coriolis_parameter: output error corrected.

[livialmg]

The former coriolis_parameter function throws an error: "AttributeError: 'Variable' object has no attribute 'to'"
With a minor change, it was corrected.

[CLAassistant]

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[dopplershift]

Thanks for the contribution. It looks like something went wrong, though. There's an extra basic.py added here in the base of the repository. Also, the version of src/metpy/calc/basic.py looks out of date (it's restoring some old parameter names). Can you update and fix?

[lgtm-com]

This pull request introduces 2 alerts when merging 285b474 into 51a4172 - view on LGTM.com

new alerts:

• 2 for Unused import

[jthielen]

This unfortunately looks like it would be a mutually conflicting change with the upcoming xarray updates for 1.0 (#1353). Would you be willing to write a test case or short example of the error you had encountered, so that I can verify that it is resolved in the updated version of #1353? Thank you!

[livialmg]

`import numpy as np
from metpy import calc as mpcalc
import xarray as xr

file_in = 'test.nc'

file = xr.open_dataset(file_in)
lat = file.variables['lat'][:]
f = mpcalc.coriolis_parameter(np.deg2rad(lat))`

I wrote this small piece of code to demonstrate the error I'm getting.
test.nc is a sample from my actual data:

------------------------------------------------------------------------
dimensions:
	lon = 321 ;
	lat = 281 ;
variables:
	double lon(lon) ;
		lon:units = "degrees_east" ;
		lon:long_name = "Longitude" ;
	double lat(lat) ;
		lat:units = "degrees_north" ;
		lat:long_name = "Latitude" ;
	double temp(lat, lon) ;
		temp:_FillValue = -999000000. ;
}
---------------------------------------------------------------------

The error message is:
"File "/home/livia/MetPy/src/metpy/xarray.py", line 1067, in wrapper
return func(*args, **kwargs)

File "/home/livia/MetPy/src/metpy/calc/basic.py", line 588, in coriolis_parameter
return (2. * mpconsts.omega * np.sin(latitude)).to('1/s')

AttributeError: 'Variable' object has no attribute 'to'"

When chaging the line 588 in basic.py, from:

return (2. * mpconsts.omega * np.sin(latitude)).to('1/s')

to

f = (2. * mpconsts.omega * np.sin(latitude))
return f.data.to('1/s')

then, I get an array if the coriolis parameter.

[jthielen]

Ah, it looks like you've found a corner-case in our existing xarray handling! xarray Variable objects are low-level objects that are not meant to be used on the user level, and so, we never built handling for them explicitly. This would be a good thing to document in MetPy's xarray guide as well.

I would recommend the following to fix your issue:

file = xr.open_dataset(file_in)
lat = file['lat']
f = mpcalc.coriolis_parameter(lat)

This will allow MetPy's handling of DataArrays to be used, with units automatically handled. Also, unfortunately while your proposed fix makes the calculation work for Variables, it breaks it for the standard case of pint.Quantity.

Hopefully this resolves your issue, and we can close this PR.