Plotting utilities library

This library file contains six functions:

quantile_normalise_t2m:
Converts a cube of T2m in kelvin to a cube of approximately on the range 0-1. If the cube is on the conventional equiangular projectionj (pole at 90N), then the cube will have an approximately flat distribution (each value occurs the same number of times. This is convenient for colourmap plotting.
plot_cube:
Generate an iris cube with the given resolution, range and pole location (data are all zero). Most useful as a common arrangement to regrid other cubes onto.
wind_field:
Take a field of white noise, and advect it along with a wind field. This turns the speckles into stripes in the direction of the wind. They can be made to move from frame to frame by advecting most them a little more each timestep (and resetting a few of them to zero advection)
make_wind_seed:
Make the field of white noise needed by the ‘wind_field’ function.
draw_lat_lon:
Draw lines of latitude and longitude (copes with rotated poles).
get_precip_colours:
Make a colour map for plotting precip. Uses the ‘algae’ colour scale from cmocean, adjusted to taper to transparency at the bottom end
# Utility functions used in various plots - particularly multi-variate
#  reanalysis videos.

import iris
import numpy
import matplotlib
from matplotlib.lines import Line2D

# Normalise a T2m field so it is approximately uniformly distributed
#  over the range 0-1 (when projected in a conventional equirectangular
#  projection). (If you then plot a colour map of the normalised data,
#  there will be an approximately equal amount of each colour).
# Takes an iris cube as input and returns one as output
def quantile_normalise_t2m(p):
   res=p.copy()
   res.data[res.data>300.10]=0.95
   res.data[res.data>299.9]=0.90
   res.data[res.data>298.9]=0.85
   res.data[res.data>297.5]=0.80
   res.data[res.data>295.7]=0.75
   res.data[res.data>293.5]=0.70
   res.data[res.data>290.1]=0.65
   res.data[res.data>287.6]=0.60
   res.data[res.data>283.7]=0.55
   res.data[res.data>280.2]=0.50
   res.data[res.data>277.2]=0.45
   res.data[res.data>274.4]=0.40
   res.data[res.data>272.3]=0.35
   res.data[res.data>268.3]=0.30
   res.data[res.data>261.4]=0.25
   res.data[res.data>254.6]=0.20
   res.data[res.data>249.1]=0.15
   res.data[res.data>244.9]=0.10
   res.data[res.data>240.5]=0.05
   res.data[res.data>0.95]=0.0
   return res

# Make a dummy iris Cube for plotting.
# Makes a cube in equirectangular projection.
# Takes resolution, plot range, and pole location
#  (all in degrees) as arguments, returns an
#  iris cube.
def plot_cube(resolution,xmin=-180,xmax=180,
                         ymin=-90,ymax=90,
                         pole_latitude=90,
                         pole_longitude=180,
                         npg_longitude=0):

    cs=iris.coord_systems.RotatedGeogCS(pole_latitude,
                                        pole_longitude,
                                        npg_longitude)
    lat_values=numpy.arange(ymin,ymax+resolution,resolution)
    latitude = iris.coords.DimCoord(lat_values,
                                    standard_name='latitude',
                                    units='degrees_north',
                                    coord_system=cs)
    lon_values=numpy.arange(xmin,xmax+resolution,resolution)
    longitude = iris.coords.DimCoord(lon_values,
                                     standard_name='longitude',
                                     units='degrees_east',
                                     coord_system=cs)
    dummy_data = numpy.zeros((len(lat_values), len(lon_values)))
    plot_cube = iris.cube.Cube(dummy_data,
                               dim_coords_and_dims=[(latitude, 0),
                                                    (longitude, 1)])
    return plot_cube

