Recipe: Matlab program to read data from downloaded file

Back to G5NR Data Access Guide.

Problem

We want to read a downloaded data file using Matlab.

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 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.

The following code has been tested with Matlab version 2014a.

Code

This code reads the global temperature data from file and computes the maximum and minimum temperatures.

% NetCDF-4 file
opdfile = 'c1440_NR.inst01hr_3d_T_Cv.20060918_0900z.nc4';

% read global data
im = 720;
jm = 361;
lm = 72;
fprintf('Reading T (global)...');
T = ncread(opdfile, 'T');
fprintf('done.\n')

% compute max/min of T
fprintf('max(T): %f\n', max(T(:)));
fprintf('min(T): %f\n', min(T(:)));

Run

Running this Matlab script

>> run g5nr_reader.m

produces the output

Reading T (global)...done.
max(T): 315.651245
min(T): 180.366745

Discussions

Modifications to read a subset of the data

The above Matlab script can be easily modified to read a subset of the temperature data instead. To read the temperature inside the box bounded by latitudes 25^oN, 50^oN and longitudes -130^oW, -65^oW, we need to modify the call to ncread. We replace lines 4-10 of the above script with

% bounding box
%   lons = -130:0.5:-65
%   lats =   25:0.5:50
imin = round((-130. + 180.)/0.5);
imax = round(( -65. + 180.)/0.5);
jmin = round((  25 +  90.)/0.5);
jmax = round((  50 +  90.)/0.5);

% corresponding array sizes
im = imax-imin+1;
jm = jmax-jmin+1;
lm = 72; % read all 72 levels

% start/count for ncread
start = [imin, jmin, 1, 1];
count = [im, jm, lm, 1];

% now read
fprintf('Reading T (subset)...');
T = ncread(opdfile, 'T', start, count);
fprintf('done.\n')

The modified version of the code is g5nr_reader_subset.m.

Running this modified version

>> run g5nr_reader_subset.m

produces the output

Reading T (subset)...done.
max(T): 305.604828
min(T): 191.695648

See Also

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