Adding hurs (realtive humidity) derivation script (#2397)

Co-authored-by: Elizaveta Malinina <[email protected]>

Author: malininae

esmvalcore/preprocessor/_derive/hurs.py

@@ -0,0 +1,54 @@
+"""Derivation of variable `hurs`."""
+
+import cf_units
+import dask.array as da
+import iris
+from iris import NameConstraint
+
+from ._baseclass import DerivedVariableBase
+
+# Constants
+GAS_CONSTANT_WV = 461.5  # JK-1kg-1
+ENTALPY_OF_VAPORIZATION = 2.501e6  # Jkg-1
+
+
+class DerivedVariable(DerivedVariableBase):
+    """Derivation of variable `hurs`."""
+
+    @staticmethod
+    def required(project):
+        """Declare the variables needed for derivation."""
+        required = [
+            {"short_name": "tdps"},
+            {"short_name": "tas"},
+        ]
+        return required
+
+    @staticmethod
+    def calculate(cubes):
+        """Compute relative humidity.
+
+        Relative humidity computed from dewpoint temperature and
+        surface air temperature following Bohren and Albrecht 1998.
+        """
+        tdps_cube = cubes.extract_cube(NameConstraint(var_name="tdps"))
+        tas_cube = cubes.extract_cube(NameConstraint(var_name="tas"))
+
+        cubes_difference = tas_cube - tdps_cube
+        cubes_product = tas_cube * tdps_cube
+
+        log_humidity_cube = (
+            -ENTALPY_OF_VAPORIZATION
+            * cubes_difference
+            / (GAS_CONSTANT_WV * cubes_product)
+        )
+
+        hurs_cube = 100 * iris.analysis.maths.exp(log_humidity_cube)
+
+        hurs_cube.units = cf_units.Unit("%")
+
+        hurs_cube.data = da.ma.where(
+            hurs_cube.core_data() > 100.0, 100.0, hurs_cube.core_data()
+        )
+
+        return hurs_cube

tests/unit/preprocessor/_derive/test_hurs.py

@@ -0,0 +1,70 @@
+"""Test derivation of ``hurs``."""
+
+import iris
+import numpy as np
+import pytest
+from iris.cube import Cube, CubeList
+
+from esmvalcore.preprocessor._derive import hurs
+
+
+@pytest.fixture
+def cubes():
+    """Input cubes for derivation of ``hurs``."""
+    time_coord = iris.coords.DimCoord(
+        [0.0, 1.0, 2.0, 3.0],
+        standard_name="time",
+        var_name="time",
+        units="days since 1950-01-01 00:00:00",
+    )
+    lat_coord = iris.coords.DimCoord(
+        [45.0], standard_name="latitude", var_name="lat", units="degrees"
+    )
+    lon_coord = iris.coords.DimCoord(
+        [10.0], standard_name="longitude", var_name="lon", units="degrees"
+    )
+
+    coord_specs = [(time_coord, 0), (lat_coord, 1), (lon_coord, 2)]
+
+    tdps_cube = Cube(
+        [[[279.17]], [[282.73]], [[288.15]], [[288.25]]],
+        dim_coords_and_dims=coord_specs,
+        standard_name="dew_point_temperature",
+        var_name="tdps",
+        units="K",
+    )
+    tas_cube = Cube(
+        [[[288.15]], [[288.15]], [[288.15]], [[288.15]]],
+        dim_coords_and_dims=coord_specs,
+        standard_name="air_temperature",
+        var_name="tas",
+        units="K",
+    )
+    return CubeList([tdps_cube, tas_cube])
+
+
+def test_hurs_calculate(cubes):
+    """Test function ``calculate``."""
+    derived_var = hurs.DerivedVariable()
+    required_vars = derived_var.required("CMIP6")
+    expected_required_vars = [
+        {"short_name": "tdps"},
+        {"short_name": "tas"},
+    ]
+    assert required_vars == expected_required_vars
+    out_cube = derived_var.calculate(cubes)
+    assert out_cube.shape == (4, 1, 1)
+    assert out_cube.units == "%"
+    assert out_cube.coords("time")
+    assert out_cube.coords("latitude")
+    assert out_cube.coords("longitude")
+    np.testing.assert_allclose(
+        out_cube.data,
+        [[[54.6093]], [[69.7301]], [[100.0]], [[100.0]]],
+        rtol=0.00005,
+    )
+    np.testing.assert_allclose(
+        out_cube.coord("time").points, [0.0, 1.0, 2.0, 3.0]
+    )
+    np.testing.assert_allclose(out_cube.coord("latitude").points, [45.0])
+    np.testing.assert_allclose(out_cube.coord("longitude").points, [10.0])