Add derivation of sea ice extent

esmvalcore/cmor/tables/custom/CMOR_siextent.dat

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+SOURCE: CMIP5
+!============
+variable_entry:    siextent
+!============
+modeling_realm:    seaIce
+!----------------------------------
+! Variable attributes:
+!----------------------------------
+standard_name:
+units:             m2
+cell_methods:      area: mean where sea time: mean
+cell_measures:     area: areacello
+long_name:         Sea Ice Extent
+comment:           
+!----------------------------------
+! Additional variable information:
+!----------------------------------
+dimensions:        longitude latitude time
+type:              real
+valid_min:
+valid_max:
+!----------------------------------
+!

esmvalcore/preprocessor/_derive/siextent.py

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+"""Derivation of variable `sithick`."""
+import logging
+
+import dask.array as da
+import iris
+from iris import Constraint
+
+from esmvalcore.exceptions import RecipeError
+from ._baseclass import DerivedVariableBase
+
+logger = logging.getLogger(__name__)
+
+
+class DerivedVariable(DerivedVariableBase):
+    """Derivation of variable `siextent`."""
+
+    @staticmethod
+    def required(project):
+        """Declare the variables needed for derivation."""
+        required = [
+            {
+                'short_name': 'sic',
+                'optional': 'true'
+            },
+            {
+                'short_name': 'siconca',
+                'optional': 'true'
+            }]
+        return required
+
+    @staticmethod
+    def calculate(cubes):
+        """Compute sea ice extent.
+
+        Returns an array of ones in every grid point where
+        the sea ice area fraction has values > 15 .
+
+        Use in combination with the preprocessor
+        `area_statistics(operator='sum')` to weigh by the area and
+        compute global or regional sea ice extent values.
+
+        Arguments
+        ---------
+            cubes: cubelist containing sea ice area fraction.
+
+        Returns
+        -------
+            Cube containing sea ice extent.
+        """
+        try:
+            sic = cubes.extract_cube(Constraint(name='sic'))
+        except iris.exceptions.ConstraintMismatchError:
+            try:
+                sic = cubes.extract_cube(Constraint(name='siconca'))
+            except iris.exceptions.ConstraintMismatchError as exc:
+                raise RecipeError(
+                    'Derivation of siextent failed due to missing variables '
+                    'sic and siconca.') from exc
+
+        ones = da.ones_like(sic)
+        siextent_data = da.ma.masked_where(sic.lazy_data() < 15., ones)
+        siextent = sic.copy(siextent_data)
+        siextent.units = 'm2'
+
+        return siextent

tests/unit/preprocessor/_derive/test_siextent.py

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+"""Test derivation of `ohc`."""
+import cf_units
+import iris
+import numpy as np
+import pytest
+
+import esmvalcore.preprocessor._derive.siextent as siextent
+from esmvalcore.exceptions import RecipeError
+
+
+@pytest.fixture
+def cubes_sic():
+    sic_name = 'sea_ice_area_fraction'
+    time_coord = iris.coords.DimCoord([0., 1., 2.],
+                                      standard_name='time')
+    sic_cube = iris.cube.Cube([[[20, 10], [10, 10]],
+                               [[10, 10], [10, 10]],
+                               [[10, 10], [10, 10]]],
+                              units='%',
+                              standard_name=sic_name,
+                              var_name='sic',
+                              dim_coords_and_dims=[(time_coord, 0)])
+    return iris.cube.CubeList([sic_cube])
+
+
+@pytest.fixture
+def cubes_siconca():
+    sic_name = 'sea_ice_area_fraction'
+    time_coord = iris.coords.DimCoord([0., 1., 2.],
+                                      standard_name='time')
+    sic_cube = iris.cube.Cube([[[20, 10], [10, 10]],
+                               [[10, 10], [10, 10]],
+                               [[10, 10], [10, 10]]],
+                              units='%',
+                              standard_name=sic_name,
+                              var_name='siconca',
+                              dim_coords_and_dims=[(time_coord, 0)])
+    return iris.cube.CubeList([sic_cube])
+
+
+@pytest.fixture
+def cubes():
+    sic_name = 'sea_ice_area_fraction'
+    time_coord = iris.coords.DimCoord([0., 1., 2.],
+                                      standard_name='time')
+    sic_cube = iris.cube.Cube([[[20, 10], [10, 10]],
+                               [[10, 10], [10, 10]],
+                               [[10, 10], [10, 10]]],
+                              units='%',
+                              standard_name=sic_name,
+                              var_name='sic',
+                              dim_coords_and_dims=[(time_coord, 0)])
+    siconca_cube = iris.cube.Cube([[[20, 10], [10, 10]],
+                                   [[10, 10], [10, 10]],
+                                   [[10, 10], [10, 10]]],
+                                  units='%',
+                                  standard_name=sic_name,
+                                  var_name='siconca',
+                                  dim_coords_and_dims=[(time_coord, 0)])
+    return iris.cube.CubeList([sic_cube, siconca_cube])
+
+
+def test_siextent_calculation_sic(cubes_sic):
+    """Test function ``calculate`` when sic is available."""
+    derived_var = siextent.DerivedVariable()
+    out_cube = derived_var.calculate(cubes_sic)
+    assert out_cube.units == cf_units.Unit('m2')
+    out_data = out_cube.data
+    expected = np.ma.ones_like(cubes_sic[0].data)
+    expected.mask = True
+    expected[0][0][0] = 1.
+    np.testing.assert_array_equal(out_data.mask, expected.mask)
+    np.testing.assert_array_equal(out_data[0][0][0], expected[0][0][0])
+
+
+def test_siextent_calculation_siconca(cubes_siconca):
+    """Test function ``calculate`` when siconca is available."""
+    derived_var = siextent.DerivedVariable()
+    out_cube = derived_var.calculate(cubes_siconca)
+    assert out_cube.units == cf_units.Unit('m2')
+    out_data = out_cube.data
+    expected = np.ma.ones_like(cubes_siconca[0].data)
+    expected.mask = True
+    expected[0][0][0] = 1.
+    np.testing.assert_array_equal(out_data.mask, expected.mask)
+    np.testing.assert_array_equal(out_data[0][0][0], expected[0][0][0])
+
+
+def test_siextent_calculation(cubes):
+    """Test function ``calculate`` when sic and siconca are available."""
+    derived_var = siextent.DerivedVariable()
+    out_cube = derived_var.calculate(cubes)
+    assert out_cube.units == cf_units.Unit('m2')
+    out_data = out_cube.data
+    expected = np.ma.ones_like(cubes[0].data)
+    expected.mask = True
+    expected[0][0][0] = 1.
+    np.testing.assert_array_equal(out_data.mask, expected.mask)
+    np.testing.assert_array_equal(out_data[0][0][0], expected[0][0][0])
+
+
+def test_siextent_no_data(cubes_sic):
+    derived_var = siextent.DerivedVariable()
+    cubes_sic[0].var_name = 'wrong'
+    msg = ('Derivation of siextent failed due to missing variables '
+           'sic and siconca.')
+    with pytest.raises(RecipeError, match=msg):
+        derived_var.calculate(cubes_sic)
+
+
+def test_siextent_required():
+    """Test function ``required``."""
+    derived_var = siextent.DerivedVariable()
+    output = derived_var.required(None)
+    assert output == [
+        {'short_name': 'sic', 'optional': 'true'},
+        {'short_name': 'siconca', 'optional': 'true'}
+    ]