# To plot wind vectors, take a field of white noise and advect it
#  along with the wind, integrating it as you go.
#  Then plot this field as a colour map
#  (or as a perturbation to the temperature colour map).
# If we need to make the vectors move between timesteps (for a 
#  video), offset the start and end point of the integration by
#  a sequence number that can be increased from frame to frame. 
#  (Also - use the same noise field for each frame in the video).
# This function returns the integrated noise field (as iris cube).
def wind_field(uw, # E-W wind speed (iris cube).
               vw, # N-S wind speed (iris cube - same grid as uw)
               z,  # Noise field (iris cube)
               sequence=None, # Set to no.of frame for video effect
               iterations=50, # Bigger = longer vectors
               epsilon=0.003, # Biger = longer vectors
               sscale=1):     # Bigger = bigger colour contrast
    z=z.regrid(uw,iris.analysis.Linear())
    (width,height)=z.data.shape
    # Each point in this field has an index location (i,j)
    #  and a real (x,y) position
    xmin=numpy.min(uw.coords()[0].points)
    xmax=numpy.max(uw.coords()[0].points)
    ymin=numpy.min(uw.coords()[1].points)
    ymax=numpy.max(uw.coords()[1].points)
    # Convert between index and real positions
    def i_to_x(i):
        return xmin + (i/width) * (xmax-xmin)
    def j_to_y(j):
        return ymin + (j/height) * (ymax-ymin)
    def x_to_i(x):
        return numpy.minimum(width-1,numpy.maximum(0, 
                numpy.floor((x-xmin)/(xmax-xmin)*(width-1)))).astype(int)
    def y_to_j(y):
        return numpy.minimum(height-1,numpy.maximum(0, 
                numpy.floor((y-ymin)/(ymax-ymin)*(height-1)))).astype(int)
    i,j=numpy.mgrid[0:width,0:height]
    x=i_to_x(i)
    y=j_to_y(j)
    # Result is a distorted version of the random field
    result=z.copy()
    # Repeatedly, move the x,y points according to the vector field
    #  and update result with the random field at their locations
    ss=uw.copy()
    ss.data=numpy.sqrt(uw.data**2+vw.data**2)
    if sequence is not None:
        startsi=numpy.arange(0,iterations,3)
        endpoints=numpy.tile(startsi,1+(width*height)//len(startsi))
        endpoints += sequence%iterations
        endpoints[endpoints>=iterations] -= iterations
        startpoints=endpoints-25
        startpoints[startpoints<0] += iterations
        endpoints=endpoints[0:(width*height)].reshape(width,height)
        startpoints=startpoints[0:(width*height)].reshape(width,height)
    else:
        endpoints=iterations+1 
        startpoints=-1       
    for k in range(iterations):
        x += epsilon*vw.data[i,j]
        x[x>xmax]=xmax
        x[x<xmin]=xmin
        y += epsilon*uw.data[i,j]
        y[y>ymax]=y[y>ymax]-ymax+ymin
        y[y<ymin]=y[y<ymin]-ymin+ymax
        i=x_to_i(x)
        j=y_to_j(y)
        update=z.data*ss.data/sscale
        update[(endpoints>startpoints) & ((k>endpoints) | (k<startpoints))]=0
        update[(startpoints>endpoints) & ((k>endpoints) & (k<startpoints))]=0
        result.data[i,j] += update
    return result

# Draw lines of latitude and longitude
def draw_lat_lon(ax,lwd=0.75,
                    pole_longitude=180,
                    pole_latitude=90,
                    npg_longitude=0,
                    zorder=10):
    for lat in range(-90,95,5):
        x=[]
        y=[]
        for lon in range(-180,181,1):
            rp=iris.analysis.cartography.rotate_pole(numpy.array(lon),
                                                     numpy.array(lat),
                                                     pole_longitude,
                                                     pole_latitude)
            nx=rp[0]+npg_longitude
            if nx>180: nx -= 360
            ny=rp[1]
            if(len(x)==0 or (abs(nx-x[-1])<10 and abs(ny-y[-1])<10)):
                x.append(nx)
                y.append(ny)
            else:
                ax.add_line(Line2D(x, y, linewidth=lwd, color=(0.4,0.4,0.4,1),
                                   zorder=zorder))
                x=[]
                y=[]
        if(len(x)>1):        
            ax.add_line(Line2D(x, y, linewidth=lwd, color=(0.4,0.4,0.4,1),
                               zorder=zorder))

    for lon in range(-180,185,5):
        x=[]
        y=[]
        for lat in range(-90,90,1):
            rp=iris.analysis.cartography.rotate_pole(numpy.array(lon),
                                                     numpy.array(lat),
                                                     pole_longitude,
                                                     pole_latitude)
            nx=rp[0]+npg_longitude
            if nx>180: nx -= 360
            ny=rp[1]
            if(len(x)==0 or (abs(nx-x[-1])<10 and abs(ny-y[-1])<10)):
                x.append(nx)
                y.append(ny)
            else:
                ax.add_line(Line2D(x, y, linewidth=lwd, color=(0.4,0.4,0.4,1),
                                   zorder=zorder))
                x=[]
                y=[]
        if(len(x)>1):        
            ax.add_line(Line2D(x, y, linewidth=lwd, color=(0.4,0.4,0.4,1),
                               zorder=zorder))

