LFC calc: if EL is above top of sounding, use top of sounding as EL

[wx4stg]

If attempting to calculate the LFC of a partial sounding, particularly one where the data is cut off below the EL, calculating the LFC or CAPE causes a value error. This commit uses the top of the sounding as the EL if the EL is absent (for the LFC calculation only).

This may or may not be a fix for the issue described in #2315, I'm not sure.

[wx4stg]

hmm. This was somewhat of a quick fix for an exception getting raised when I threw a particular dataset at cape_cin a few weeks ago, and it made sense to me at the time, but now that I'm actually going about writing a test for this, it's returning an LFC that's above the profile I'm providing....

I think crashing is clearly the wrong behavior, but so is providing an LFC based on interpolated data. I'm going to think about this some more

[dopplershift]

The scenario you're describing fits that of the May 3rd sounding we have in the test data (may4_sounding.txt) and that works fine with surface_based_cape_cin. I wonder if there's something else going on here.

[wx4stg]

the "sounding" that caused the crash in my case was very much an edge case...

from metpy import calc as mpcalc
import numpy as np
from metpy.units import units

level = np.array([733.0, 732.0, 731.0, 730.0]) * units.hPa
temp = np.array([16.740580530590336, 16.645407342070797, 16.550234153551262, 16.39797906332763]) * units.degC
dew = np.array([-1.5180807300997776, -1.3453806517560651, -1.1726805734123527, -1.0063032908045617]) * units.degC

# this will cause a traceback
mpcalc.surface_based_cape_cin(levels, temp, dew)

The traceback is:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/stgardner4/Downloads/metpy/src/metpy/xarray.py", line 1328, in wrapper
    result = func(*bound_args.args, **bound_args.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/units.py", line 325, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/calc/thermo.py", line 2865, in surface_based_cape_cin
    return cape_cin(p, t, td, profile)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/xarray.py", line 1328, in wrapper
    result = func(*bound_args.args, **bound_args.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/units.py", line 325, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/calc/thermo.py", line 2363, in cape_cin
    lfc_pressure, _ = lfc(pressure, temperature, dewpoint,
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/xarray.py", line 1328, in wrapper
    result = func(*bound_args.args, **bound_args.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/units.py", line 325, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/stgardner4/Downloads/metpy/src/metpy/calc/thermo.py", line 722, in lfc
    if np.min(el_pressure) > this_lcl[0]:
       ^^^^^^^^^^^^^^^^^^^
  File "<__array_function__ internals>", line 200, in amin
  File "/opt/mamba/envs/HDWX/lib/python3.11/site-packages/pint/facets/numpy/quantity.py", line 66, in __array_function__
    return numpy_wrap("function", func, args, kwargs, types)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/mamba/envs/HDWX/lib/python3.11/site-packages/pint/facets/numpy/numpy_func.py", line 1034, in numpy_wrap
    return handled[name](*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/mamba/envs/HDWX/lib/python3.11/site-packages/pint/facets/numpy/numpy_func.py", line 828, in implementation
    ret = func(*bound_args.args, **bound_args.kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<__array_function__ internals>", line 200, in amin
  File "/opt/mamba/envs/HDWX/lib/python3.11/site-packages/numpy/core/fromnumeric.py", line 2946, in amin
    return _wrapreduction(a, np.minimum, 'min', axis, None, out,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/mamba/envs/HDWX/lib/python3.11/site-packages/numpy/core/fromnumeric.py", line 86, in _wrapreduction
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: zero-size array to reduction operation minimum which has no identity

There is a point to this, I promise... I'm searching for the bottom and top of the effective inflow layer of an ACARS profile:
https://github.com/wx4stg/acars-sounding-server/blob/5f63295525aceb5a3ef1d89287a0d5449dc1c40c/soundingPlot.py#L88

This led me to investigate why el_pressure was empty... which led to my fix of just checking "if it's empty use the top pressure available". This (correctly) returns 0 J/kg CAPE and CIN, but internally, uses an LFC of 556 hPa, and if the user asks for the LFC, that's what it returns. I don't see how that's possible, given that is above the provided environmental profile.

>>> mpcalc.lfc(level, temp, dew)
<stdin>:1: UserWarning: Interpolation point out of data bounds encountered
(<Quantity(556.424774, 'hectopascal')>, <Quantity(-5.21169013, 'degree_Celsius')>)

I haven't had time to look into why this happens and probably won't until next week... This might be better suited for an issue instead of a draft PR.

[wx4stg]

I cannot reproduce this now, so I'll close this and if it comes back up I'll do a better job of recording what happened.