ENH: Avoid returning negative saturation mixing ratios

This corresponds to being outside the region of liquid phase water
equilibrium, where e_s >= p. Detect this case, warn, and instead return
NaN.

src/metpy/calc/thermo.py

@@ -1748,11 +1748,24 @@ def saturation_mixing_ratio(total_press, temperature):
 
     .. math:: r_s = \epsilon \frac{e_s}{p - e_s}
 
+    By definition, this value is only defined for conditions where the saturation vapor
+    pressure (:math:`e_s`) for the given temperature is less than the given total pressure
+    (:math:`p`). Otherwise, liquid phase water cannot exist in equilibrium and there is only
+    water vapor present. For any value pairs that fall under this condition, the function will
+    warn and return NaN.
+
     .. versionchanged:: 1.0
        Renamed ``tot_press`` parameter to ``total_press``
 
     """
-    return mixing_ratio._nounit(saturation_vapor_pressure._nounit(temperature), total_press)
+    e_s = saturation_vapor_pressure._nounit(temperature)
+    undefined = e_s >= total_press
+    if np.any(undefined):
+        _warnings.warn('Saturation mixing ratio is undefined for some requested pressure/'
+                       'temperature combinations. Total pressure must be greater than the '
+                       'water vapor saturation pressure for liquid water to be in '
+                       'equilibrium.')
+    return np.where(undefined, np.nan, mixing_ratio._nounit(e_s, total_press))
 
 
 @exporter.export

tests/calc/test_thermo.py

@@ -259,6 +259,7 @@ def test_moist_lapse_starting_points(start, direction):
 @pytest.mark.filterwarnings('ignore:overflow encountered in exp:RuntimeWarning')
 @pytest.mark.filterwarnings(r'ignore:invalid value encountered in \w*divide:RuntimeWarning')
 @pytest.mark.filterwarnings(r'ignore:.*Excess accuracy requested.*:UserWarning')
+@pytest.mark.filterwarnings(r'ignore:Saturation mixing ratio is undefined.*:UserWarning')
 def test_moist_lapse_failure():
     """Test moist_lapse under conditions that cause the ODE solver to fail."""
     p = np.logspace(3, -1, 10) * units.hPa
@@ -829,6 +830,14 @@ def test_saturation_mixing_ratio_with_xarray():
     xr.testing.assert_identical(result['x'], temperature['x'])
 
 
+def test_saturation_mixing_ratio_bad_value_handling():
+    """Test that saturation mixing ratio issues a warning and returns nan with bad values."""
+    with pytest.warns(UserWarning, match='undefined'):
+        e_s = saturation_mixing_ratio(10 * units.hPa, 295 * units.kelvin)
+
+    assert np.isnan(e_s)
+
+
 def test_equivalent_potential_temperature():
     """Test equivalent potential temperature calculation."""
     p = 1000 * units.mbar