Area average time series

src/CSET/operators/collapse.py

@@ -29,6 +29,7 @@
 
 from CSET._common import iter_maybe
 from CSET.operators.aggregate import add_hour_coordinate
+from CSET.operators.misc import guess_bounds
 
 
 def collapse(
@@ -112,6 +113,18 @@ def collapse(
                 cube_max = cube.collapsed(coordinate, iris.analysis.MAX)
                 cube_min = cube.collapsed(coordinate, iris.analysis.MIN)
                 collapsed_cubes.append(cube_max - cube_min)
+            elif method == "AREA_AVG":
+                # Mask nans
+                if np.count_nonzero(np.isnan(cube.data)) > 0:
+                    cube.data = np.ma.masked_invalid(cube.data)
+
+                # Compute grid cell areas
+                cube = guess_bounds(cube)
+                grid_areas = iris.analysis.cartography.area_weights(cube)
+
+                collapsed_cubes.append(
+                    cube.collapsed(coordinate, iris.analysis.MEAN, weights=grid_areas)
+                )
             else:
                 collapsed_cubes.append(
                     cube.collapsed(coordinate, getattr(iris.analysis, method))

src/CSET/operators/misc.py

@@ -428,3 +428,39 @@ def convert_units(cubes: iris.cube.Cube | iris.cube.CubeList, units: str):
         return new_cubelist[0]
     else:
         return new_cubelist
+
+
+def guess_bounds(cube):
+    """
+    Guess bounds for x and y coordinates on a cube.
+
+    Arguments
+    ---------
+    cube: iris.cube.Cube
+        Input cube whose x and y coordinate bounds will be guessed if missing.
+
+    Returns
+    -------
+    iris.cube.Cube
+        The same cube with bounds added to x and y coordinates where absent.
+
+    Raises
+    ------
+    ValueError
+        If the cube uses a variable resolution grid where bounds cannot be
+        guessed reliably.
+    """
+    # Loop over spatial coordinates
+    for axis in ["x", "y"]:
+        coord = cube.coord(axis=axis)
+        try:
+            _ = iris.util.regular_step(coord)
+        except ValueError as e:
+            logging.warning(
+                "Cannot guess bounds for a variable resolution (non-regular) grid: %s",
+                e,
+            )
+        # Guess bounds if there aren't any
+        if coord.bounds is None:
+            coord.guess_bounds()
+    return cube

src/CSET/recipes/surface_fields/generic_area_average_time_series.yaml

@@ -0,0 +1,35 @@
+category: Surface Time Series
+title: Domain area average $VARNAME time series
+description: Plots a time series of the domain area average $VARNAME.
+
+steps:
+  - operator: read.read_cubes
+    file_paths: $INPUT_PATHS
+    model_names: $MODEL_NAME
+    constraint:
+      operator: constraints.combine_constraints
+      varname_constraint:
+        operator: constraints.generate_var_constraint
+        varname: $VARNAME
+      cell_methods_constraint:
+        operator: constraints.generate_cell_methods_constraint
+        cell_methods: []
+        varname: $VARNAME
+      pressure_level_constraint:
+        operator: constraints.generate_level_constraint
+        coordinate: "pressure"
+        levels: []
+    subarea_type: $SUBAREA_TYPE
+    subarea_extent: $SUBAREA_EXTENT
+
+  # Area-weighted average
+  - operator: collapse.collapse
+    coordinate: [grid_latitude, grid_longitude]
+    method: AREA_AVG
+
+  # Make a single NetCDF with all the data inside it.
+  - operator: write.write_cube_to_nc
+    overwrite: True
+
+  # Plot the data.
+  - operator: plot.plot_line_series