Menu
I am trying to extract a variable from a multi-variable netcdf file, by entering the command:
ncks -v ta temp1.nc out.nc
However then I look at the out.nc header, all the variables are still there.
The headers of both temp1.nc and out.nc are below:
--- temp1.nc header ---
dimensions:
time = UNLIMITED ; // (124 currently)
lon = 256 ;
bnds = 2 ;
lat = 128 ;
lev = 40 ;
variables:
double time(time) ;
time:standard_name = "time" ;
time:long_name = "time" ;
time:units = "days since 1850-1-1" ;
time:calendar = "365_day" ;
time:axis = "T" ;
double lon(lon) ;
lon:standard_name = "longitude" ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:axis = "X" ;
lon:bounds = "lon_bnds" ;
double lon_bnds(lon, bnds) ;
double lat(lat) ;
lat:standard_name = "latitude" ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:axis = "Y" ;
lat:bounds = "lat_bnds" ;
double lat_bnds(lat, bnds) ;
double p0 ;
p0:long_name = "reference pressure" ;
p0:units = "Pa" ;
double lev(lev) ;
lev:standard_name = "atmosphere_hybrid_sigma_pressure_coordinat
lev:axis = "Z" ;
lev:positive = "down" ;
lev:long_name = "hybrid sigma pressure coordinate" ;
lev:units = "1" ;
lev:formula_terms = "a: a b: b p0: p0 ps: ps" ;
lev:bounds = "lev_bnds" ;
lev:formula = "p = a*p0 + b*ps" ;
double lev_bnds(lev, bnds) ;
lev_bnds:standard_name = "atmosphere_hybrid_sigma_pressure_coor
lev_bnds:units = "1" ;
lev_bnds:formula_terms = "a: a_bnds b: b_bnds p0: p0 ps: ps" ;
double a(lev) ;
a:long_name = "vertical coordinate formula term: ap(k)" ;
a:units = "Pa" ;
double b(lev) ;
b:long_name = "vertical coordinate formula term: b(k)" ;
b:units = "1" ;
double a_bnds(lev, bnds) ;
a_bnds:long_name = "vertical coordinate formula term: ap(k+1/2)
a_bnds:units = "Pa" ;
double b_bnds(lev, bnds) ;
b_bnds:long_name = "vertical coordinate formula term: b(k+1/2)"
b_bnds:units = "1" ;
float ps(time, lat, lon) ;
ps:standard_name = "surface_air_pressure" ;
ps:long_name = "Surface Air Pressure" ;
ps:units = "Pa" ;
ps:comment = "surface pressure, not mean sea level pressure" ;
ps:original_units = "hPa" ;
ps:history = "2011-10-17T12:45:37Z altered by CMOR";
ps:cell_measures = "area: areacella" ;
float ta(time, lev, lat, lon) ;
ta:standard_name = "air_temperature" ;
ta:long_name = "Air Temperature" ;
ta:units = "K" ;
ta:_FillValue = 1.e+20f ;
ta:missing_value = 1.e+20f ;
ta:original_name = "T" ;
ta:cell_measures = "area: areacella" ;
ta:history = "2011-10-17T12:45:37Z altered by CMOR ;
// global attributes:
--- out.nc header ---
dimensions:
lev = 40 ;
bnds = 2 ;
lat = 128 ;
lon = 256 ;
time = UNLIMITED ; // (124 currently)
variables:
double a(lev) ;
a:long_name = "vertical coordinate formula term: ap(k)" ;
a:units = "Pa" ;
double a_bnds(lev, bnds) ;
a_bnds:long_name = "vertical coordinate formula term: ap(k+1/2)" ;
a_bnds:units = "Pa" ;
double b(lev) ;
b:long_name = "vertical coordinate formula term: b(k)" ;
b:units = "1" ;
double b_bnds(lev, bnds) ;
b_bnds:long_name = "vertical coordinate formula term: b(k+1/2)" ;
b_bnds:units = "1" ;
double lat(lat) ;
lat:standard_name = "latitude" ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:axis = "Y" ;
lat:bounds = "lat_bnds" ;
double lat_bnds(lat, bnds) ;
double lev(lev) ;
lev:standard_name = "atmosphere_hybrid_sigma_pressure_coordinate" ;
lev:axis = "Z" ;
lev:positive = "down" ;
lev:long_name = "hybrid sigma pressure coordinate" ;
lev:units = "1" ;
lev:formula_terms = "a: a b: b p0: p0 ps: ps" ;
lev:bounds = "lev_bnds" ;
lev:formula = "p = a*p0 + b*ps" ;
double lev_bnds(lev, bnds) ;
lev_bnds:standard_name = "atmosphere_hybrid_sigma_pressure_coordinate" ;
lev_bnds:units = "1" ;
lev_bnds:formula_terms = "a: a_bnds b: b_bnds p0: p0 ps: ps" ;
double lon(lon) ;
lon:standard_name = "longitude" ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:axis = "X" ;
lon:bounds = "lon_bnds" ;
double lon_bnds(lon, bnds) ;
double p0 ;
p0:long_name = "reference pressure" ;
p0:units = "Pa" ;
float ps(time, lat, lon) ;
ps:standard_name = "surface_air_pressure" ;
ps:long_name = "Surface Air Pressure" ;
ps:units = "Pa" ;
ps:comment = "surface pressure, not mean sea level pressure" ;
ps:original_units = "hPa" ;
ps:history = "2011-10-17T12:45:37Z altered by CMOR" ;
ps:cell_measures = "area: areacella" ;
float ta(time, lev, lat, lon) ;
ta:standard_name = "air_temperature" ;
ta:long_name = "Air Temperature" ;
ta:units = "K" ;
ta:_FillValue = 1.e+20f ;
ta:missing_value = 1.e+20f ;
ta:original_name = "T" ;
ta:cell_measures = "area: areacella" ;
ta:history = "2011-10-17T12:45:37Z altered by CMOR" ;
double time(time) ;
time:standard_name = "time" ;
time:long_name = "time" ;
time:units = "days since 1850-1-1" ;
time:calendar = "365_day" ;
time:axis = "T" ;
// global attributes:
I expected to have only ta on out.nc. I don't get any error message from ncks.
What am I doing wrong here? I appreciate any help.
Thank you.
574
asked Mar 24 '18 00:03
Use the parameter 'name=' to select variables by name: [...] As all CDO operators with an unlimited number of input files this operator can't be combined with other operators.
You can read netCDF data in R using the ncdf4 package. You can use the nc_open function from the ncdf4 package to open a connection to a netCDF file.
The ncdump tool generates an ASCII representation of a netCDF file on standard output, either with or without an ASCII representation of the variable data in the file. The ASCII representation used is the CDL notation that ncgen accepts as input.
This behavior occurs because, by default, NCO extracts all the variables "associated" with a requested variable. That way users get everything they need without having to look for it themselves. In other words, it's a feature, not a bug. To extract only the variables explicitly requested, use the -C switch, documented here, e.g.
ncks -C -v ta temp1.nc out.nc
The docs also show how to strip away the CF-conventions-associated variables, while retainining, say, the coordinates, with a variety of longer options.
160
Just to give another command line alterative to NCO, you can also use CDO,
the command is:
cdo selvar,variable_name input.nc output.nc
if you don't have it, you can install it with ubuntu, mint etc with
sudo apt-get install cdo
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
