From 3e543cca08f6333ab11c4494dea238ab68bb2e21 Mon Sep 17 00:00:00 2001
From: Bouwe Andela <b.andela@esciencecenter.nl>
Date: Wed, 12 Jun 2019 16:50:15 +0200
Subject: [PATCH] Lazy derivation of gtfgco2

---
 esmvalcore/preprocessor/_derive/gtfgco2.py | 55 ++--------------------
 1 file changed, 4 insertions(+), 51 deletions(-)

diff --git a/esmvalcore/preprocessor/_derive/gtfgco2.py b/esmvalcore/preprocessor/_derive/gtfgco2.py
index f8295c8c6e..81b67a3424 100644
--- a/esmvalcore/preprocessor/_derive/gtfgco2.py
+++ b/esmvalcore/preprocessor/_derive/gtfgco2.py
@@ -1,47 +1,9 @@
 """Derivation of variable `gtfgco2`."""
 import iris
-import numpy as np
 
 from ._baseclass import DerivedVariableBase
 
 
-def calculate_total_flux(fgco2_cube, cube_area):
-    """
-    Calculate the area of unmasked cube cells.
-
-    Requires a cube with two spacial dimensions. (no depth coordinate).
-
-    Parameters
-    ----------
-    cube: iris.cube.Cube
-        Data Cube
-    cube_area: iris.cube.Cube
-        Cell area Cube
-
-    Returns
-    -------
-    numpy.array:
-        An numpy array containing the total flux of CO2.
-
-    """
-    data = []
-    times = fgco2_cube.coord('time')
-
-    fgco2_cube.data = np.ma.array(fgco2_cube.data)
-    for time_itr in np.arange(len(times.points)):
-
-        total_flux = fgco2_cube[time_itr].data * cube_area.data
-
-        total_flux = np.ma.masked_where(fgco2_cube[time_itr].data.mask,
-                                        total_flux)
-        data.append(total_flux.sum())
-
-    ######
-    # Create a small dummy output array
-    data = np.array(data)
-    return data
-
-
 class DerivedVariable(DerivedVariableBase):
     """Derivation of variable `gtfgco2`."""
 
@@ -62,20 +24,11 @@ def calculate(cubes):
         fgco2_cube = cubes.extract_strict(
             iris.Constraint(name='surface_downward_mass_flux_of_carbon_dioxide'
                             '_expressed_as_carbon'))
+        area_cube = cubes.extract_strict(iris.Constraint(name='cell_area'))
 
-        try:
-            cube_area = cubes.extract_strict(iris.Constraint(name='cell_area'))
-        except iris.exceptions.ConstraintMismatchError:
-            pass
-
-        total_flux = calculate_total_flux(fgco2_cube, cube_area)
-
-        # Dummy result cube
-        result = fgco2_cube.collapsed(
+        total_flux = (fgco2_cube * area_cube).collapsed(
             ['latitude', 'longitude'],
-            iris.analysis.MEAN,
+            iris.analysis.SUM,
         )
-        result.units = fgco2_cube.units * cube_area.units
 
-        result.data = total_flux
-        return result
+        return total_flux