Can't get area weights of cube with additional auxillary coordinates of lat and lon #3916
Description
🐛 Bug Report
Iris is unable to calculate area weights for rotated pole grids if auxiliary coordinates with true latitude and longitude have been added to the cube.
How To Reproduce
- load data on a rotated pole grid
# load rotated pole grids
fname = iris.sample_data_path("rotated_pole.nc")
cube = iris.load_cube(fname)
- attach the aux coords thus:
# get cube's coordinate system
cs = cube.coord_system()
# get coord names
# Longitude
xcoord = cube.coord(axis="X", dim_coords=True)
# Latitude
ycoord = cube.coord(axis="Y", dim_coords=True)
# read in the grid lat/lon points from the cube
glat = cube.coord(ycoord).points
glon = cube.coord(xcoord).points
# create a rectangular grid out of an array of
# glon and glat values, shape will be len(glat)xlen(glon)
x, y = np.meshgrid(glon, glat)
# get the cube dimensions which corresponds to glon and glat
x_dim = cube.coord_dims(xcoord)[0]
y_dim = cube.coord_dims(ycoord)[0]
# define two new variables to hold the unrotated coordinates
rlongitude, rlatitude = iris.analysis.cartography.unrotate_pole(
x, y, cs.grid_north_pole_longitude, cs.grid_north_pole_latitude
)
# create two new auxillary coordinates to hold
# the values of the unrotated coordinates
reg_long = iris.coords.AuxCoord(rlongitude, long_name="longitude", units="degrees")
reg_lat = iris.coords.AuxCoord(rlatitude, long_name="latitude", units="degrees")
# add two auxilary coordinates to the cube holding
# regular(unrotated) lat/lon values
cube.add_aux_coord(reg_long, [y_dim, x_dim])
cube.add_aux_coord(reg_lat, [y_dim, x_dim])
- print the cube:
air_temperature / (K) (grid_latitude: 550; grid_longitude: 700)
Dimension coordinates:
grid_latitude x -
grid_longitude - x
Auxiliary coordinates:
latitude x x
longitude x x
4. try and get area weights:
weights =iris.analysis.cartography.area_weights(cube)
Traceback (most recent call last):
File "area_bug.py", line 49, in
weights =iris.analysis.cartography.area_weights(cube)
File ".../lib/python3.6/site-packages/iris/analysis/cartography.py", line 399, in area_weights
lon, lat = _get_lon_lat_coords(cube)
File ".../python3.6/site-packages/iris/analysis/cartography.py", line 189, in _get_lon_lat_coords
"Calling _get_lon_lat_coords with multiple lat or lon coords"
ValueError: Calling _get_lon_lat_coords with multiple lat or lon coords is currently disallowed
5. remove the extra coords - works fine:
cube.remove_coord('longitude')
cube.remove_coord('latitude')
weights =iris.analysis.cartography.area_weights(cube)
Expected behaviour
I expect that a cube like this should return the correct area weights
Environment
- RHEL 7.9
- Iris Version: 2.2.0
Additional context
Click to expand this section...
https://github.com/SciTools/iris/blob/master/lib/iris/analys...
I would suggest that this:
coord for coord in cube.coords() if "latitude" in coord.name()
should actually be:
coord for coord in cube.coords(dim_coords=True) if "latitude" in coord.name()
See here for further details.