Index function interday diurnal temperature range (closes #154)

climix/etc/metadata.yml

@@ -39,6 +39,13 @@ index_functions:
       daily minimum and maximum temperature.
       From this it calculated the average diurnal temperature range.
 
+  interday_diurnal_temperature_range:
+    description: |
+      This index function takes two inputs, low_data and high_data, i.e.
+      daily minimum and maximum temperature.
+      From this it calculates the average day-to-day difference in
+      diurnal temperature range.
+
   first_occurrence:
     description: |
       Calculates the first time some condition is met.

climix/index_functions/__init__.py

@@ -4,6 +4,7 @@ from .index_functions import (  # noqa: F401
     CountLevelCrossings,
     CountOccurrences,
     DiurnalTemperatureRange,
+    InterdayDiurnalTemperatureRange,
     FirstOccurrence,
     LastOccurrence,
     SpellLength,

climix/index_functions/index_functions.py

@@ -75,6 +75,29 @@ class DiurnalTemperatureRange(IndexFunction):
     lazy_func = call_func
 
 
+class InterdayDiurnalTemperatureRange(IndexFunction):
+    def __init__(self):
+        super().__init__(units=Unit('degree_Celsius'))
+
+    def prepare(self, input_cubes):
+        props = {(cube.dtype, cube.units, cube.standard_name)
+                 for cube in input_cubes.values()}
+        assert len(props) == 1
+        dtype, units, standard_name = props.pop()
+        assert units in [Unit('Kelvin'), Unit('degree_Celsius')]
+        super().prepare(input_cubes)
+
+    def call_func(self, data, axis, **kwargs):
+        res = np.absolute(np.diff(data['high_data'] -
+                                  data['low_data'], axis=axis)).mean(axis=axis)
+        return res.astype('float32')
+
+    def lazy_func(self, data, axis, **kwargs):
+        res = da.absolute(da.diff(data['high_data'] -
+                                  data['low_data'], axis=axis)).mean(axis=axis)
+        return res.astype('float32')
+
+
 class FirstOccurrence(ThresholdMixin, IndexFunction):
     def __init__(self, threshold, condition):
         super().__init__(threshold, condition, units=Unit('days'))

setup.py

@@ -61,6 +61,7 @@ setuptools.setup(
             'thresholded_statistics=climix.index_functions:ThresholdedStatistics',
             'temperature_sum=climix.index_functions:TemperatureSum',
             'diurnal_temperature_range=climix.index_functions:DiurnalTemperatureRange',
+            'interday_diurnal_temperature_range=climix.index_functions:InterdayDiurnalTemperatureRange',
         ],
     },
     project_urls={