectors(u, v, ds, dx2, dy2)
         if apply_mask:
-            ut = ma.asanyarray(ut)
-            ut[mask] = ma.masked
-            vt = ma.asanyarray(vt)
-            vt[mask] = ma.masked
-        ut_cube.data[index] = ut
-        vt_cube.data[index] = vt
+            if lazy_output:
+                ut = da.ma.masked_array(ut, mask=mask)
+                vt = da.ma.masked_array(vt, mask=mask)
+            else:
+                ut = ma.asanyarray(ut)
+                ut[mask] = ma.masked
+                vt = ma.asanyarray(vt)
+                vt[mask] = ma.masked
+        ut_cube.core_data()[index] = ut
+        vt_cube.core_data()[index] = vt
 
     # Calculate new coords of locations in target coordinate system.
     xyz_tran = target_crs.transform_points(src_crs, x, y)

lib/iris/tests/unit/analysis/cartography/test_rotate_winds.py

@@ -511,5 +511,62 @@ def test_non_earth_semimajor_axis(self):
         rotate_winds(u, v, other_cs)
 
 
+class TestLazyRotateWinds(tests.IrisTest):
+    def _compare_lazy_rotate_winds(self, masked):
+        # Compute wind rotation with lazy data and compare results
+
+        # Choose target coord system that will (not) lead to masked results
+        if masked:
+            coord_sys = iris.coord_systems.OSGB()
+        else:
+            coord_sys = iris.coord_systems.GeogCS(6371229)
+
+        u, v = uv_cubes()
+
+        # Create deep copy of the cubes with rechunked lazy data to check if
+        # input data is modified, and if Dask metadata is preserved
+        u_lazy = u.copy(data=u.copy().lazy_data().rechunk([2, 1]))
+        v_lazy = v.copy(data=v.copy().lazy_data().rechunk([1, 2]))
+
+        ut_ref, vt_ref = rotate_winds(u, v, coord_sys)
+        self.assertFalse(ut_ref.has_lazy_data())
+        self.assertFalse(vt_ref.has_lazy_data())
+        # Ensure that choice of target coordinates leads to (no) masking
+        self.assertTrue(ma.isMaskedArray(ut_ref.data) == masked)
+
+        # Results are lazy if at least one component is lazy
+        ut, vt = rotate_winds(u_lazy, v, coord_sys)
+        self.assertTrue(ut.has_lazy_data())
+        self.assertTrue(vt.has_lazy_data())
+        self.assertTrue(ut.core_data().chunksize == (2, 1))
+        self.assertArrayAllClose(ut.data, ut_ref.data, rtol=1e-5)
+        self.assertArrayAllClose(vt.data, vt_ref.data, rtol=1e-5)
+
+        ut, vt = rotate_winds(u, v_lazy, coord_sys)
+        self.assertTrue(ut.has_lazy_data())
+        self.assertTrue(vt.has_lazy_data())
+        self.assertTrue(vt.core_data().chunksize == (1, 2))
+        self.assertArrayAllClose(ut.data, ut_ref.data, rtol=1e-5)
+        self.assertArrayAllClose(vt.data, vt_ref.data, rtol=1e-5)
+
+        ut, vt = rotate_winds(u_lazy, v_lazy, coord_sys)
+        self.assertTrue(ut.has_lazy_data())
+        self.assertTrue(vt.has_lazy_data())
+        self.assertTrue(ut.core_data().chunksize == (2, 1))
+        self.assertTrue(vt.core_data().chunksize == (1, 2))
+        self.assertArrayAllClose(ut.data, ut_ref.data, rtol=1e-5)
+        self.assertArrayAllClose(vt.data, vt_ref.data, rtol=1e-5)
+
+        # Ensure that input data has not been modified
+        self.assertArrayAllClose(u.data, u_lazy.data, rtol=1e-5)
+        self.assertArrayAllClose(v.data, v_lazy.data, rtol=1e-5)
+
+    def test_lazy_rotate_winds_masked(self):
+        self._compare_lazy_rotate_winds(True)
+
+    def test_lazy_rotate_winds_notmasked(self):
+        self._compare_lazy_rotate_winds(False)
+
+
 if __name__ == "__main__":
     tests.main()