Metpy mpcalc.absolute_vorticity() with incompatible grids

Question

I'm attempting to follow this training example to calculate QG omega on NCEP/NCAR data but I'm getting hung up on mpcalc.absolute_vorticity.

import xarray as xr
import metpy.calc as mc
import metpy.constants as mpconstants
from metpy.units import units
import numpy as np

path='./'
uf = 'uwnd.2018.nc'
vf = 'vwnd.2018.nc'
af = 'air.2018.nc'

ads = xr.open_dataset(path+af).metpy.parse_cf()
uds = xr.open_dataset(path+uf).metpy.parse_cf()
vds = xr.open_dataset(path+vf).metpy.parse_cf()

a700 = ads['air'].metpy.sel(
        level=700 * units.hPa,
        time='2018-01-04T12')
u700 = uds['uwnd'].metpy.sel(
        level=700 * units.hPa,
        time='2018-01-04T12')
v700 = vds['vwnd'].metpy.sel(
        level=700 * units.hPa,
        time='2018-01-04T12')

lats = ads['lat'].metpy.unit_array
lons = ads['lon'].metpy.unit_array
X, Y = np.meshgrid(lons,lats)
dx, dy = mc.lat_lon_grid_deltas(lons,lats)

avort = mc.absolute_vorticity(u700,v700,dx,dy,lats)

I get the error, "ValueError: operands could not be broadcast together with shapes (73,144) (73,) ".

Full error traceback below:

Traceback (most recent call last):
  File "metpy.decomp.py", line 43, in <module>
    avort = mc.absolute_vorticity(u700,v700,dx,dy,lats) 
  File "/work1/jsa/anconda3/envs/earth/lib/python3.7/site-packages/metpy/xarray.py", line 570, in wrapper
    return func(*args, **kwargs)
  File "/work1/jsa/anconda3/envs/earth/lib/python3.7/site-packages/metpy/units.py", line 312, in wrapper
    return func(*args, **kwargs)
  File "/work1/jsa/anconda3/envs/earth/lib/python3.7/site-packages/metpy/calc/kinematics.py", line 639, in absolute_vorticity
    return relative_vorticity + f
  File "/work1/jsa/anconda3/envs/earth/lib/python3.7/site-packages/pint/quantity.py", line 754, in __add__
    return self._add_sub(other, operator.add)
  File "/work1/jsa/anconda3/envs/earth/lib/python3.7/site-packages/pint/quantity.py", line 75, in wrapped
    result = f(self, *args, **kwargs)
  File "/work1/jsa/anconda3/envs/earth/lib/python3.7/site-packages/pint/quantity.py", line 686, in _add_sub
    magnitude = op(self._magnitude, other._magnitude)
ValueError: operands could not be broadcast together with shapes (73,144) (73,)

It would be really helpful to see the full error traceback and not just the last error so that the failing line can actually be seen. — DopplerShift, Jul 21 '21 at 21:54
Apologies for not including originally, the full error traceback is now included. — Stuivenvoltage, Jul 21 '21 at 21:58

score 1 · Accepted Answer · answered Jul 21 '21 at 21:58

Without access to the data or the full error, my guess is the problem is that lats doesn't have the right shape to obey NumPy's broadcasting rules. Try changing to:

avort = mc.absolute_vorticity(u700, v700, dx, dy, lats[:, None])

Essentially, the trailing dimensions need to line up. Before, NumPy was trying to line up 73 (lats) with the lons dimension (144). Adding the [:, None] creates a size-1 dimension as the last dimension for lats, and then the alignment is trying to match shape (73, 1) with (73, 144), which works.

Metpy mpcalc.absolute_vorticity() with incompatible grids

1 Answers1