Extracting netCDF time series for each lat/long in Matlab - matlab

Question

I'm currently working with netCDF output from climate models and would like to obtain a text file of the time series for each latitude/longitude combination in the netCDF. For example, if the netCDF has 10 latitudes and 10 longitudes I would obtain 100 text files, each with a time series in a column format. I'm fairly familiar with the Matlab/netCDF language, but I can't seem to wrap my head around this. Naming the text files is not important; I will rename them "latitude_longitude_PCP.txt", where PCP is precipitation at the latitude and longitude location.
Any help would be appreciated. Thanks.
--Darren

Answer 1

There are several ways this problem could be solved.
Method 1. If you were able to put your netcdf file on a THREDDS Data Server, you could use the NetCDF Subset Service Grid as Point to specify a longitude/latitude point and get back the data in CSV or XML format. Here's an example from Unidata's THREDDS Data Server: http://thredds.ucar.edu/thredds/ncss/grid/grib/NCEP/GFS/Global_0p5deg/best/pointDataset.html
Method 2. If you wanted to use Matlab to extract a time series at a specific longitude/latitude location you could use the "nj_tseries" function from NCTOOLBOX, available at: http://nctoolbox.github.io/nctoolbox/
Method 3. If you really want to write an ASCII time series at every i,j location in your [time,lon,lat] grid using Matlab, you could do something like this (using NCTOOLBOX):
url='http://thredds.ucar.edu/thredds/dodsC/grib/NCEP/GFS/Global_2p5deg/best';
nc = ncgeodataset(url);
nc.variables
var='Downward_Short-Wave_Radiation_Flux_surface_12_Hour_Average';
lon = nc.data('lon');
lat = nc.data('lat');
jd = nj_time(nc,var);
ncvar = nc.variable(var);
for j=1:length(lat)
for i=1:length(lon)
v=ncvar.data(:,j,i);
outfile=sprintf('%6.2flon%6.2flat.csv',lon(i),lat(j))
fid=fopen(outfile,'wt')
data= [datevec(jd) v]
fprintf(fid,'%2.2d %2.2d %2.2d %2.2d %2.2d %2.2d %7.2f\n',data')
fclose(fid)
disp([outfile ' created.'])
end
end
If you had enough memory to read all the data into matlab, you could read outside the double loop, which would be a lot faster. But writing ASCII is slow anyway, so it might not matter that much.

Answer 2

%% Create demo data
data = reshape(1:20*30*40,[20 30 40]);
nccreate('t.nc','data','Dimensions',{'lat', 20, 'lon',30, 'time', inf});
ncwrite('t.nc', 'data',data);
ncdisp('t.nc');
%% Write timeseries to ASCII files
% Giving an idea of the size of your data can help people
% recommend different approaches tailored to the data size.
% For smaller data, it might be faster to read in the full
% 3D data into memory
varInfo = ncinfo('t.nc','data');
disp(varInfo);
for latInd =1:varInfo.Size(1)
for lonInd =1:varInfo.Size(2)
fileName = ['t_ascii_lat',num2str(latInd),'_lon',num2str(lonInd),'.txt'];
tSeries = ncread('t.nc','data',[latInd, lonInd, 1],[1,1,varInfo.Size(3)]);
dlmwrite(fileName,squeeze(tSeries));
end
end
%% spot check
act = dlmread('t_ascii_lat10_lon29.txt');
exp = squeeze(data(10,29,:));
assert(isequal(act,exp));