Provided by: nco_4.4.2-1_amd64 bug

NAME

       ncbo - netCDF Binary Operator

SYNTAX

       ncbo  [-3]  [-4]  [-6]  [-7]  [-A]  [--bfr  sz][-C][-c]  [--cnk_byt  sz][--cnk_dmn  nm,sz]
       [--cnk_map map] [--cnk_plc plc] [--cnk_scl sz][-D dbg_lvl] [-d dim,[ min][,[  max]]]  [-F]
       [-G  gpe_dsc]  [-g  grp[,...]]  [-h] [--hdf] [--hdr_pad sz] [-L dfl_lvl] [-l path] [--msa]
       [--no_tmp_fl] [-O] [-p path] [-R] [-r] [--ram_all] [-t  thr_nbr]  [--unn]  [-v  var[,...]]
       [-X box] [-x] file_1 file_2 file_3

DESCRIPTION

       ncbo  subtracts  variables in file_2 from the corresponding variables (those with the same
       name) in file_1 and stores the results in file_3.  Variables in file_2  are  broadcast  to
       conform  to  the  corresponding  variable in file_1 if necessary.  Broadcasting a variable
       means creating data in non-existing dimensions from the data in existing dimensions.   For
       example, a two dimensional variable in file_2 can be subtracted from a four, three, or two
       (but not  one  or  zero)  dimensional  variable  (of  the  same  name)  in  file_1.   This
       functionality  allows the user to compute anomalies from the mean.  Note that variables in
       file_1 are not broadcast to conform to the dimensions in file_2.  Thus, ncbo,  the  number
       of  dimensions, or rank, of any processed variable in file_1 must be greater than or equal
       to the rank of the same variable in file_2.   Furthermore,  the  size  of  all  dimensions
       common to both file_1 and file_2 must be equal.

       When  computing  anomalies  from  the mean it is often the case that file_2 was created by
       applying an averaging operator to a file with the same dimensions as file_1, if not file_1
       itself.   In  these  cases, creating file_2 with ncra rather than ncwa will cause the ncbo
       operation to fail.  For concreteness say the record  dimension  in  file_1  is  time.   If
       file_2  were  created  by  averaging file_1 over the time dimension with the ncra operator
       rather than with the ncwa operator, then file_2 will have  a  time  dimension  of  size  1
       rather  than  having no time dimension at all In this case the input files to ncbo, file_1
       and file_2, will have unequally sized time dimensions  which  causes  ncbo  to  fail.   To
       prevent this from occuring, use ncwa to remove the time dimension from file_2.  An example
       is given below.

       ncbo will never difference coordinate variables or variables of type NC_CHAR  or  NC_BYTE.
       This  ensures  that  coordinates  like  (e.g.,  latitude  and  longitude)  are  physically
       meaningful in the output file, file_3.  This behavior is hardcoded.  ncbo applies  special
       rules  to  some  NCAR  CSM  fields  (e.g.,  ORO).  See NCAR CSM Conventions for a complete
       description.  Finally, we note that ncflint (ncflint netCDF File Interpolator) can be also
       perform file subtraction (as well as addition, multiplication and interpolation).

EXAMPLES

       Say files 85_0112.nc and 86_0112.nc each contain 12 months of data.  Compute the change in
       the monthly averages from 1985 to 1986:
              ncbo 86_0112.nc 85_0112.nc 86m85_0112.nc

       The following examples demonstrate the broadcasting feature  of  ncbo.   Say  we  wish  to
       compute  the monthly anomalies of T from the yearly average of T for the year 1985.  First
       we create the 1985 average from  the  monthly  data,  which  is  stored  with  the  record
       dimension time.
              ncra 85_0112.nc 85.nc
              ncwa -O -a time 85.nc 85.nc
       The  second  command,  ncwa,  gets  rid  of the time dimension of size 1 that ncra left in
       85.nc.  Now none of the variables in 85.nc  has  a  time  dimension.   A  quicker  way  to
       accomplish this is to use ncwa from the beginning:
              ncwa -a time 85_0112.nc 85.nc
       We are now ready to use ncbo to compute the anomalies for 1985:
              ncbo -v T 85_0112.nc 85.nc t_anm_85_0112.nc
       Each  of  the  12 records in t_anm_85_0112.nc now contains the monthly deviation of T from
       the annual mean of T for each gridpoint.

