Recipe: IDL program to read data from downloaded file

Back to G5NR Data Access Guide.

Problem

We want to read a downloaded data file using IDL.

Solution

For the purpose of this example, we assume that we have already downloaded the file c1440_NR.inst01hr_3d_T_Cv.20060918_0900z.nc4 from the ftp/http server. For more information about file naming conventions, and how to download a file from the ftp server, please follow the links in the #See Also section.

Code

The code below reads the global temperature data from file, computes the maximum and minimum temperatures and plots (using the matplotlib package) the aire temperature at the surface (level=71). It has been tested with IDL v 8.3.

; file
file = 'c1440_NR.inst01hr_3d_T_Cv.20060918_0900z.nc4'

; read data from file
ncid = ncdf_open(file)
ncdf_varget, ncid, 'T', T
ncdf_close, ncid

; max/min of T
print, 'max(T):', max(T)
print, 'min(T):', min(T)

; plot T at the surface
lons = findgen(720)*0.5-180.
lats = findgen(361)*0.5-90.
basemap = map('Cylindrical Equal Area', limit=[-90., -180., 90., 180.])
cntrplot = contour( $
           T[*,*,71], $ ;; level=71 => surface
           lons, $
           lats, $
           grid_units=2, $ ;; degrees
           n_levels=20, $
           rgb_table=34, $
           ;; /fill, $
           /overplot, $
           title='surface air temperature' $
                  )
cntnt = mapcontinents(/countries)
END

Output

Running this IDL script

IDL> .run g5nr_reader.pro

we get the text output

max(T):      315.651
min(T):      180.367

and the plot

Discussions

Modifications to read a subset of the data

The above python code can be easily modified to read a subset of the data instead. For example, to read temperature data inside the box bounded by latitudes 25^oN, 50^oN and longitudes -130^oN, -65^oN, we will replace line 15 in the code above by

T = rootgrp.variables['T'][0,:,229:280,99:230]

The conversion of lat/lon co-ordinates to indices is left as an exercise for the user.

Running the modified script, produces the text output

rootgrp.variables['T'].shape (1, 72, 361, 720)
Reading T... done.
T.shape: (72, 51, 131)
min(T): 191.6956
max(T): 305.6048

See Also

1. File Spec: File:G5NR-Ganymed-7km FileSpec-ON6-V1.0.pdf
2. Recipe: Retrieve (global) data from FTP server