# Colour map for precip - algae from cmocean but with taper in alpha
def get_precip_colours():
    cols=[]
    for ci in range(200):
        cols.append([0.06885643,0.14208946,0.07903363,0.0])
    for ci in range(200):
        cols.append([0.06885643,0.14208946,0.07903363,ci/200])
    cm_data = [[ 0.06885643, 0.14208946, 0.07903363],
               [ 0.07022733, 0.145481  , 0.08182252],
               [ 0.07158041, 0.14886551, 0.08459735],
               [ 0.07289843, 0.15224692, 0.08735815],
               [ 0.07419261, 0.15562329, 0.09010536],
               [ 0.07545752, 0.15899606, 0.0928391 ],
               [ 0.07669294, 0.16236561, 0.09555954],
               [ 0.0779044 , 0.16573113, 0.09826708],
               [ 0.07908113, 0.16909506, 0.10096151],
               [ 0.08023865, 0.17245461, 0.10364358],
               [ 0.0813567 , 0.17581401, 0.1063126 ],
               [ 0.08245885, 0.17916893, 0.10896978],
               [ 0.08351901, 0.18252464, 0.11161396],
               [ 0.08456274, 0.18587646, 0.1142466 ],
               [ 0.08556724, 0.18922894, 0.11686652],
               [ 0.08655243, 0.19257854, 0.119475  ],
               [ 0.08750046, 0.19592877, 0.12207102],
               [ 0.08842694, 0.19927694, 0.12465567],
               [ 0.08931759, 0.20262583, 0.12722804],
               [ 0.09018515, 0.2059733 , 0.1297891 ],
               [ 0.09101742, 0.20932169, 0.13233799],
               [ 0.09182583, 0.21266913, 0.13487564],
               [ 0.09259864, 0.21601782, 0.13740115],
               [ 0.09334765, 0.21936582, 0.13991552],
               [ 0.09405985, 0.22271554, 0.14241766],
               [ 0.09474918, 0.22606468, 0.14490881],
               [ 0.09539954, 0.22941612, 0.14738752],
               [ 0.0960289 , 0.23276689, 0.14985548],
               [ 0.09661612, 0.23612068, 0.15231064],
               [ 0.09718523, 0.23947356, 0.15475537],
               [ 0.09770794, 0.2428303 , 0.1571868 ],
               [ 0.09821309, 0.24618625, 0.15960787],
               [ 0.09867327, 0.24954595, 0.16201567],
               [ 0.09911166, 0.25290573, 0.16441267],
               [ 0.09951033, 0.25626852, 0.16679686],
               [ 0.09988043, 0.25963262, 0.16916945],
               [ 0.1002173 , 0.26299882, 0.17152986],
               [ 0.10051752, 0.2663677 , 0.17387765],
               [ 0.10079233, 0.2697376 , 0.17621409],
               [ 0.10102103, 0.27311168, 0.17853665],
               [ 0.10122817, 0.27648631, 0.18084823],
               [ 0.10138903, 0.2798652 , 0.18314573],
               [ 0.10152195, 0.28324567, 0.18543137],
               [ 0.10161961, 0.28662882, 0.18770415],
               [ 0.10167654, 0.29001546, 0.18996328],
               [ 0.10170933, 0.29340326, 0.19221085],
               [ 0.10169001, 0.29679616, 0.19444309],
               [ 0.1016439 , 0.30019064, 0.19666326],
               [ 0.10156046, 0.30358817, 0.19886988],
               [ 0.10143361, 0.30698958, 0.201062  ],
               [ 0.10127976, 0.31039267, 0.20324181],
               [ 0.1010766 , 0.31380039, 0.20540608],
               [ 0.10083896, 0.31721078, 0.20755678],
               [ 0.10057112, 0.32062332, 0.20969446],
               [ 0.10024781, 0.32404118, 0.21181533],
               [ 0.09989443, 0.32746115, 0.21392297],
               [ 0.0995047 , 0.33088402, 0.21601636],
               [ 0.09906254, 0.3343117 , 0.218093  ],
               [ 0.0985884 , 0.33774172, 0.2201558 ],
               [ 0.09807546, 0.34117488, 0.22220362],
               [ 0.09750943, 0.34461278, 0.22423417],
               [ 0.09690948, 0.3480532 , 0.22625023],
               [ 0.09627239, 0.35149649, 0.22825119],
