[python #KPU-861209]: Bulk wind shear calculation for gridded data

Greetings!

So unfortunately, in the code sample you sent you're using xarray's own 
`.sel()` method, which doesn't support units and apparently silently discards 
them. So the calls to `.sel()` you're making are requesting the 500 and 1000 
levels in the native coordinates of the data, which is Pa. That means you're 
actually selecting the 5 hPa and 10 hPa levels. To properly use the unit 
support, you need to use MetPy's accessor, as in (note the `.metpy`):

  u_500 = ds['u-component_of_wind_isobaric'].metpy.sel(isobaric=1000*units.hPa)

Alternatively, you can just make sure you request the 100000 Pa surface 
directly:

  u_500 = ds['u-component_of_wind_isobaric'].sel(isobaric=100000)

I know this is confusing, it's unfortunate that we don't have a way to signal 
that the units are silently discarded.1

I'm not sure from what you sent if this will resolve all of your issues, but it 
should definitely help results be more like what you expect I think.

Ryan

> Hello
> 
> Thanks very much for your work on metpy and support.
> 
> I'm having trouble with calculating the bulk wind difference
> (1000hPa-500hPa) for gridded GFS data.
> 
> This is the simple calculation I'm doing:
> 
> [image: image.png]
> [image: image.png]
> The following plot of wind barbs shows that something is wrong (red=500hPa
> winds, black=1000hPa and green is the difference):
> 
> [image: image.png]
> The vector differences are not correct. Could you please help me with this
> problem?


Ticket Details
===================
Ticket ID: KPU-861209
Department: Support Python
Priority: Low
Status: Closed
===================
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