Adds support for forecast reference metadata in CF checks. (#779)

Adds support for forecast reference metadata in CF checks.

This commit updates cfutil's feature detection to ignore variables used
to declare forecast metadata. Specifically, variables with standard
names 'forecast_period' and 'forecast_reference_time' are used in
conjunction with 'time' to indicate to dataset consumers the appropriate
model initialization time, forecast hour, and valid time. This is
useful for datasets that aggregate multiple forecast runs, such as FMRC.

compliance_checker/cfutil.py

@@ -194,6 +194,9 @@ def is_geophysical(ds, variable):
     if variable in get_auxiliary_coordinate_variables(ds):
         return False
 
+    if variable in get_forecast_metadata_variables(ds):
+        return False
+
     # Is it dimensionless and unitless?
     if len(ncvar.shape) == 0 and unitless:
         return False
@@ -296,6 +299,26 @@ def get_auxiliary_coordinate_variables(ds):
     return ret_val
 
 
+def get_forecast_metadata_variables(ds):
+    '''
+    Returns a list of variables that represent forecast reference time
+    metadata.
+
+    :param netCDF4.Dataset ds: An open netCDF4 Dataset.
+    :rtype: list
+    '''
+    forecast_metadata_standard_names = {
+        'forecast_period',
+        'forecast_reference_time',
+    }
+    forecast_metadata_variables = []
+    for varname in ds.variables:
+        standard_name = getattr(ds.variables[varname], 'standard_name', None)
+        if standard_name in forecast_metadata_standard_names:
+            forecast_metadata_variables.append(varname)
+    return forecast_metadata_variables
+
+
 def get_cell_boundary_map(ds):
     '''
     Returns a dictionary mapping a variable to its boundary variable. The

compliance_checker/tests/data/forecast_reference.cdl

@@ -0,0 +1,32 @@
+netcdf forecast_reference.cdl {
+dimensions:
+    time = 2 ;
+    lat = 4 ;
+    lon = 4 ;
+variables:
+    float time(time) ;
+        time:units = "seconds since 1970-01-01" ;
+        time:standard_name = "time" ;
+        time:calendar = "gregorian" ;
+        time:axis = "T" ;
+    float lat(lat) ;
+        lat:units = "degrees_north" ;
+        lat:standard_name = "latitude" ;
+        lat:axis = "Y" ;
+    float lon(lon) ;
+        lon:units = "degrees_east" ;
+        lon:standard_name = "longitude" ;
+        lon:axis = "X" ;
+    float air_temp(time, lon, lat) ;
+        air_temp:units = "deg_C" ;
+        air_temp:standard_name = "air_temperature";
+    float forecast_reference_time(time) ;
+        forecast_reference_time:units = "seconds since 1970-01-01" ;
+        forecast_reference_time:standard_name = "forecast_reference_time" ;
+    float forecast_hour(time) ;
+        forecast_hour:units = "hours" ;
+        forecast_hour:standard_name = "forecast_period" ;
+
+    :featureType = "grid" ;
+
+}

compliance_checker/tests/resources.py

@@ -48,6 +48,7 @@ def generate_dataset(cdl_path, nc_path):
     'dimension_order'                      : get_filename('tests/data/dimension_order.cdl'),
     'example-grid'                         : get_filename('tests/data/example-grid.cdl'),
     'featureType'                          : get_filename('tests/data/example-grid.cdl'),
+    'forecast_reference'                   : get_filename('tests/data/forecast_reference.cdl'),
     'fvcom'                                : get_filename('tests/data/examples/fvcom.cdl'),
     'ghrsst'                               : get_filename('tests/data/20160919092000-ABOM-L3S_GHRSST-SSTfnd-AVHRR_D-1d_dn_truncate.cdl'),
     'glcfs'                                : get_filename('tests/data/examples/glcfs.cdl'),

compliance_checker/tests/test_feature_detection.py

@@ -226,6 +226,20 @@ def test_auxiliary_coordinates(self):
             aux_coord_vards = util.get_auxiliary_coordinate_variables(nc)
             assert set(['lat', 'lon']) == set(aux_coord_vards)
 
+    def test_forecast_reference_metadata(self):
+        '''
+        Tests variables used for forecast reference metadata to ensure they are
+        not misclassified as geophysical variables.
+        '''
+        with Dataset(resources.STATIC_FILES['forecast_reference']) as nc:
+            self.assertFalse(util.is_geophysical(nc, 'forecast_reference_time'))
+            self.assertFalse(util.is_geophysical(nc, 'forecast_hour'))
+            self.assertTrue(util.is_geophysical(nc, 'air_temp'))
+            self.assertFalse(util.is_geophysical(nc, 'time'))
+
+            assert len(util.get_coordinate_variables(nc)) == 3
+            assert len(util.get_geophysical_variables(nc)) == 1
+
     def test_rotated_pole_grid(self):
         with Dataset(resources.STATIC_FILES['rotated_pole_grid']) as nc:
             latitudes = util.get_latitude_variables(nc)