Check that pressure decreases monotonically in parcel calculations

src/metpy/calc/__init__.py

@@ -5,6 +5,7 @@
 
 from .basic import *  # noqa: F403
 from .cross_sections import *  # noqa: F403
+from .exceptions import *  # noqa: F403
 from .indices import *  # noqa: F403
 from .kinematics import *  # noqa: F403
 from .thermo import *  # noqa: F403

src/metpy/calc/exceptions.py

@@ -0,0 +1,8 @@
+# Copyright (c) 2021 MetPy Developers.
+# Distributed under the terms of the BSD 3-Clause License.
+# SPDX-License-Identifier: BSD-3-Clause
+"""Contains specific exceptions raised by calculations."""
+
+
+class InvalidSoundingError(ValueError):
+    """Raise when a sounding does not meet thermodynamic calculation expectations."""

src/metpy/calc/thermo.py

@@ -10,6 +10,7 @@
 import scipy.optimize as so
 import xarray as xr
 
+from .exceptions import InvalidSoundingError
 from .tools import (_greater_or_close, _less_or_close, _remove_nans, find_bounding_indices,
                     find_intersections, first_derivative, get_layer)
 from .. import constants as mpconsts
@@ -873,13 +874,28 @@ def parcel_profile_with_lcl_as_dataset(pressure, temperature, dewpoint):
     )
 
 
+def _check_pressure(pressure):
+    """Check that pressure decreases monotonically.
+
+    Returns True if the pressure decreases monotonically; otherwise, returns False.
+    """
+    return np.all(pressure[:-1] > pressure[1:])
+
+
 def _parcel_profile_helper(pressure, temperature, dewpoint):
     """Help calculate parcel profiles.
 
     Returns the temperature and pressure, above, below, and including the LCL. The
     other calculation functions decide what to do with the pieces.
 
     """
+    # Check that pressure is monotonic decreasing.
+    if not _check_pressure(pressure):
+        msg = """
+        Pressure does not decrease monotonically in your sounding.
+        Using scipy.signal.medfilt may fix this."""
+        raise InvalidSoundingError(msg)
+
     # Find the LCL
     press_lcl, temp_lcl = lcl(pressure[0], temperature, dewpoint)
     press_lcl = press_lcl.to(pressure.units)

tests/calc/test_thermo.py

@@ -11,10 +11,10 @@
                         brunt_vaisala_period, cape_cin, density, dewpoint,
                         dewpoint_from_relative_humidity, dewpoint_from_specific_humidity,
                         dry_lapse, dry_static_energy, el, equivalent_potential_temperature,
-                        exner_function, gradient_richardson_number, isentropic_interpolation,
-                        isentropic_interpolation_as_dataset, lcl, lfc, lifted_index,
-                        mixed_layer, mixed_layer_cape_cin, mixed_parcel, mixing_ratio,
-                        mixing_ratio_from_relative_humidity,
+                        exner_function, gradient_richardson_number, InvalidSoundingError,
+                        isentropic_interpolation, isentropic_interpolation_as_dataset, lcl,
+                        lfc, lifted_index, mixed_layer, mixed_layer_cape_cin, mixed_parcel,
+                        mixing_ratio, mixing_ratio_from_relative_humidity,
                         mixing_ratio_from_specific_humidity, moist_lapse, moist_static_energy,
                         most_unstable_cape_cin, most_unstable_parcel, parcel_profile,
                         parcel_profile_with_lcl, parcel_profile_with_lcl_as_dataset,
@@ -190,6 +190,16 @@ def test_parcel_profile_lcl():
     assert_array_almost_equal(prof, true_prof, 2)
 
 
+def test_parcel_profile_lcl_not_monotonic():
+    """Test parcel profile with lcl calculation."""
+    with pytest.raises(InvalidSoundingError):
+        p = np.array([1004., 1000., 943., 925., 928., 850., 839., 749., 700.]) * units.hPa
+        t = np.array([24.2, 24., 20.2, 21.6, 21.4, 20.4, 20.2, 14.4, 13.2]) * units.degC
+        td = np.array([21.9, 22.1, 19.2, 20.5, 20.4, 18.4, 17.4, 8.4, -2.8]) * units.degC
+
+        _ = parcel_profile_with_lcl(p, t, td)
+
+
 def test_parcel_profile_with_lcl_as_dataset():
     """Test parcel profile with lcl calculation with xarray."""
     p = np.array([1004., 1000., 943., 928., 925., 850., 839., 749., 700., 699.]) * units.hPa

tutorials/xarray_tutorial.py

@@ -260,21 +260,24 @@
 #########################################################################
 # For profile-based calculations (and most remaining calculations in the ``metpy.calc``
 # module), xarray ``DataArray``\s are accepted as inputs, but the outputs remain Pint
-# Quantities (typically scalars)
+# Quantities (typically scalars). Note that MetPy's profile calculations (such as CAPE and
+# CIN) require the sounding to be ordered from highest to lowest pressure. As seen earlier
+# in this tutorial, this data is ordered the other way, so we need to reverse the inputs
+# to ``mpcalc.surface_based_cape_cin``.
 
 data_at_point = data.metpy.sel(
     time1='2017-09-05 12:00',
-    latitude=40 * units.degrees_north,
+    latitude=30 * units.degrees_north,
     longitude=260 * units.degrees_east
 )
 dewpoint = mpcalc.dewpoint_from_relative_humidity(
     data_at_point['Temperature_isobaric'],
     data_at_point['Relative_humidity_isobaric']
 )
 cape, cin = mpcalc.surface_based_cape_cin(
-    data_at_point['isobaric3'],
-    data_at_point['Temperature_isobaric'],
-    dewpoint
+    data_at_point['isobaric3'][::-1],
+    data_at_point['Temperature_isobaric'][::-1],
+    dewpoint[::-1]
 )
 cape