Model level constraint operator

src/CSET/operators/constraints.py

@@ -62,6 +62,26 @@ def generate_var_constraint(varname: str, **kwargs) -> iris.Constraint:
     return varname_constraint
 
 
+def generate_model_level_constraint(model_level_name: str, **kwargs) -> iris.Constraint:
+    """Generate constraint from variable name.
+
+    Operator that takes a CF compliant model_level_number string, and uses iris to
+    generate a constraint to be passed into the read operator to minimize the
+    CubeList the read operator loads and speed up loading.
+
+    Arguments
+    ---------
+    model_level_name: str
+        CF compliant model level name of variable. Needed later for LFRic.
+
+    Returns
+    -------
+    model_level_number_constraint: iris.Constraint
+    """
+    model_level_number_constraint = iris.Constraint(model_level_number=model_level_name)
+    return model_level_number_constraint
+
+
 def generate_cell_methods_constraint(cell_methods: list, **kwargs) -> iris.Constraint:
     """Generate constraint from cell methods.
 

src/CSET/recipes/extract_instant_model_level_air_temp.yaml

@@ -0,0 +1,36 @@
+title: Extract Meaned Model level Air Temperature
+description: |
+  Extracts out the instantaneous model level air temperature from a file and writes it
+  to a new one.
+section: Grid Stat
+major_cat: Major Category 1
+minor_cat: Minor Category 1
+
+steps:
+  - operator: read.read_cubes
+    constraint:
+      operator: constraints.generate_var_constraint
+      varname: air_temperature
+
+  - operator: filters.filter_cubes
+    constraint:
+      operator: constraints.combine_constraints
+      varname_constraint:
+        operator: constraints.generate_var_constraint
+        varname: air_temperature
+      model_level_number_constraint:
+        operator: constraints.generate_model_level_constraint
+        model_level_name: 2
+      cell_methods_constraint:
+        operator: constraints.generate_cell_methods_constraint
+        cell_methods: []
+
+  - operator: collapse.collapse
+    coordinate: time
+    method: MEAN
+
+  - operator: plot.spatial_contour_plot
+    file_path: CSET_OUTPUT_PATH
+
+  - operator: write.write_cube_to_nc
+    file_path: CSET_OUTPUT_PATH

tests/operators/test_constraints.py

@@ -33,6 +33,15 @@ def test_generate_var_constraint():
     assert repr(var_constraint) == expected_var_constraint
 
 
+def test_generate_model_level_constraint():
+    """Generate iris cube constraint for str variable name."""
+    var_constraint = constraints.generate_model_level_constraint("2")
+    expected_model_level_constraint = (
+        "Constraint(coord_values={'model_level_number': 2})"
+    )
+    assert repr(var_constraint) == expected_model_level_constraint
+
+
 def test_generate_cell_methods_constraint():
     """Generate iris cube constraint for cell methods."""
     cell_methods_constraint = constraints.generate_cell_methods_constraint([])