Adding hurs (realtive humidity) derivation script

esmvalcore/preprocessor/_derive/hurs.py

@@ -0,0 +1,45 @@
+"""Derivation of variable `hurs`."""
+
+from iris import NameConstraint
+import iris
+import numpy as np
+import cf_units
+
+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 = iris.analysis.maths.subtract(tas_cube, tdps_cube) 
+        cubes_product = iris.analysis.maths.multiply(tas_cube, tdps_cube)
+
+        log_humidity_cube = iris.analysis.maths.divide(-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('%')
+
+        return hurs_cube