               [ 0.09557752, 0.35494489, 0.23023372],
               [ 0.09484678, 0.35839595, 0.23220107],
               [ 0.09407947, 0.36184972, 0.23415298],
               [ 0.09325665, 0.36530817, 0.23608636],
               [ 0.09239033, 0.36876996, 0.23800293],
               [ 0.09148554, 0.37223456, 0.23990332],
               [ 0.09053798, 0.37570238, 0.2417867 ],
               [ 0.08953167, 0.3791749 , 0.24365037],
               [ 0.08848513, 0.38265026, 0.2454971 ],
               [ 0.08739776, 0.3861285 , 0.24732662],
               [ 0.08626466, 0.38961004, 0.24913796],
               [ 0.08507286, 0.39309597, 0.25092886],
               [ 0.08383877, 0.39658475, 0.25270175],
               [ 0.0825619 , 0.40007639, 0.25445635],
               [ 0.08124183, 0.4035709 , 0.25619236],
               [ 0.07986463, 0.40706939, 0.25790734],
               [ 0.07843839, 0.41057109, 0.25960234],
               [ 0.07696833, 0.41407555, 0.26127794],
               [ 0.07545435, 0.41758275, 0.26293384],
               [ 0.07389642, 0.42109266, 0.26456976],
               [ 0.07228966, 0.42460561, 0.26618462],
               [ 0.07062863, 0.42812193, 0.26777721],
               [ 0.06892492, 0.43164076, 0.26934902],
               [ 0.06717918, 0.43516204, 0.27089974],
               [ 0.06539233, 0.43868571, 0.27242909],
               [ 0.06356557, 0.44221171, 0.27393677],
               [ 0.06170041, 0.44573995, 0.27542249],
               [ 0.05979877, 0.44927035, 0.27688596],
               [ 0.05785702, 0.45280318, 0.27832599],
               [ 0.05588066, 0.45633814, 0.27974269],
               [ 0.05387746, 0.45987489, 0.28113643],
               [ 0.05185174, 0.4634133 , 0.28250695],
               [ 0.04980869, 0.46695324, 0.28385397],
               [ 0.04775453, 0.47049459, 0.28517723],
               [ 0.04569662, 0.47403719, 0.28647649],
               [ 0.04364369, 0.47758089, 0.28775151],
               [ 0.04160606, 0.48112551, 0.28900203],
               [ 0.0395867 , 0.48467089, 0.29022786],
               [ 0.03763814, 0.48821682, 0.29142876],
               [ 0.03580155, 0.49176311, 0.29260454],
               [ 0.03408899, 0.49530953, 0.29375502],
               [ 0.03251323, 0.49885586, 0.29488003],
               [ 0.03108771, 0.50240186, 0.29597942],
               [ 0.02982666, 0.50594726, 0.29705307],
               [ 0.02874508, 0.50949179, 0.29810087],
               [ 0.02785177, 0.51303546, 0.29912154],
               [ 0.02716441, 0.51657792, 0.3001151 ],
               [ 0.02670582, 0.52011863, 0.30108235],
               [ 0.02649479, 0.52365722, 0.30202332],
               [ 0.02655111, 0.52719334, 0.30293807],
               [ 0.0268957 , 0.53072659, 0.30382669],
               [ 0.02754542, 0.53425678, 0.30468824],
               [ 0.02851433, 0.53778386, 0.30552072],
               [ 0.02984163, 0.54130675, 0.30632735],
               [ 0.03155357, 0.54482496, 0.30710846],
               [ 0.03367174, 0.54833822, 0.3078629 ],
               [ 0.03621581, 0.55184641, 0.30858828],
               [ 0.03923368, 0.55534819, 0.3092893 ],
               [ 0.04267874, 0.55884313, 0.30996532],
               [ 0.04642777, 0.56233118, 0.31061247],
               [ 0.05049037, 0.56581074, 0.31123743],
               [ 0.05483677, 0.56928169, 0.31183596],
               [ 0.05945722, 0.57274288, 0.31241135],
               [ 0.06433628, 0.57619346, 0.3129641 ],
               [ 0.0694603 , 0.57963256, 0.31349439],
               [ 0.07481987, 0.58305904, 0.31400453],
               [ 0.08040588, 0.58647176, 0.31449636],
               [ 0.08620977, 0.58986967, 0.3149695 ],
