Wind vector rotation troubles (rotate_vector)

Personne n'a encore pris cette issue.

I am currently working with a gridded data set that has its wind vectors already in an earth-relative frame. When looking to rotate the wind vectors appropriately for plotting on a Basemap projection I've run into a strange problem where the vectors, quite simply, look rather odd. I ran a few tests with simple test points to make sure the `rotate_vector` routine appeared to be working correctly, and it seemed fine. But when I run a whole model output grid through the `rotate_vector` routine, it seems to produce wind fields which don't look quite right (see below).


Here are some stats associated with the point that has the red star on the plot with the title 'Rotated vectors'

```
Lat and Lon: 39.11, -70.0144

Original U and V: 1.30, 7.14
Rotated U and V: -1.33 , 7.13
```

Now note how the rotated wind at this point is aligned along the -70 meridian, which would suggest that in earth relative terms we might expect the wind components to be more along the lines of u=0 and v=7. However, judging from above that's certainly not the case.

Now, if I rotate that same vector, _by itself_, I get the following rotated wind:

`Rotated U and V: -0.03 7.26`

And see the second plot for a visual:


It would seem desirable to have the same behavior for the single point that we have for the gridded set of winds.

The only thing special about the gridded data, that I have noticed, is that the latitudes can vary in a non-standard way with increasing x dimension, west to east (e.g. 35.5N, 34.2N, 33.1N, 36.2N, etc.). It wasn't clear if this was okay within `rotate_vector`. In some simple tests it didn't seem to be a problem.

It would be nice to be able to use this routine for not only a set of gridded data, but observations as well. It's worth noting that observations wouldn't necessarily be ordered in a nice, regular way given the nature that observations tend to be irregularly spaced (e.g. surface stations co-located with airports).

Unfortunately I don't have any suggestions for a solution, but am hopeful others may have an idea.

Below is the snippet of code used to generate the first plot above.

Thanks!
Jacob

```
import nemsio
import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

# Get the input file
f='restart_file'
nio=nemsio.nemsfile(f)

skip=25
u10=u[::skip,::skip,0]
v10=v[::skip,::skip,0]
lats=nio.lats[::skip,::skip]
lons=nio.lons[::skip,::skip]

# Create the figure
fig=plt.figure(figsize=(18, 6))

# Domain covers NE CONUS
llcrnrlon=-84.0
llcrnrlat=35.0
urcrnrlon=-60.0
urcrnrlat=49.0
res='l'

m = Basemap(llcrnrlon=llcrnrlon,llcrnrlat=llcrnrlat,urcrnrlon=urcrnrlon,urcrnrlat=urcrnrlat,\
rsphere=(6378137.00,6356752.3142),\
resolution=res,projection='lcc',\
lat_1=25.0,lon_0=-95.0)

#The vector rotation to the Basemap projection just specified
u10_rot, v10_rot, x, y = m.rotate_vector(u10, v10, lons, lats, returnxy=True)

parallels = np.arange(-80.,90,5.)
meridians = np.arange(0.,360.,5.)

# - First sublot is without rotation
ax = fig.add_subplot(121)
ax.set_title('Without rotation')

m.drawmapboundary(fill_color='aqua')
m.fillcontinents(color='#cc9955', lake_color='aqua', zorder = 0)
m.drawcoastlines(color = '0.15')

m.drawparallels(parallels)
m.drawmeridians(meridians)

m.barbs(x, y, u10, v10, pivot='middle', barbcolor='black',zorder=10)

# - Second subplot is with rotation

ax = fig.add_subplot(122)
ax.set_title('Rotated vectors')

m.drawmapboundary(fill_color='aqua')
m.fillcontinents(color='#cc9955', lake_color='aqua', zorder = 0)
m.drawcoastlines(color = '0.15')
m.drawparallels(parallels)
m.drawmeridians(meridians)
m.barbs(x, y, u10_rot, v10_rot,
pivot='middle', barbcolor='black',zorder=10)

m.scatter(-70.0144,39.11,s=175,color='red',marker='*',latlon=True)
```

Commencez par reproduire l’exemple Basemap fourni et comparez les résultats de m.rotate_vector pour le point unique et les tableaux maillés découpés. Étudiez la manière dont la routine gère la grille de latitudes bidimensionnelle non standard et les coordonnées d’observation irrégulières ; le travail est considéré comme terminé lorsque le comportement de la rotation est cohérent ou que les contraintes de coordonnées prises en charge sont clairement établies.

Rédigé par le modèle d'indexation à partir du texte de l'issue.