CCL Calculation Issue

[Cjseng]

Hello! I'm trying to use the metpy.calc.ccl functionality and keep getting this traceback:

Traceback (most recent call last):

File ~\anaconda3\envs\MetPy\lib\site-packages\spyder_kernels\py3compat.py:356 in compat_exec
exec(code, globals, locals)

File c:\users\cjsen\desktop\upper air profiles - working\winndex data grabber.py:104
ccl_array = mpcalc.ccl(pressure, temperature, dewpoint)

File ~\anaconda3\envs\MetPy\lib\site-packages\metpy\xarray.py:1328 in wrapper
result = func(*bound_args.args, **bound_args.kwargs)

File ~\anaconda3\envs\MetPy\lib\site-packages\metpy\units.py:325 in wrapper
return func(*args, **kwargs)

File ~\anaconda3\envs\MetPy\lib\site-packages\metpy\calc\thermo.py:574 in ccl
x, y = x[-1], y[-1]

File ~\anaconda3\envs\MetPy\lib\site-packages\pint\facets\numpy\quantity.py:238 in getitem
return type(self)(self._magnitude[key], self._units)

IndexError: index -1 is out of bounds for axis 0 with size 0

CCL from CSV.txt

As far as I can tell, my arrays for Pressure, Temperature, and Dewpoint are all fine. If I set "which = 'all'", the IndexError drops but it returns an empty array that looks like this: (<Quantity([], 'millibar')>, <Quantity([], 'degree_Celsius')>, <Quantity([], 'degree_Celsius')>)

I'm just super lost at this point and am hoping someone here can point me in the right direction! I've attached my current code with the only change being the file path on line 41. I can gladly provide more information if needed.

[dopplershift]

Can you share the source data so we can try to reproduce? My best guess right now is that you're triggering a corner case that we missed.

[Cjseng]

Here you go! Basically the intent is to loop through a bunch of these CSV files to calculate the CCL for each, and then once I get the CCL calculating correctly I'll be exporting it to another CSV file for a future use case.

20150501_12_profile.csv

[Z-Richard]

Hi @Cjseng, I am sorry for the late response -- but the issue is due to the fact MetPy implicitly assumes that the pressure array is in descending order (such that it mimics the real atmosphere, from ground to TOA). To make your code work, you only need to change

pressure = df['Pres (mb)'].values * units.mbar
temperature = df['Temp (C)'].values * units.degC
dewpoint = df['Dewpoint (C)'].values * units.degC

to

pressure = np.flip(df['Pres (mb)'].values) * units.mbar
temperature = np.flip(df['Temp (C)'].values) * units.degC
dewpoint = np.flip(df['Dewpoint (C)'].values) * units.degC

I believe MetPy should have better specifications for these functions and provide more useful error messages when things go astray. I will submit an issue shortly.

[Cjseng]

Good morning! No worries at all, I appreciate you looking into that. I did find another way to calculate CCL in the meantime that seems to have gotten me pretty close, but I'm sure needing CCL will come up again and this will definitely be useful. Thank you very much!