NCDIFF(1)						      General Commands Manual							 NCDIFF(1)

NAME

       ncdiff - netCDF Differencer

SYNTAX

       ncdiff  [-3]  [-4]  [-6]  [-A]  [-C]  [-c]  [-D dbg] [-d dim,[ min][,[ max]]] [-F] [-h] [-L dfl_lvl] [-l path] [-O] [-p path] [-R] [-r] [-v
       var[,...]]  [-x] file_1 file_2 file_3

DESCRIPTION

       ncdiff 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,  ncdiff, 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 ncdiff opera-
       tion 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 ncdiff, file_1 and file_2, will have unequally sized  time  dimensions  which  causes
       ncdiff to fail.	To prevent this from occuring, use ncwa to remove the time dimension from file_2.  An example is given below.

       ncdiff will never difference coordinate variables or variables of type NC_CHAR or NC_BYTE.  This ensures that coordinates like (e.g., lati-
       tude and longitude) are physically meaningful in the output file, file_3.  This behavior is hardcoded.  ncdiff  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:
	      ncdiff 86_0112.nc 85_0112.nc 86m85_0112.nc

       The following examples demonstrate the broadcasting feature of ncdiff.  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 ncdiff to compute the anomalies for 1985:
	      ncdiff -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 ncdiff to subtract the zonal annual means from the monthly gridpoint data:
	      ncwa -a lon 85.nc xavg_85.nc
	      ncdiff 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 con-
       tains  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 ncdiff to create a file containing the difference of each month's data from the five-year mean:
	      ncdiff 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 Brian Mays.

REPORTING BUGS

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

COPYRIGHT

       Copyright (C) 1995-2011 Charlie Zender
       This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICU-
       LAR 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.

																	 NCDIFF(1)