Recipe: Python program as OPeNDAP client

Back to G5NR Data Access Guide.

Problem

By accessing the collection inst01hr_3d_T_Cv via the OPeNDAP server

http://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km

we want to read the surface temperature data inside the box bound by latitudes 25^oN, 50^oN and longitudes -130^oW, -65^oW for 2006/Sep/18, 9z, compute its min/max and plot the temperature data.

Solution

Here we use the netcdf4-python package. If the command

> python -c "import netCDF4"

does not return any error, we have the package installed.

Code

This code accesses the collection inst01hr_3d_T_Cv from the OPeNDAP server and reads a subset of the temperature data (all levels inside the bounding box specified above) and computes its max/min. It then computes the max/min of the above data at the surface (level=72).

NOTE:

1. Instead of reading a downloaded NetCDF-4 file, we read an OPeNDAP URL.
2. While in the downloaded file, the temperature variable appears in the uppercase (T), on the OPeNDAP server, this variable is in lowercase.
3. Via the OPeNDAP URL, we now have access to all times for which data exists. The hourly inst files are available starting at 2005/15/15, 2200z. Our desired time, 2006/09/18, 0900z is then the 11772^th step.

#!/usr/bin/env python

import sys
import numpy as np
import netCDF4 as nc4
import matplotlib.pyplot as plt

from mpl_toolkits.basemap import Basemap

rootgrp = nc4.Dataset('http://opendap.nccs.nasa.gov:9090/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv', 'r')
print "rootgrp.variables['t'].shape", rootgrp.variables['t'].shape

# read subset of air temperature
print 'Reading T (subset)...',; sys.stdout.flush()
T = rootgrp.variables['t'][11771,:,229:280,99:230]
print 'done.'; sys.stdout.flush()
print 'T.shape:', T.shape

# max/min
print 'max(T): %.4f' % np.max(T)
print 'min(T): %.4f' % np.min(T)

# set up cylindrical map
m = Basemap(
    projection='cyl',
    llcrnrlat=25, urcrnrlat=50,
    llcrnrlon=-130, urcrnrlon=-65,
    resolution='c'
    )
m.drawcoastlines(linewidth=1.5)
m.drawmapboundary()
 
# plot contour
level = 71 # surface
X = np.arange(-130.0, -64.99, .5) # -65 is the last element
Y = np.arange(25.0, 50.01, .5) # 50 is the last element
cp = plt.contour(X, Y, T[level,:,:], 20, zorder=2)
plt.clabel(cp, inline=1, fontsize=9)
plt.title('Air temperature at the surface')
plt.show()

Output

Running this python script, we get the text output

rootgrp.variables['t'].shape (18288, 72, 361, 720)
Reading T (subset)... done.
T.shape: (72, 51, 131)
max(T): 305.6048
min(T): 191.6956

and the plot

See Also

1. File Spec: File:G5NR-Ganymed-7km FileSpec-ON6-V1.0.pdf
2. Recipe: Python program to read data from downloaded file