Trying to change the variable dimensions when combining two netcdf files

In the code below, attempt has been made to combine the two netcdf files and also change the dimensions of variable ua and va from 144 x 143 x 8 x 3650 to 144 x 143 x 1 x 3650 i.e. ua and va value corresponding 3rd position first index will only be taken. For this, the code has saved all values of ua from netcdf file to a variable ua and similarly for va. Then " ua_data = ua(:, :, plev_index, :); " has been used. But in final combined file the dimensions of ua and va are same as earlier. Please help if anyone can suugest something.

Code is as given below:

% Define input file names

file1 = 'ua_day_IPSL-CM5A-MR_rcp45_r1i1p1_20260101-20351231.nc';

file2 = 'va_day_IPSL-CM5A-MR_rcp45_r1i1p1_20260101-20351231.nc';

% Define output file name

outputFile = 'Netcdfcombined_file.nc';

% Open first file and read variables and attributes

ua = ncread(file1, 'ua');

attributes1 = ncinfo(file1);

dims1 = attributes1.Dimensions;

% Open second file and read variable and attributes

va = ncread(file2, 'va');

attributes2 = ncinfo(file2);

dims2 = attributes2.Dimensions;

% Find the index of pressure level corresponding to 100000 Pa

plev_values = ncread(file1, 'plev');

plev_index = find(plev_values == 100000);

if isempty(plev_index)

error('Pressure level corresponding to 100000 Pa not found in file.');

end

% Write dimensions and attributes to output file

ncwriteschema(outputFile, attributes1);

% Create 'va' variable in the output file

varDims = {'lon', 'lat', 'plev', 'time'};

varSize = [size(va,1), size(va,2), 1, size(va,4)]; % Assuming plev is 1 level

nccreate(outputFile, 'va', 'Dimensions', varDims, 'Datatype', 'double');

ua_data = ua(:, :, plev_index, :);

varSize(3) = 1; % Set plev dimension size to 1

ncwrite(outputFile, 'ua', ua_data, [1, 1, 1, 1]); % Specify start position only

% Transfer remaining variables from the first file to the output file

variables1 = {attributes1.Variables.Name};

variables1 = setdiff(variables1, 'ua'); % Remove ua

for i = 1:length(variables1)

data = ncread(file1, variables1{i});

ncwrite(outputFile, variables1{i}, data);

end

% Write va variable from the second file to the output file

plev_index_double = double(plev_index); % Convert plev_index to double

va_data = va(:, :, plev_index, :);

varSize(3) = 1; %Set plev dimension size to 1

if ~isempty(va)

ncwrite(outputFile, 'va', va_data, [1, 1, 1, 1]); % Specify start position only

end

[Shivam]

Hi,

Can you attach the related files for me to look into the query?

[Prashant]

Hello Sir,

Actually I had replied to ur comment last time but somehow it did not get posted.

Anyways the problem is the files are two large to be shared, their size is around 4.5 GB

Thanks for the comment

with bedt regards,

Prashant M Soni

Answer 1

[Ronit]

Hi Prashant,

It seems like you are trying to update the dimensions of the “ua” and “va” variables in the output file from their original dimensions (144 x 143 x 8 x 3650) to the new dimensions (144 x 143 x 1 x 3650). Specifically, when you create the “va” variable in the output file and attempt to write the “ua” data, you need to ensure that the dimensions defined in the output file match the new dimensions of your data.

Here's a corrected version of the relevant parts of your code:

% Define output file name 
outputFile = 'Netcdfcombined_file.nc'; 
 
% Create the output file 
ncid = netcdf.create(outputFile, 'CLOBBER'); 
 
% Define dimensions in the output file 
lonDimId = netcdf.defDim(ncid, 'lon', size(ua, 1)); 
latDimId = netcdf.defDim(ncid, 'lat', size(ua, 2)); 
plevDimId = netcdf.defDim(ncid, 'plev', 1); % Only one pressure level 
timeDimId = netcdf.defDim(ncid, 'time', size(ua, 4)); 
 
% Define the variables with the correct dimensions in the output file 
uaVarId = netcdf.defVar(ncid, 'ua', 'double', [lonDimId, latDimId, plevDimId, timeDimId]); 
vaVarId = netcdf.defVar(ncid, 'va', 'double', [lonDimId, latDimId, plevDimId, timeDimId]); 
 
% End definition mode 
netcdf.endDef(ncid); 
 
% Prepare the data 
ua_data = ua(:, :, plev_index, :); 
va_data = va(:, :, plev_index, :); 
 
% Write the data to the variables 
netcdf.putVar(ncid, uaVarId, ua_data); 
netcdf.putVar(ncid, vaVarId, va_data); 
 
% Close the file 
netcdf.close(ncid); 

To learn more about functions like “netcdf.defDim”, “netcdf.defVar” and “netcdf.putVar”, please refer to the following documentation link - https://www.mathworks.com/help/matlab/network-common-data-form.html

Hope this helps!

[Prashant]

Hello Ronit Sir,

Thanks for the help, your code works well for the described problem. But I am facing issue while copying the attributes from the first file to the combined file. There is some issue with the file format as shown by matlab. Could you please suggest some solution for the same.

Thank you very much for your helping hand

with best regards,

Prashant Soni