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Fix Q Vector example stability and scaling (backport) #3878

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Merged
dopplershift merged 1 commit into from
Jul 23, 2025
Merged

Fix Q Vector example stability and scaling (backport) #3878

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18 changes: 13 additions & 5 deletions examples/calculations/QVector.py
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
Dataset and plotting using Matplotlib.
"""
import matplotlib.pyplot as plt
import numpy as np

import metpy.calc as mpcalc
from metpy.cbook import example_data
Expand All @@ -26,7 +27,7 @@
# Calculate the q-vectors. Passing in a fixed value of static stability, but could also
# use `mpcalc.static_stability()`.
u_qvect, v_qvect = mpcalc.q_vector(ds.uwind, ds.vwind, ds.temperature, 850 * units.hPa,
static_stability=0.02 * units('J / kg / Pa^2'))
static_stability=2e-6 * units('J / kg / Pa^2'))

# start figure and set axis
fig, ax = plt.subplots(figsize=(5, 5))
Expand All @@ -41,15 +42,22 @@
cmap=plt.cm.bwr, alpha=0.75)
plt.colorbar(cf, pad=0, aspect=50)

# calculate a scale length quantity of our vectors based on their mean
scale = (np.hypot(u_qvect, v_qvect).mean() * np.sqrt(u_qvect.size)).data

# plot Q-vectors as arrows, every other arrow
qvec = ax.quiver(ds.lon.values[::2], ds.lat.values[::2],
u_qvect[::2, ::2] * 1e13, v_qvect[::2, ::2] * 1e13,
color='black', scale=1000, alpha=0.5, width=0.01)
u_qvect[::2, ::2], v_qvect[::2, ::2],
color='black', scale=scale.m, alpha=0.5, width=0.01)

# calculate representative arrow for key
key = scale / 10

qk = ax.quiverkey(qvec, 0.8, 0.9, 200, r'$200 m^2/kg/s$', labelpos='E',
# add key for arrow length
qk = ax.quiverkey(qvec, 0.65, 0.905, key.m, f'{key:0.2~P}', labelpos='E',
coordinates='figure')

ax.set(xlim=(260, 270), ylim=(30, 40))
ax.set_title('Q-Vector Calculation')
ax.set_title('Q-Vector Calculation', loc='left')

plt.show()
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