How to create a netcdf in matlab which can be read by grads without using descriptor file?

I have data containing lat, lon, time and precipitation values:

lat: 128*1double

lon 256*1double

time 3652*1double

pr 256*128*3652double

I have masked my original data and now I want to create netcdf file which has all the information of original files and most importantly it can be read by grads without the use of descriptor file.

Answer 1

[ANKUR KUMAR]

There are often times when MATLAB created NetCDF files are not readable in earth and atmospheric science visualization softwares like GrADS or NCL. Or one needs to write its seprate descriptor file to get it read in GrADS. Refer this function nccreatewrite on file exchagne which enables MATLAB created nc file readable in GrDAS. Here is an example for you.

clc
clear

random_data = normrnd(3,10,[20,30,5])+273;

filename='sample.nc';

delete(filename)

Warning: File 'sample.nc' not found.

lon_att.standard_name='longitude';
lon_att.long_name='longitude';
lon_att.units='degrees_east';
lon_att.axis='X';
nccreatewrite(filename,'lon',{'lon'},[65:84],lon_att)

att_fields = 4×2 cell array
    {'standard_name'}    {'longitude'   }
    {'long_name'    }    {'longitude'   }
    {'units'        }    {'degrees_east'}
    {'axis'         }    {'X'           }

-------------------------------------------
| Succesfully written lon in file sample.nc |
--------------------------------------------

lat_att.standard_name='latitude';
lat_att.long_name='latitude';
lat_att.units='degrees_north';
lat_att.axis='Y';
nccreatewrite(filename,'lat',{'lat'},[1:30],lat_att)

att_fields = 4×2 cell array
    {'standard_name'}    {'latitude'     }
    {'long_name'    }    {'latitude'     }
    {'units'        }    {'degrees_north'}
    {'axis'         }    {'Y'            }

-------------------------------------------
| Succesfully written lat in file sample.nc |
--------------------------------------------

lev_att.long_name='generic';
lev_att.units='level';
lev_att.axis='Z';
nccreatewrite(filename,'lev',{'lev'},[1:5],lev_att)

att_fields = 3×2 cell array
    {'long_name'}    {'generic'}
    {'units'    }    {'level'  }
    {'axis'     }    {'Z'      }

-------------------------------------------
| Succesfully written lev in file sample.nc |
--------------------------------------------

nccreatewrite(filename,'TC',{'lon','lat','lev'},random_data)

Warning: You are not writing any Attributes to TC variable

-------------------------------------------
| Succesfully written TC in file sample.nc |
--------------------------------------------

ncdisp(filename)

Source:
           /users/mss.system.TaLGpO/sample.nc
Format:
           netcdf4
Dimensions:
           lon = 20
           lat = 30
           lev = 5
Variables:
    lon
           Size:       20x1
           Dimensions: lon
           Datatype:   double
           Attributes:
                       standard_name = 'longitude'
                       long_name     = 'longitude'
                       units         = 'degrees_east'
                       axis          = 'X'
    lat
           Size:       30x1
           Dimensions: lat
           Datatype:   double
           Attributes:
                       standard_name = 'latitude'
                       long_name     = 'latitude'
                       units         = 'degrees_north'
                       axis          = 'Y'
    lev
           Size:       5x1
           Dimensions: lev
           Datatype:   double
           Attributes:
                       long_name = 'generic'
                       units     = 'level'
                       axis      = 'Z'
    TC 
           Size:       20x30x5
           Dimensions: lon,lat,lev
           Datatype:   double

Lets plot the data from created nc file

filename='sample.txt'; % This is actually a netCDF file. I have just manually changed the 
% %file extension in order to attach the file hee in the answers.

lon=ncread(filename,'lon');
lat=ncread(filename,'lat');
lev=ncread(filename,'lev');
tc=ncread(filename,'TC');

Let use contourf to plot how our data looks like

contourf(lon,lat,tc(:,:,3)')
hold on
daspect(ones(1,3))
load coastlines
plot(coastlon, coastlat, 'r')
axis([65 84 1 30])
colormap(parula)
colorbar

You can use the same .nc file to plot in GrDAS. Here is an example for you.

% sdfopen sample.nc
% q file
% set lev 3
% set gxout shaded
% d tc

You will get a warning like above, and you can ignore it for a moment. I am working on it to resolve this warning.

You will get an image like this in GrADS.

Answer 2

[KSSV]

% nc filename to be written
file = 'myfile.nc' ;

%% Write lon and lat variables
% Get data
lon = 1:10 ;
lat = 1:10 ;
nx = length(lon) ;
nccreate(file,'lon','Dimensions',{'lon',1,nx},'DeflateLevel',7) ;
ny = length(lat) ;
nccreate(file,'lat','Dimensions',{'lat',1,ny},'DeflateLevel',7) ;
nccreate(file,'time','Dimensions',{'time',1,Inf},'DeflateLevel',7) ;
nccreate(file,'z','Dimensions',{'lon','lat','time'},'DeflateLevel',7) ;
for i = 1:10
    ncwrite(file,'time',i,i)   % write time 
    data = rand(10) ; 
    ncwrite(file,'z',data,[1,1,i]) ;   % write 3D data 
end

[UTKARSH VERMA]

Hi KSSV,

Thanks for replying.

I ran the above code and I tried to open the same file in Grads but it's not working.

Grads shows while opening the above generated nc file:

ga-> sdfopen myfile.nc

Scanning self-describing file: myfile.nc

gadsdf: SDF file has no discernable X coordinate.

To open this file with GrADS, use a descriptor file with an XDEF entry.

Documentation is at http://iges.org/grads/gadoc/SDFdescriptorfile.html

ga->

Can you suggest some changes or addition to above code?

Best,

Utkarsh