如何使用cartopy在Map上绘制NetCDF文件中变量“r”（相对湿度）的数据？

from netCDF4 import Dataset
import cartopy.crs as ccrs
import numpy as np
import xarray as xr
import cartopy.crs as ccrs
import matplotlib.pyplot as plt
ds = Dataset('relative_humidityEne_Dic2003_2020CMAS.nc', 'r')
lon = ds.variables["longitude"]
lat = ds.variables["latitude"]
level = ds.variables["level"]
time = ds.variables["time"]
r = ds.variables["r"]
ax = plt.axes(projection=ccrs.PlateCarree())
r_ds = ds.variables['r'][0, :, :, :]
plt.contourf(lon, lat, r_ds, transform=ccrs.PlateCarree())
ax.coastlines()
plt.show()

root group (NETCDF3_64BIT_OFFSET data model, file format NETCDF3):
    Conventions: CF-1.6
    history: 2021-02-19 10:03:12 GMT by grib_to_netcdf-2.16.0: /opt/ecmwf/eccodes/bin/grib_to_netcdf -S param -o /cache/data7/adaptor.marsdev.internal-1613728991.1847723-6796-17-440f19fd-bb8f-478f-bc36-959e3e9d9c42.nc /cache/tmp/440f19fd-bb8f-478f-bc36-959e3e9d9c42-adaptor.marsdev.internal-1613728966.210842-6796-20-tmp.grib
    dimensions(sizes): longitude(480), latitude(241), level(8), time(53)
    variables(dimensions): float32 longitude(longitude), float32 latitude(latitude), int32 level(level), int32 time(time), int16 r(time,level,latitude,longitude)
    groups: 
dict_keys(['longitude', 'latitude', 'level', 'time', 'r'])
las dimenciones variable r es:  ('time', 'level', 'latitude', 'longitude')
(53, 8, 241, 480)

----------------------------------------------------------------------
<class 'netCDF4._netCDF4.Variable'>
int16 r(time, level, latitude, longitude)
    scale_factor: 0.001964639476266317
    add_offset: 64.43876426873042
    _FillValue: -32767
    missing_value: -32767
    units: %
    long_name: Relative humidity
    standard_name: relative_humidity
unlimited dimensions: 
current shape = (53, 8, 241, 480)
filling on
las dimenciones variable r es:  ('time', 'level', 'latitude', 'longitude')
(53, 8, 241, 480)

错误信息：

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/IPython/core/interactiveshell.py", line 3331, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-2-403ea023eab8>", line 1, in <module>
    runfile('/home/leo/Documentos/Universidad/Trabajo_de_investigación/PerfilesVerticalesContaminantesAtmosfera/Datos/readNetdcf42003_2020CMAS.py', wdir='/home/leo/Documentos/Universidad/Trabajo_de_investigación/PerfilesVerticalesContaminantesAtmosfera/Datos')
  File "/snap/pycharm-professional/230/plugins/python/helpers/pydev/_pydev_bundle/pydev_umd.py", line 197, in runfile
    pydev_imports.execfile(filename, global_vars, local_vars)  # execute the script
  File "/snap/pycharm-professional/230/plugins/python/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "/home/leo/Documentos/Universidad/Trabajo_de_investigación/PerfilesVerticalesContaminantesAtmosfera/Datos/readNetdcf42003_2020CMAS.py", line 65, in <module>
    plt.contourf(lon, lat, r_ds, transform=ccrs.PlateCarree())
  File "/home/leo/.local/lib/python3.8/site-packages/matplotlib/pyplot.py", line 2577, in contourf
    __ret = gca().contourf(
  File "/home/leo/.local/lib/python3.8/site-packages/cartopy/mpl/geoaxes.py", line 321, in wrapper
    return func(self, *args, **kwargs)
  File "/home/leo/.local/lib/python3.8/site-packages/cartopy/mpl/geoaxes.py", line 1586, in contourf
    result = matplotlib.axes.Axes.contourf(self, *args, **kwargs)
  File "/home/leo/.local/lib/python3.8/site-packages/matplotlib/__init__.py", line 1447, in inner
    return func(ax, *map(sanitize_sequence, args), **kwargs)
  File "/home/leo/.local/lib/python3.8/site-packages/matplotlib/axes/_axes.py", line 6335, in contourf
    contours = mcontour.QuadContourSet(self, *args, **kwargs)
  File "/home/leo/.local/lib/python3.8/site-packages/matplotlib/contour.py", line 816, in __init__
    kwargs = self._process_args(*args, **kwargs)
  File "/home/leo/.local/lib/python3.8/site-packages/matplotlib/contour.py", line 1430, in _process_args
    x, y, z = self._contour_args(args, kwargs)
  File "/home/leo/.local/lib/python3.8/site-packages/matplotlib/contour.py", line 1488, in _contour_args
    x, y, z = self._check_xyz(args[:3], kwargs)
  File "/home/leo/.local/lib/python3.8/site-packages/matplotlib/contour.py", line 1514, in _check_xyz
    x = np.asarray(x, dtype=np.float64)
  File "/usr/local/lib/python3.8/dist-packages/numpy/core/_asarray.py", line 102, in asarray
    return array(a, dtype, copy=False, order=order)
TypeError: __array__() takes no arguments (1 given)