使用netcdf文件在python中制作填充轮廓图

Question

I'm attempting to make a plot emissions from a model, using Basemap and matplotlib.pyplot. I'm very new to python so was attempting to use someone else's example and adjust it for my data but finding numerous errors.

from mpl_toolkits.basemap import Basemap, cm
from netCDF4 import Dataset as NetCDFFile
import numpy as np
import matplotlib.pyplot as plt

# plot rainfall from NWS using special precipitation
# colormap used by the NWS, and included in basemap.

nc = NetCDFFile('file.nc')
pm25var = nc.variables['emis_all']
data = 0.01*pm25var[:]
latcorners = nc.variables['lat'][:]
loncorners = -nc.variables['lon'][:]
lon_0 = -nc.variables['true_lon'].getValue()
lat_0 = nc.variables['true_lat'].getValue()
# create figure and axes instances
fig = plt.figure(figsize=(8,8))
ax = fig.add_axes([0.1,0.1,0.8,0.8])
# create polar stereographic Basemap instance.
m = Basemap(projection='stere',lon_0=lon_0,lat_0=90.,lat_ts=lat_0,\
        llcrnrlat=latcorners[0],urcrnrlat=latcorners[2],\
        llcrnrlon=loncorners[0],urcrnrlon=loncorners[2],\
        rsphere=6371200.,resolution='l',area_thresh=10000)
# draw coastlines, state and country boundaries, edge of map.
m.drawcoastlines()
m.drawstates()
m.drawcountries()
# draw parallels.
parallels = np.arange(0.,90,10.)
m.drawparallels(parallels,labels=[1,0,0,0],fontsize=10)
# draw meridians
meridians = np.arange(0.,60.,10.)
m.drawmeridians(meridians,labels=[0,0,0,1],fontsize=10)
ny = data.shape[0]; nx = data.shape[1]
lons, lats = m.makegrid(nx, ny) # get lat/lons of ny by nx evenly space grid.
x, y = m(lons, lats) # compute map proj coordinates.
# draw filled contours.
clevs = [0,1,2.5,5,7.5,10,15,20,30,40,50,70,100,150,200,250,300,400,500,600,750]
cs = m.contourf(x,y,data,clevs,cmap=cm.s3pcpn)
# add colorbar.
cbar = m.colorbar(cs,location='bottom',pad="5%")
cbar.set_label('mm')
# add title
plt.title(pm25var.long_name+' for period ending '+pm25var.dateofdata)
plt.show()

I keep getting "KeyError: 'true_lon'" and have no idea how to resolve it. The data has 3 keys (lat, lon and time). I have shown the details of the lon variable below.

>>>print dataset.variables['lon']
<type 'netCDF4._netCDF4.Variable'>
float64 lon(lon)
long_name: longitude
units: degrees_east
comment: centre of grid cell
unlimited dimensions: 
current shape = (720,)
filling on, default _FillValue of 9.96920996839e+36 used

The data is global. The details of variable I'm trying to plot (emis_all) are below.

>>>print dataset.variables['emis_all']
<type 'netCDF4._netCDF4.Variable'>
float64 emis_all(time, lat, lon)
long_name: PM25 - Total
pollutant: PM25
sector: Total
units: kt/year
unlimited dimensions: 
current shape = (11, 360, 720)
filling on, default _FillValue of 9.96920996839e+36 used

Any help/advice much appreciated. Like I said I am a beginner just trying to get started and practice making a few plots with my own data.

Answer 1

The error you get is because you don't have that variable in your netCDF file. You should start from "the beggining" and try to figure out which does each line of your code. Then, select what do you actually need. I've simplified your code, just try it out:

# read netcdf file
nc = NetCDFFile('file.nc')
emis = nc.variables['emis_all'][:] 
lats = nc.variables['lat'][:]
lons = nc.variables['lon'][:]
nc.close()

data = emis[0,:,:] #emissions for the first hour only

# create figure and axes instances
fig = plt.figure(figsize=(8,8))
ax = fig.add_axes([0.1,0.1,0.8,0.8])

m = Basemap()
m.drawcoastlines()
m.drawstates()
m.drawcountries()
# draw parallels.
parallels = np.arange(0.,90,10.)
m.drawparallels(parallels,labels=[1,0,0,0],fontsize=10)
# draw meridians
meridians = np.arange(0.,60.,10.)
m.drawmeridians(meridians,labels=[0,0,0,1],fontsize=10)


x, y = m(lons, lats) # compute map proj coordinates.

# draw filled contours.
clevs = [0,1,2.5,5,7.5,10,15,20,30,40,50,70,100,150,200,250,300,400,500,600,750]

cs = m.contourf(x,y,data,clevs)
# add colorbar.
cbar = m.colorbar(cs,location='bottom',pad="5%")
cbar.set_label('mm')
# add title
plt.title('emissions')
plt.show()

---

some usefull comands: print nc.variables will print you the list of variables of your netCDF file emi.shape will return the shape of your array. In my example I chose only the 1st hour (or other time lapse) of the data. You can do everything you want with your array (like sum, mean), search for numpy module.