Plot streamlines on a matplotlib contourf plot

Question

I have a matplotlib contourf plot of longitudes and pressure levels in the vertical. I am trying to plot streamlines on this using the plt.streamplot function in matplotlib and using U and V wind data. If I plot only the streamplot, it works fine. But I cannot get the streamlines to overlay on the contour plot. Here is my code:-

fig, axes = plt.subplots(nrows, ncols, sharex=True, sharey=True)
if (nrows==1 and ncols==1):
    axes=[axes]
else:
    axes=axes.flat
for i, ax in enumerate(axes):
    X,Y = np.meshgrid(x[i],y[i])
    levels=np.arange(vmin,vmax,step)
    h = ax.contourf(X,Y,z[i],cmap=cmap,levels=levels,extend='both')
    w = ax.streamplot(X, Y, W[i], Z[i], linewidth=0.2, color='gray')

And this is the plot I get:

And this is the plot I get

The following is the streamline plot, not sure why the y axis is from 0-120 instead of 0 to 1000:

This is the streamline plot, not sure why the y axis is from 0-120 instead of 0 to 1000

You must be using different coordinates. The coordinates of the streamline plot are 0 to 120 and the coordinates of the contour are 100 to 1000. Hence you see some of the arrows from the streamplot still in the upper part of the contour, i.e the range between 100 and 120 where both coordinate ranges overlap. — ImportanceOfBeingErnest, Feb 14 '19 at 01:41

Serenity · Answer 1 · 2019-02-14T04:45:34.580

You use curvilinear coordinate system for contour plot (lat-p). You have to convert u,v to coordinate system of contour something like here (this is example for lat-lon you have to modify it to use pressure levels):

def myStreamPlot(lon,lat,u,v,color='k',density=2.5):
        from scipy.interpolate import griddata

        n,m = u.shape[1],u.shape[0]
        x = np.linspace(np.nanmin(lon), np.nanmax(lon), n)
        y = np.linspace(np.nanmin(lat), np.nanmax(lat), m)
        xi, yi = np.meshgrid(x,y)

        lon = lon.ravel()
        lat = lat.ravel()
        u   = u.ravel()
        v   = v.ravel()

        gu = griddata(zip(lon,lat), u, (xi,yi))
        gv = griddata(zip(lon,lat), v, (xi,yi))
        gspd = np.sqrt(gu**2 + gv**2)
        SL = plt.streamplot(x,y,gu,gv,linewidth=1.,color=color,density=density)

This code use griddata function of scipy.interpolate: https://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.griddata.html

Plot streamlines on a matplotlib contourf plot

1 Answers1