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Fix q_vector units w/ default static stability #3690

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Fix q_vector units w/ default static stability #3690

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12 changes: 8 additions & 4 deletions src/metpy/calc/kinematics.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
*,
Expand Down Expand Up @@ -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.
Comment on lines +1329 to +1330
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With the insight of @kgoebber's comment here, I think it would be helpful to include a bit more detail here (or elsewhere in the docstring) on how static stability is important for this calculation, and perhaps recommend, when possible, to not rely on this default?

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I think that's reasonable, and I'm also inclined to at least pick a default that has a sensible scale, even if it does change results.

x_dim : int, optional
Axis number of x dimension. Defaults to -1 (implying [..., Y, X] order). Automatically
parsed from input if using `xarray.DataArray`.
Expand All @@ -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)
Expand Down
8 changes: 4 additions & 4 deletions tests/calc/test_kinematics.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand Down Expand Up @@ -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],
Expand Down Expand Up @@ -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)

Expand Down
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