               [ 0.09222724, 0.59325117, 0.31543058],
               [ 0.0984523 , 0.59661516, 0.31587743],
               [ 0.10488194, 0.59995997, 0.31631578],
               [ 0.11151261, 0.60328396, 0.31675059],
               [ 0.11834151, 0.60658549, 0.31718585],
               [ 0.12536778, 0.6098628 , 0.31762468],
               [ 0.13258816, 0.61311399, 0.31807501],
               [ 0.13999895, 0.6163372 , 0.31854464],
               [ 0.14759587, 0.61953055, 0.31904185],
               [ 0.15537304, 0.62269226, 0.31957617],
               [ 0.16332236, 0.62582066, 0.32015839],
               [ 0.17143307, 0.62891434, 0.32080039],
               [ 0.17969137, 0.63197223, 0.32151494],
               [ 0.18808025, 0.63499369, 0.32231537],
               [ 0.19658051, 0.63797853, 0.32321469],
               [ 0.20516717, 0.6409274 , 0.32422681],
               [ 0.21381461, 0.64384147, 0.32536378],
               [ 0.22249597, 0.64672254, 0.32663605],
               [ 0.23118418, 0.64957297, 0.32805203],
               [ 0.23985293, 0.65239558, 0.32961779],
               [ 0.24847602, 0.65519373, 0.33133728],
               [ 0.25703342, 0.65797057, 0.33321113],
               [ 0.26550717, 0.66072943, 0.3352381 ],
               [ 0.2738829 , 0.66347352, 0.33741504],
               [ 0.28214978, 0.66620584, 0.33973735],
               [ 0.29030037, 0.66892916, 0.34219931],
               [ 0.29833024, 0.67164589, 0.34479452],
               [ 0.30623756, 0.67435818, 0.34751622],
               [ 0.31401727, 0.67706862, 0.35035755],
               [ 0.32167446, 0.67977832, 0.35331149],
               [ 0.32921313, 0.68248841, 0.35637146],
               [ 0.33663762, 0.6851999 , 0.35953132],
               [ 0.34394207, 0.68791541, 0.36278437],
               [ 0.35114051, 0.69063416, 0.36612568],
               [ 0.35823533, 0.69335721, 0.36954994],
               [ 0.36522563, 0.69608614, 0.37305153],
               [ 0.37212492, 0.69881984, 0.37662736],
               [ 0.37892642, 0.70156084, 0.38027132],
               [ 0.3856453 , 0.70430747, 0.38398168],
               [ 0.39227735, 0.70706166, 0.38775317],
               [ 0.39883103, 0.70982284, 0.39158356],
               [ 0.40531204, 0.71259083, 0.39547048],
               [ 0.41171522, 0.7153677 , 0.39940867],
               [ 0.41805337, 0.71815166, 0.40339835],
               [ 0.42432867, 0.72094309, 0.40743704],
               [ 0.43053587, 0.72374398, 0.41151964],
               [ 0.43668612, 0.72655264, 0.41564705],
               [ 0.44278167, 0.72936931, 0.41981738],
               [ 0.44882469, 0.73219416, 0.42402884],
               [ 0.45481188, 0.73502867, 0.42827737],
               [ 0.46075114, 0.73787164, 0.43256386],
               [ 0.46664458, 0.74072315, 0.43688699],
               [ 0.47249405, 0.74358334, 0.44124545],
               [ 0.47830134, 0.74645233, 0.44563802],
               [ 0.48406816, 0.74933022, 0.45006358],
               [ 0.48979527, 0.75221736, 0.45452056],
               [ 0.49548385, 0.75511394, 0.45900769],
               [ 0.50113714, 0.75801961, 0.46352502],
               [ 0.50675657, 0.76093444, 0.46807172],
               [ 0.51234353, 0.76385852, 0.47264701],
               [ 0.51789934, 0.76679191, 0.47725017],
               [ 0.52342527, 0.76973468, 0.48188052],
               [ 0.52892256, 0.77268689, 0.48653744],
               [ 0.53439237, 0.7756486 , 0.49122035],
               [ 0.53983582, 0.77861987, 0.4959287 ],
               [ 0.54525401, 0.78160076, 0.500662  ],
               [ 0.55064796, 0.78459132, 0.50541978],
               [ 0.55601867, 0.78759161, 0.5102016 ],
               [ 0.56136709, 0.79060167, 0.51500705],
