Check that pressure decreases monotonically in parcel calculations

This commit addresses #1339 using ideas from #1654.  A new exception
is added on the theory that parcel calculations are complicated, so
there may be other problems with input soundings (extreme
supersaturation?) that might want to be flagged in the future.

src/metpy/calc/exceptions.py

@@ -0,0 +1,2 @@
+class InvalidSoundingError(ValueError):
+    """Raise when a sounding does not meet thermodynamic calculation expectations."""

src/metpy/calc/thermo.py

@@ -12,6 +12,7 @@
 
 from .tools import (_greater_or_close, _less_or_close, _remove_nans, find_bounding_indices,
                     find_intersections, first_derivative, get_layer)
+from .exceptions import InvalidSoundingError
 from .. import constants as mpconsts
 from ..cbook import broadcast_indices
 from ..interpolate.one_dimension import interpolate_1d
@@ -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

@@ -31,6 +31,7 @@
                         vertical_velocity, vertical_velocity_pressure,
                         virtual_potential_temperature, virtual_temperature,
                         wet_bulb_temperature)
+from metpy.calc.exceptions import InvalidSoundingError
 from metpy.calc.thermo import _find_append_zero_crossings
 from metpy.testing import assert_almost_equal, assert_array_almost_equal, assert_nan
 from metpy.units import masked_array, units
@@ -190,6 +191,16 @@ 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
+
+        pressure, temp, dewp, prof = 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