Remove a Variable from a nc file
Added by Jeisson Javier Leal Rojas almost 2 years ago
Hi all.
I have two nc files: precipitation (pre.nc) and temperature(temp.nc).
My temp.nc files has a extra variable that I do not need.
This is how my pre.nc file looks like when doing ncdump -h pre.nc:
dimensions:
time = UNLIMITED ; // (750 currently)
lon = 3600 ;
lat = 1400 ;
variables:
int time(time) ;
time:standard_name = "time" ;
time:units = "minutes since 2020-1-20 00:00:00" ;
time:calendar = "proleptic_gregorian" ;
time:axis = "T" ;
double lon(lon) ;
lon:standard_name = "longitude" ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:axis = "X" ;
double lat(lat) ;
lat:standard_name = "latitude" ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:axis = "Y" ;
double pre(time, lat, lon) ;
pre:standard_name = "pr" ;
pre:long_name = "precipitation flux" ;
pre:units = "kg m-2 s-1" ;
pre:param = "52.1.0" ;
pre:_FillValue = -9999. ;
pre:missing_value = -9999. ;
pre:level_type = "atmosphere" ;
pre:cell_methods = "time: mean" ;
And this is how my temp.nc file looks like when doing ncdump -h temp.nc:
dimensions:
time = UNLIMITED ; // (750 currently)
lon = 3600 ;
lat = 1400 ;
height = 1 ;
variables:
int time(time) ;
time:standard_name = "time" ;
time:units = "minutes since 2020-1-20 00:00:00" ;
time:calendar = "proleptic_gregorian" ;
time:axis = "T" ;
double lon(lon) ;
lon:standard_name = "longitude" ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:axis = "X" ;
double lat(lat) ;
lat:standard_name = "latitude" ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:axis = "Y" ;
double height(height) ;
height:standard_name = "height" ;
height:long_name = "height" ;
height:units = "m" ;
height:positive = "up" ;
height:axis = "Z" ;
double tavg(time, height, lat, lon) ;
tavg:standard_name = "tas" ;
tavg:long_name = "temperature in 2m" ;
tavg:units = "K" ;
tavg:param = "0.0.0" ;
tavg:_FillValue = -9999. ;
tavg:missing_value = -9999. ;
tavg:cell_methods = "time: mean" ;
As you can see for temp.nc I have this double height(height) variable I want to remove so temp.nc will look like pre.nc.
I've checked previous posts such as https://code.mpimet.mpg.de/boards/2/topics/12103, but that one in particular uses sellevel to remove one value of a variable, and not the variable itself.
Thanks in advance
Replies (5)
RE: Remove a Variable from a nc file - Added by Estanislao Gavilan almost 2 years ago
Hi,
you can either use the command "-selname" or "-delname". The first selects a variable, and the second removes it.
Regards,
Estanislao
RE: Remove a Variable from a nc file - Added by Karin Meier-Fleischer almost 2 years ago
Hi Jeisson,
another approach is to reduce the 1-size height dimension:
cdo --reduce_dim -copy temp.nc temp_1.nc
See Options https://code.mpimet.mpg.de/projects/cdo/embedded/cdo.pdf#subsection.1.2.1
RE: Remove a Variable from a nc file - Added by Jeisson Javier Leal Rojas almost 2 years ago
Dear Estanislao,
I tried before the delname and tried your suggestion with selname, but it seems that height is not a valid Variable name:
cdo selname,height temp.nc temp_out.nc
propt:
cdo selname (Warning): Variable name height not found!
Dear Karin,
I tried your suggestion, and it seems to work!
cdo --reduce_dim -copy temp.nc temp_out.nc
And this is how temp_out looks like:
dimensions:
time = UNLIMITED ; // (750 currently)
lon = 3600 ;
lat = 1400 ;
variables:
int time(time) ;
time:standard_name = "time" ;
time:units = "minutes since 2020-1-20 00:00:00" ;
time:calendar = "proleptic_gregorian" ;
time:axis = "T" ;
double lon(lon) ;
lon:standard_name = "longitude" ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:axis = "X" ;
double lat(lat) ;
lat:standard_name = "latitude" ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:axis = "Y" ;
double tavg(time, lat, lon) ;
tavg:standard_name = "tas" ;
tavg:long_name = "temperature in 2m" ;
tavg:units = "K" ;
tavg:param = "0.0.0" ;
tavg:_FillValue = -9999. ;
tavg:missing_value = -9999. ;
tavg:cell_methods = "time: mean" ;
Do you happen to know why delname or selname did not work?
Do you mind to explain me a bit more what cdo --reduce_dim -copy does on the background?
Thank you all,
Jeisson Leal.
RE: Remove a Variable from a nc file - Added by Ralf Mueller almost 2 years ago
Jeisson Javier Leal Rojas wrote in RE: Remove a Variable from a nc file:
Dear Estanislao,
[...]
Do you happen to know why delname or selname did not work?
selname and delname like most other operators work on data variables. heights is a coordinate variable (according to CF-convention), so CDO does not access them directly - only via coordinate related operations, e.g. intlevel or so.
there is nothing much to tell:Do you mind to explain me a bit more what cdo --reduce_dim -copy does on the background?
- copy performs a copy
- reduce_dim deletes dimensions of size 1
you need both here, because --reduce_dim is an option that can be given to every operators. In order to apply it to your data, you need a simple operators, which lets your data untouched. hence: copy
Thank you all,
Jeisson Leal.
RE: Remove a Variable from a nc file - Added by Jeisson Javier Leal Rojas almost 2 years ago
Thank you!