               [ 0.56669416, 0.79362156, 0.51983577],
               [ 0.57200073, 0.79665133, 0.5246874 ],
               [ 0.57728767, 0.79969103, 0.52956161],
               [ 0.5825556 , 0.80274077, 0.53445795],
               [ 0.587804  , 0.80580103, 0.53937491],
               [ 0.59303502, 0.80887141, 0.54431345],
               [ 0.59824938, 0.81195197, 0.54927334],
               [ 0.60344779, 0.81504274, 0.55425434],
               [ 0.60863092, 0.8181438 , 0.55925625],
               [ 0.61379944, 0.82125518, 0.56427887],
               [ 0.61895395, 0.82437694, 0.56932201],
               [ 0.62409506, 0.82750913, 0.57438551],
               [ 0.62922216, 0.83065222, 0.5794679 ],
               [ 0.63433574, 0.83380631, 0.58456889],
               [ 0.63943758, 0.83697102, 0.58968966],
               [ 0.64452821, 0.84014641, 0.59483008],
               [ 0.64960814, 0.84333252, 0.59999004],
               [ 0.65467785, 0.84652942, 0.60516941],
               [ 0.65973664, 0.8497376 , 0.61036667],
               [ 0.66478454, 0.8529573 , 0.61558114],
               [ 0.66982361, 0.85618797, 0.62081462],
               [ 0.67485427, 0.85942969, 0.62606703],
               [ 0.67987695, 0.8626825 , 0.6313383 ],
               [ 0.68489103, 0.86594688, 0.63662701],
               [ 0.68989598, 0.86922324, 0.64193178],
               [ 0.69489414, 0.87251091, 0.64725509],
               [ 0.69988588, 0.87580994, 0.65259689],
               [ 0.70487156, 0.87912041, 0.6579571 ],
               [ 0.70984902, 0.88244339, 0.6633321 ],
               [ 0.71482059, 0.88577817, 0.66872453],
               [ 0.71978715, 0.88912459, 0.67413514],
               [ 0.72474903, 0.89248272, 0.67956388],
               [ 0.72970443, 0.89585351, 0.68500748],
               [ 0.73465479, 0.89923659, 0.69046752],
               [ 0.73960143, 0.90263159, 0.69594546],
               [ 0.74454464, 0.90603859, 0.70144129],
               [ 0.74948217, 0.90945876, 0.70695082],
               [ 0.75441627, 0.91289135, 0.71247711],
               [ 0.75934783, 0.91633616, 0.71802108],
               [ 0.76427661, 0.91979348, 0.72358186],
               [ 0.76920074, 0.92326438, 0.72915567],
               [ 0.77412316, 0.92674772, 0.73474699],
               [ 0.77904414, 0.93024359, 0.74035579],
               [ 0.78396177, 0.93375306, 0.74597816],
               [ 0.78887747, 0.93727567, 0.75161605],
               [ 0.79379251, 0.94081106, 0.75727124],
               [ 0.79870604, 0.9443598 , 0.76294165],
               [ 0.80361734, 0.94792244, 0.76862535],
               [ 0.80852873, 0.9514981 , 0.77432618],
               [ 0.81344004, 0.95508703, 0.7800433 ],
               [ 0.81834917, 0.95869047, 0.78577205],
               [ 0.82325913, 0.96230716, 0.79151775],
               [ 0.82817006, 0.96593722, 0.79728024],
               [ 0.8330792 , 0.96958222, 0.80305326],
               [ 0.83798987, 0.97324073, 0.80884306],
               [ 0.84290226, 0.97691284, 0.81464957]]
    for c in cm_data:
        c.append(1.0)
    cols=cols+cm_data
    return cols    

# Make a random scatter field to seed the wind vectors
# (the 'z' value needed by 'wind_field').
# The 'resolution' parameter (in degrees) varies the coarseness of the 
#  vectors. The 'seed' parameter makes the function deterministic
#  calling the function with the same value (not none) will give
#  the same field (needed for videos).
def make_wind_seed(resolution,seed=None):
    if seed is not None:
        numpy.random.seed(seed=seed)
    z=plot_cube(resolution)
    (width,height)=z.data.shape
    z.data=numpy.random.rand(width,height)-0.5
    return z