Write netCDF data file

Several steps are needed to create a netCDF data file and write data in it:

• Create a writable data file object using addfile() function. The first argument is

file name and the second one is 'c' which means creating data file.

• Add dimensions using adddim() function of the data file object. The two arguments

are dimension name and length.

• Add global attributes using addgroupattr() function of the data file object. The

two arguments are attribute name and value.

• Add variables using addvar() function of the data file object. The three arguments

are variable name, data type and dimensions.

• Create netCDF file using create() function of the data file object.

• Write data array to the netCDF file using write() function of the data file object.

• Close netCDF data file by close() function of the data file object.

The below example shows reading 4 netCDF files and joint them into a new netCDF data file.

datadir = 'D:/Temp/nc'
outfn = os.path.join(datadir, 'join_file.nc')
#New netCDF file
ncfile = addfile(outfn, 'c')
#Add dimensions
stn = 26564
recdim = ncfile.adddim('recNum', stn)
stdim = ncfile.adddim('station', stn)
iddim = ncfile.adddim('id_len', 11)
tdim = ncfile.adddim('time', 4)
#Add global attributes
ncfile.addgroupattr('Conventions', 'Unidata Observation Dataset v1.0')
ncfile.addgroupattr('cdm_datatype', 'Station')
ncfile.addgroupattr('geospatial_lat_max', '90.0')
ncfile.addgroupattr('geospatial_lat_min', '-90.0')
ncfile.addgroupattr('geospatial_lon_max', '180.0')
ncfile.addgroupattr('geospatial_lon_min', '-180.0')
ncfile.addgroupattr('stationDimension', 'station')
ncfile.addgroupattr('missing_value', -8.9999998E15)
ncfile.addgroupattr('stream_order_output', 1)
#Add variables
variables = []
var = ncfile.addvar('latitude', 'float', [stdim])    #Latitude
var.addattr('long_name', 'station latitude')
var.addattr('units', 'degrees_north')
variables.append(var)
var = ncfile.addvar('longitude', 'float', [stdim])    #Longitude
var.addattr('long_name', 'station longitude')
var.addattr('units', 'degrees_east')
variables.append(var)
var = ncfile.addvar('altitude', 'float', [stdim])    #Altitude
var.addattr('long_name', 'station altitude')
var.addattr('units', 'meters')
variables.append(var)
var = ncfile.addvar('streamflow', 'float', [tdim, stdim])    #Stream flow - Add time dimension
var.addattr('long_name', 'River Flow')
var.addattr('units', 'meter^3 / sec')
variables.append(var)
tvar = ncfile.addvar('time', 'int', [tdim])
tvar.addattr('long_name', 'time')
tvar.addattr('units', 'hours since 1900-01-01 00:00:0.0')
#Creat netCDF file
ncfile.create()
#Write data
stime = datetime.datetime(2015,8,2,0)
etime = datetime.datetime(2015,8,2,3)
st = datetime.datetime(1900,1,1)
fi = 0
while stime <= etime:
    print stime
    fn = os.path.join(datadir, stime.strftime('%Y%m%d%H') + '00.CHRTOUT_DOMAIN2')
    if os.path.exists(fn):
        print '\t' + fn
        f = addfile(fn)
        hours = (stime - st).total_seconds() // 3600
        origin = [fi]
        ncfile.write(tvar, array([hours]), origin=origin)
        if fi == 0:
            lat = f['latitude'][:]
            ncfile.write(variables[0], lat)
            lon = f['longitude'][:]
            ncfile.write(variables[1], lon)
            alt = f['altitude'][:]
            ncfile.write(variables[2], alt)
        flow = f['streamflow'][:]
        origin = [fi, 0]
        shape = [1, stn]
        flow = flow.array.reshape(shape)
        ncfile.write(variables[3], flow, origin=origin)
        fi += 1
    stime = stime + datetime.timedelta(hours=1)

#close netCDF file
ncfile.flush()
ncfile.close()

print 'Finished!'

Read and plot joined netCDF data file:

f = addfile('D:/Temp/nc/join_file.nc')
lon = f['longitude'][:]
lat = f['latitude'][:]
var = f['streamflow']
flow = var[1,:]
axesm()
geoshow('cn_province')
levs = arange(0, 0.1, 0.005)
layer = scatter(lon, lat, flow, levs, edge=False)
colorbar(layer)
t = f.gettime(1)
title('River Flow (' + t.strftime('%Y-%m-%d %Hh)'))