Using function advection with wrf output

[iamzdy]

Sorry to bother you guys. I am using wrf-python and metpy to calculate the temperature advection. Firstly, I used wrf-python to extract variables and get 3 DataArray theta, u and v. Then, I used a function called advection and met some problems. Here is my code:

from netCDF4 import Dataset
from wrf import getvar
import metpy.calc as mpcalc

ncfile = Dataset('wrfout')
theta = getvar(ncfile, units='K')
u = getvar(ncfile,'ua')
v = getvar(ncfile,'va')

adv = mpcalc.advection(theta,u,v)

I got errors like that:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/metpy/xarray.py", line 652, in wrapper
    args = tuple(a.metpy.unit_array if isinstance(a, xr.DataArray) else a for a in args)
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/metpy/xarray.py", line 652, in <genexpr>
    args = tuple(a.metpy.unit_array if isinstance(a, xr.DataArray) else a for a in args)
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/metpy/xarray.py", line 135, in unit_array
    return self._data_array.values * self.units
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/metpy/xarray.py", line 128, in units
    return units(self._units)
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/pint/registry.py", line 1341, in parse_expression
    lambda x: self._eval_token(x, case_sensitive=case_sensitive, **values)
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/pint/pint_eval.py", line 109, in evaluate
    return bin_op[op_text](left, self.right.evaluate(define_op, bin_op, un_op))
  File "/public/home/zhongwei_vivian/anaconda3/lib/python3.7/site-packages/pint/quantity.py", line 1161, in __sub__
    return self._add_sub(other, operator.sub)
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/pint/quantity.py", line 140, in wrapped
    return f(self, *args, **kwargs)
  File "/public/home/vivian/anaconda3/lib/python3.7/site-packages/pint/quantity.py", line 1048, in _add_sub
    raise DimensionalityError(self._units, "dimensionless")
pint.errors.DimensionalityError: Cannot convert from 'meter * second' to 'dimensionless'

Is something wrong with the extracted data(theta, u, v)? Did I use the function wrongly?

[dopplershift]

1. Are u, v, and theta xarray DataArray instances?
2. What are the units on u and v?

[iamzdy]

1. Function type returns to xarray.core.dataarray.DataArray
2. u.units returns to 'm s-1'

[dopplershift]

This looks like a problem in our conversion of UDUNITS syntax "m s-1" to proper pint units. @jthielen can you add the case of "m s-1" to #2380 ?

[jthielen]

"m s-1" is correctly handled as "meter/second" with the current version of MetPy, but yes, I can still easily add this string to test in @2380.

@iamzdy what MetPy version are you using?

[iamzdy]

Sorry for getting back to you so late. I used the latest MetPy version. I actually have handled this problem by updating xarray version. I think it is the problem of old units syntax.

Thank you for your reply!