ValueError: zero-size array to reduction operation minimum which has no identity

[leaver2000]

What went wrong?

Example

Given this example in calculating the most_unstable_cape_cin...

P = [
    101300.0,
    100000.0,
    97500.0,
    95000.0,
    92500.0,
    90000.0,
    87500.0,
    85000.0,
    82500.0,
    80000.0,
    77500.0,
    75000.0,
    72500.0,
    70000.0,
    65000.0,
    60000.0,
    55000.0,
    50000.0,
    45000.0,
    40000.0,
    35000.0,
    30000.0,
    25000.0,
    20000.0,
]
T = [
    297.54987283445246,
    296.88002814889177,
    295.7800726652342,
    294.78940763027333,
    292.9596739891891,
    293.0196074610551,
    291.93022847243265,
    290.53941937530794,
    288.93002938078973,
    287.759563930231,
    287.01920927776916,
    286.03030699198086,
    284.6906176208595,
    283.3897087315337,
    281.5599750904495,
    276.50969871115626,
    272.8995884058186,
    267.7999550496946,
    261.2795458605129,
    255.03060004390053,
    248.24049023132204,
    239.7704804573242,
    229.55006075500586,
    217.17027331634054,
]
Td = [
    295.903247655174,
    295.69122116536846,
    295.0838908693815,
    293.5371919545537,
    292.24457112007633,
    288.04982069988165,
    285.37780607791353,
    284.17380677750697,
    282.68806839907216,
    276.70189149107654,
    269.4555485732638,
    263.42704064619716,
    258.0106646774842,
    249.26453828772148,
    239.24704466847052,
    244.3324971464268,
    238.1753929086274,
    234.09412244735233,
    231.72751436757792,
    232.84305312811804,
    241.34976858460612,
    236.3692283618347,
    225.98551449499607,
    213.61268518282787,
]
P *= units.Pa
T *= units.K
Td *= units.K
assert np.all(T >= Td)

mpcalc.most_unstable_cape_cin(P, T, Td)

An error is triggered when evaluating the LFC https://github.com/Unidata/MetPy/blob/main/src/metpy/calc/thermo.py#L976

if np.min(el_pressure) > this_lcl[0]:

Seemingly because the profile contains no EL . This can be solved with a size check on el_pressure as such.

if el_pressure.size and np.min(el_pressure) > this_lcl[0]: # the LFC is this_lcl

or

if not el_pressure.size or np.min(el_pressure) > this_lcl[0]: # the LFC is nan

Operating System

Windows

Version

MetPy==1.6.3

Python Version

3.12

Code to Reproduce

P = [
    101300.0,
    100000.0,
    97500.0,
    95000.0,
    92500.0,
    90000.0,
    87500.0,
    85000.0,
    82500.0,
    80000.0,
    77500.0,
    75000.0,
    72500.0,
    70000.0,
    65000.0,
    60000.0,
    55000.0,
    50000.0,
    45000.0,
    40000.0,
    35000.0,
    30000.0,
    25000.0,
    20000.0,
]
T = [
    297.54987283445246,
    296.88002814889177,
    295.7800726652342,
    294.78940763027333,
    292.9596739891891,
    293.0196074610551,
    291.93022847243265,
    290.53941937530794,
    288.93002938078973,
    287.759563930231,
    287.01920927776916,
    286.03030699198086,
    284.6906176208595,
    283.3897087315337,
    281.5599750904495,
    276.50969871115626,
    272.8995884058186,
    267.7999550496946,
    261.2795458605129,
    255.03060004390053,
    248.24049023132204,
    239.7704804573242,
    229.55006075500586,
    217.17027331634054,
]
Td = [
    295.903247655174,
    295.69122116536846,
    295.0838908693815,
    293.5371919545537,
    292.24457112007633,
    288.04982069988165,
    285.37780607791353,
    284.17380677750697,
    282.68806839907216,
    276.70189149107654,
    269.4555485732638,
    263.42704064619716,
    258.0106646774842,
    249.26453828772148,
    239.24704466847052,
    244.3324971464268,
    238.1753929086274,
    234.09412244735233,
    231.72751436757792,
    232.84305312811804,
    241.34976858460612,
    236.3692283618347,
    225.98551449499607,
    213.61268518282787,
]
P *= units.Pa
T *= units.K
Td *= units.K
assert np.all(T >= Td)

mpcalc.most_unstable_cape_cin(P, T, Td)

Errors, Traceback, and Logs

728 el_pressure, _ = find_intersections(pressure[1:], parcel_temperature_profile[1:],
    729                                     temperature[1:], direction='decreasing',
    730                                     log_x=True)
    731 # if not el_pressure.size or np.min(el_pressure) > this_lcl[0]:
    732 # if el_pressure.size and np.min(el_pressure) > this_lcl[0]:
--> 733 if np.min(el_pressure) > this_lcl[0]:
    734     x = units.Quantity(np.nan, pressure.units)
    735     y = units.Quantity(np.nan, temperature.units)

[dopplershift]

Thanks for the detailed reproducer and the fix, which looks correct to me. Any interest in submitting a PR with the (second) fix? You can use this same profile to add a test for lfc.

[leaver2000]

@dopplershift I submit the PR, however I used the first rather than the second solution. In the PR I included some images to support this point. My understanding of that block of code is that when the EL is below the LCL then there is no LFC, however if no EL was found (which is the cause of the error) it is an indication of a very unstable atmosphere, maybe the EL is at the tropopause, but it's not an indication of there not being an LFC.

[dopplershift]

For completeness, as I mentioned in the PR, you're right, the first one is the correct fix.