BUG: Fix q_vector units w/ default static stability (Fixes #3689)

There's no reason the units should change in this case, just assign
proper units to the default unity value of static stability.

src/metpy/calc/kinematics.py

@@ -1266,15 +1266,16 @@ def inertial_advective_wind(
     broadcast=('u', 'v', 'temperature', 'pressure', 'static_stability', 'parallel_scale',
                'meridional_scale')
 )
-@check_units('[speed]', '[speed]', '[temperature]', '[pressure]', '[length]', '[length]')
+@check_units('[speed]', '[speed]', '[temperature]', '[pressure]', '[length]', '[length]',
+             '[energy] / [mass] / [pressure]**2')
 def q_vector(
     u,
     v,
     temperature,
     pressure,
     dx=None,
     dy=None,
-    static_stability=1,
+    static_stability=None,
     x_dim=-1,
     y_dim=-2,
     *,
@@ -1325,8 +1326,8 @@ def q_vector(
         the size of `u` along the applicable axis. Optional if `xarray.DataArray` with
         latitude/longitude coordinates used as input.
     static_stability : `pint.Quantity`, optional
-        The static stability at the pressure level. Defaults to 1 if not given to calculate
-        the Q-vector without factoring in static stability.
+        The static stability at the pressure level. Defaults to 1 J/ kg / Pa^2 if not given
+        to calculate the Q-vector without factoring in static stability.
     x_dim : int, optional
         Axis number of x dimension. Defaults to -1 (implying [..., Y, X] order). Automatically
         parsed from input if using `xarray.DataArray`.
@@ -1353,6 +1354,9 @@ def q_vector(
     static_stability
 
     """
+    if static_stability is None:
+        static_stability = units.Quantity(1, 'J kg^-1 Pa^-2')
+
     dudx, dudy, dvdx, dvdy = vector_derivative(
         u, v, dx=dx, dy=dy, x_dim=x_dim, y_dim=y_dim,
         parallel_scale=parallel_scale, meridional_scale=meridional_scale)

tests/calc/test_kinematics.py

@@ -1262,12 +1262,12 @@ def test_q_vector_without_static_stability(q_vector_data):
                           [-1.8952185e-13, -2.2269905e-14, -2.2269905e-14, -1.8952185e-13],
                           [-1.9918390e-13, -2.3370829e-14, -2.3370829e-14, -1.9918390e-13],
                           [-5.6160772e-14, -3.5145951e-13, -3.5145951e-13, -5.6160772e-14]])
-                * units('m^2 kg^-1 s^-1'))
+                * units('kg m^-2 s^-3'))
     q2_truth = (np.array([[-4.4976059e-14, -4.3582378e-13, 4.3582378e-13, 4.4976059e-14],
                           [-3.0124244e-13, -3.5724617e-14, 3.5724617e-14, 3.0124244e-13],
                           [3.1216232e-13, 3.6662900e-14, -3.6662900e-14, -3.1216232e-13],
                           [8.6038280e-14, 4.6968342e-13, -4.6968342e-13, -8.6038280e-14]])
-                * units('m^2 kg^-1 s^-1'))
+                * units('kg m^-2 s^-3'))
 
     assert_almost_equal(q1, q1_truth, 16)
     assert_almost_equal(q2, q2_truth, 16)
@@ -1840,7 +1840,7 @@ def test_q_vector_4d(data_4d):
                            [2.50227841e-13, 2.70855069e-13, 4.03362348e-13, 5.22065702e-13],
                            [3.37119836e-13, 3.17667714e-13, 2.25387106e-13, 6.46265259e-13],
                            [2.05548507e-13, 3.55426850e-13, -1.74728156e-14,
-                            5.04028133e-13]]]]) * units('m^2 kg^-1 s^-1')
+                            5.04028133e-13]]]]) * units('kg m^-2 s^-3')
     q2_truth = np.array([[[[3.34318820e-12, -1.32561232e-13, 1.01510711e-12, 6.03331800e-12],
                            [2.51737448e-13, -1.71044158e-13, -8.25290924e-13, 1.68843717e-13],
                            [-3.50533924e-12, -1.68864979e-12, 7.74026063e-13, 1.53811977e-12],
@@ -1884,7 +1884,7 @@ def test_q_vector_4d(data_4d):
                            [-1.39578146e-13, -1.36744814e-13, 3.12352497e-14, 4.55339789e-13],
                            [-1.06614836e-13, -2.19878930e-13, -8.37992151e-14, 1.87868902e-13],
                            [-2.27057581e-13, -2.74474045e-13, -1.10759455e-13,
-                            -3.90242255e-13]]]]) * units('m^2 kg^-1 s^-1')
+                            -3.90242255e-13]]]]) * units('kg m^-2 s^-3')
     assert_array_almost_equal(q1.data, q1_truth, 15)
     assert_array_almost_equal(q2.data, q2_truth, 15)