Index function extreme_temperature_range, etr (closes #166)

climix/etc/metadata.yml

@@ -46,6 +46,14 @@ index_functions:
       From this it calculates the average day-to-day difference in
       diurnal temperature range.
 
+  extreme_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 extreme temperature range as the
+      maximum of daily maximum temperature minus the minimum of daily
+      minimum temperature.
+
   first_occurrence:
     description: |
       Calculates the first time some condition is met.

climix/index_functions/__init__.py

@@ -5,6 +5,7 @@ from .index_functions import (  # noqa: F401
     CountOccurrences,
     DiurnalTemperatureRange,
     InterdayDiurnalTemperatureRange,
+    ExtremeTemperatureRange,
     FirstOccurrence,
     LastOccurrence,
     SpellLength,

climix/index_functions/index_functions.py

@@ -75,6 +75,26 @@ class DiurnalTemperatureRange(IndexFunction):
     lazy_func = call_func
 
 
+class ExtremeTemperatureRange(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 == Unit('Kelvin')) or (units == Unit('degree_Celsius'))
+        super().prepare(input_cubes)
+
+    def call_func(self, data, axis, **kwargs):
+        res = (data['high_data'].max(axis=axis) -
+               data['low_data'].min(axis=axis))
+        return res.astype('float32')
+
+    lazy_func = call_func
+
+
 class InterdayDiurnalTemperatureRange(IndexFunction):
     def __init__(self):
         super().__init__(units=Unit('degree_Celsius'))

setup.py

@@ -62,6 +62,7 @@ setuptools.setup(
             'temperature_sum=climix.index_functions:TemperatureSum',
             'diurnal_temperature_range=climix.index_functions:DiurnalTemperatureRange',
             'interday_diurnal_temperature_range=climix.index_functions:InterdayDiurnalTemperatureRange',
+            'extreme_temperature_range=climix.index_functions:ExtremeTemperatureRange',
         ],
     },
     project_urls={