Compute the LCL from actual temperature in lfc/el used by cape_cin

[gaoflow]

Description

Fixes #3857.

cape_cin applies the virtual temperature correction by replacing the environmental
and parcel temperature profiles with their virtual temperatures before calling lfc
and el:

temperature = virtual_temperature_from_dewpoint(pressure, temperature, dewpoint)
parcel_profile = virtual_temperature(parcel_profile, parcel_mixing_ratio)
lfc_pressure, _ = lfc(pressure, temperature, dewpoint, parcel_temperature_profile=parcel_profile, ...)
el_pressure, _ = el(pressure, temperature, dewpoint, parcel_temperature_profile=parcel_profile, ...)

lfc and el then recompute the LCL from those arrays — lfc from
parcel_temperature_profile[0], el from temperature[0] — i.e. they treat the
virtual temperature as if it were the actual temperature. Because virtual temperature
is always ≥ temperature, the LCL comes out at too low a pressure (too high an
altitude). Since the LFC and EL are floored at the LCL, both get biased upward; the
effect is strongest on the LFC, exactly as described in the issue.

Fix

Give lfc and el optional lcl_pressure (and, for lfc, lcl_temperature)
keywords so the caller can supply an LCL computed from the actual temperatures. When
the keywords are omitted the LCL is computed from the inputs exactly as before, so
direct callers and the public API are unchanged.

cape_cin already computes the LCL from the actual temperatures (for its below_lcl
mixing-ratio logic) before the virtual-temperature substitution, so it simply passes
that LCL through to lfc/el.

Effect

For a sounding whose LFC is floored at the LCL, the actual-temperature LCL sits about
30–45 hPa higher in pressure than the virtual one, so the LFC (and the CAPE integration
that starts there) is corrected accordingly. For example, on the profile in
test_cape_cin_value_error the surface-based CAPE goes from the previously-reported
2161.9 J/kg to 2206.7 J/kg once the LFC is floored at the correct LCL.

Tests

• New test_lfc_el_lcl_from_actual_temperature: reconstructs the virtual-temperature
  profiles cape_cin builds and shows that, without the override, lfc floors at the
  spuriously-low virtual LCL, while supplying the actual-temperature LCL floors it at
  the correct LCL (and that el honors the keyword).
• Updated the expected CAPE in test_cape_cin_value_error (whose LFC is floored at the
  LCL) with a comment explaining the change.

The full tests/calc/test_thermo.py suite passes (228 tests); ruff is clean.

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

This PR fixes a CAPE/CIN bias introduced when cape_cin substitutes virtual temperatures before calling lfc/el, causing those functions to recompute the LCL using virtual temperatures as though they were actual temperatures (GH #3857).

Changes:

• Add optional lcl_pressure (and lcl_temperature for lfc) parameters so callers can supply an LCL computed from actual temperatures.
• Update cape_cin to compute the LCL before virtual-temperature substitution and pass it through to lfc/el.
• Update and add tests to validate corrected CAPE expectations and the new LCL override behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/metpy/calc/thermo.py	Adds LCL override kwargs to lfc/el and wires actual-temperature LCL through cape_cin to prevent virtual-temperature LCL bias.
tests/calc/test_thermo.py	Updates CAPE expectation and adds regression coverage around LCL recomputation behavior.

---

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[gaoflow]

Addressed both review notes: lfc now raises if only one of lcl_pressure/lcl_temperature is supplied (the docstring already required both), and the el test now proves the keyword is honored — a LCL above the EL forces NaN, so ignoring it would fail the test.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/metpy/calc/thermo.py:972

• lfc documents that lcl_pressure must be given together with lcl_temperature, but the implementation does not enforce this. If a caller supplies only lcl_pressure, the function can return (lcl_pressure, None) when the LFC is floored at the LCL, breaking the documented return type and potentially causing downstream failures. Add an explicit validation that either both are provided or neither is.

    if (lcl_pressure is None) != (lcl_temperature is None):
        raise ValueError('lcl_pressure and lcl_temperature must be provided together')
    if lcl_pressure is None:
        this_lcl = lcl(pressure[0], parcel_temperature_profile[0], dewpoint_start)

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (3)

src/metpy/calc/thermo.py:1090

• el introduces a new lcl_pressure keyword but it isn’t validated by @check_units, so passing an incorrect type/dimensionality will raise less-informative errors at comparison time. Add a named unit check for lcl_pressure.

@check_units('[pressure]', '[temperature]', '[temperature]', '[temperature]')
def el(pressure, temperature, dewpoint, parcel_temperature_profile=None, which='top',
       lcl_pressure=None):

src/metpy/calc/thermo.py:974

• When the caller supplies lcl_pressure/lcl_temperature, lfc returns them verbatim if the LFC is floored at the LCL. If they’re provided in different units than the input pressure/temperature, the return units become inconsistent with the rest of the function (and with the default code path where lcl() is called with input units). Coerce supplied LCL values to pressure.units/temperature.units before using/returning them.

    if lcl_pressure is None:
        this_lcl = lcl(pressure[0], parcel_temperature_profile[0], dewpoint_start)
    else:
        this_lcl = (lcl_pressure, lcl_temperature)

src/metpy/calc/thermo.py:850

• New keyword arguments lcl_pressure/lcl_temperature on lfc aren’t covered by @check_units, so callers can pass unitless or wrong-dimensionality values and get confusing runtime failures later. Add named checks for these new parameters to keep behavior consistent with the rest of MetPy’s unit-validated API.

This issue also appears in the following locations of the same file:

• line 971
• line 1088

@check_units('[pressure]', '[temperature]', '[temperature]', '[temperature]')
def lfc(pressure, temperature, dewpoint, parcel_temperature_profile=None, dewpoint_start=None,
        which='top', lcl_pressure=None, lcl_temperature=None):