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 surface (level=71) temperature. It has been tested with IDL v 8.1.

; 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.
ct = colortable(72, /reverse)
cntrplot = contour( $
           T[*,*,71], $ ;; level=71 => surface
           lons, $
           lats, $
           /fill, $
           n_levels=20, $
           rgb_table=ct, $
           title='surface air temperature' $
                  )

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

File:SurfaceTempIDL.png

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