pcolormesh false returns masked array error

[lexipalmer13]

Using matplotlib with basemap to plot temperature data and getting an error saying pcolormesh cannot plot masked data. When raised on github (link) response suggested that it's a corner case and arrays get overflowed rather than an issue with the data. Putting here as a reminder as requested - happy to provide more info if helpful!

[guziy]

The problem is that the conic projection is not valid globally, and you are trying to feed it era5 global fields... Probably basemap could be more intelligent about it...

Here is the workaround...

from netCDF4 import Dataset
import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

data = Dataset('ERA5_1959.nc')

lat = data.variables['latitude'][:]
lon = data.variables['longitude'][:]

lon, lat = np.meshgrid(lon, lat)
t2m = data.variables['t2m'][1,1,:,:] #temp data

#create non-masked arrays
lon2 = lon
lat2 = lat

print(lat2.min(), lat2.max())
print(lon2.min(), lon2.max())

#plot
fig = plt.figure(figsize=(10, 8))
m = Basemap(projection='lcc', resolution='c',
            width=8E6, height=8E6, 
            lat_0=45, lon_0=-100,)
m.shadedrelief(scale=0.5)

i_arr, j_arr = np.where((lat2 >= m.latmin) & (lat2 <= m.latmax))

i_min, i_max = i_arr.min(), i_arr.max()
j_min, j_max = j_arr.min(), j_arr.max()

i_slice = slice(i_min, i_max + 1)
j_slice = slice(j_min, j_max + 1)
t2m = t2m[i_slice, j_slice]
lon2 = lon2[i_slice, j_slice]
lat2 = lat2[i_slice, j_slice]

x, y = m(lon2, lat2)
m.pcolormesh(x, y, t2m, cmap='RdBu_r')
# m.pcolormesh(lon2, lat2, t2m,
#              latlon=True, cmap='RdBu_r')
# plt.clim(-8, 8)
m.drawcoastlines(color='lightgray')

plt.title('January 2014 Temperature Anomaly')
plt.colorbar(label='temperature anomaly (°C)');
plt.show()

[guziy]

I get this warning though, it seems like it does not like something about coordinates...

test-basemap-overflow-coords/test.py:48: UserWarning: The input coordinates to pcolormesh are interpreted as cell centers, but are not monotonically increasing or decreasing. This may lead to incorrectly calculated cell edges, in which case, please supply explicit cell edges to pcolormesh.
  m.pcolormesh(x, y, t2m, cmap='RdBu_r')

[guziy]

The warning goes away if I do this:

i_slice = slice(i_min, i_max + 1)
j_slice = slice(j_min, j_max + 1)
t2m = t2m[i_min:i_max, j_min:j_max]
lon2 = lon2[i_slice, j_slice]
lat2 = lat2[i_slice, j_slice]

[lexipalmer13]

My apologies for not responding to this earlier, but this fix works! Thank you so much for your help - I really appreciate it!