       Say we wish to compute the monthly gridpoint anomalies from  the  zonal  annual  mean.   A
       zonal  mean  is  a quantity that has been averaged over the longitudinal (or x) direction.
       First we use ncwa to average over longitudinal direction  lon,  creating  xavg_85.nc,  the
       zonal mean of 85.nc.  Then we use ncbo to subtract the zonal annual means from the monthly
       gridpoint data:
              ncwa -a lon 85.nc xavg_85.nc
              ncbo 85_0112.nc xavg_85.nc tx_anm_85_0112.nc
       Assuming 85_0112.nc has dimensions time and lon, this example only works if xavg_85.nc has
       no time or lon dimension.

       As  a  final  example,  say we have five years of monthly data (i.e., 60 months) stored in
       8501_8912.nc and we wish to create a file which contains the twelve month  seasonal  cycle
       of the average monthly anomaly from the five-year mean of this data.  The following method
       is just one permutation of many which will accomplish the same result.  First use ncwa  to
       create the file containing the five-year mean:
              ncwa -a time 8501_8912.nc 8589.nc
       Next  use  ncbo  to  create a file containing the difference of each month's data from the
       five-year mean:
              ncbo 8501_8912.nc 8589.nc t_anm_8501_8912.nc
       Now use ncks to group the five January anomalies together in one file,  and  use  ncra  to
       create  the average anomaly for all five Januarys.  These commands are embedded in a shell
       loop so they are repeated for all twelve months:
              foreach idx (01 02 03 04 05 06 07 08 09 10 11 12)
              ncks -F -d time,,,12 t_anm_8501_8912.nc foo.
              ncra foo. t_anm_8589_.nc
              end
       Note that ncra understands the stride argument so the two commands inside the loop may  be
       combined into the single command
              ncra -F -d time,,,12 t_anm_8501_8912.nc foo.
       Finally,  use  ncrcat to concatenate the 12 average monthly anomaly files into one twelve-
       record file which contains the entire seasonal cycle of the monthly anomalies:
              ncrcat t_anm_8589_??.nc t_anm_8589_0112.nc

AUTHOR

       NCO manual pages written by Charlie Zender and originally formatted by Brian Mays.

REPORTING BUGS

       Report bugs to <http://sf.net/bugs/?group_id=3331>.

COPYRIGHT

       Copyright © 1995-2010 Charlie Zender
       This is free software; see the source for copying conditions.  There is NO  warranty;  not
       even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

SEE ALSO

       The  full  documentation  for  NCO is maintained as a Texinfo manual called the NCO User's
       Guide.  Because NCO is mathematical in nature, the  documentation  includes  TeX-intensive
       portions   not  viewable  on  character-based  displays.   Hence  the  only  complete  and
       authoritative versions of the NCO  User's  Guide  are  the  PDF  (recommended),  DVI,  and
       Postscript   versions  at  <http://nco.sf.net/nco.pdf>,  <http://nco.sf.net/nco.dvi>,  and
       <http://nco.sf.net/nco.ps>,  respectively.   HTML  and  XML  versions  are  available   at
       <http://nco.sf.net/nco.html> and <http://nco.sf.net/nco.xml>, respectively.

       If the info and NCO programs are properly installed at your site, the command

              info nco

       should give you access to the complete manual, except for the TeX-intensive portions.

HOMEPAGE

       The NCO homepage at <http://nco.sf.net> contains more information.

                                                                                          NCBO(1)