NAME
ncea - netCDF Ensemble Averager
SYNTAX
ncea [-3] [-4] [-6] [-A] [-C] [-c] [-D dbg] [-d dim,[ min][,[ max]]]
[-F] [-h] [-L dfl_lvl] [-l path] [-n loop] [-O] [-p path] [-R] [-r] [-t
thr_nbr] [-v var[,...]] [-X box] [-x] [-y op_typ] input-files output-
file
DESCRIPTION
ncea performs gridpoint averages of variables across an arbitrary
number (an ensemble) of input files, with each file receiving an equal
weight in the average. Each variable in the output-file will be the
same size as the same variable in any one of the in the input-files,
and all input-files must be the same size. Whereas ncra only performs
averages over the record dimension (e.g., time), and weights each
record in the record dimension evenly, ncea averages entire files, and
weights each file evenly. All dimensions, including the record
dimension, are treated identically and preserved in the output-file.
The file is the logical unit of organization for the results of many
scientific studies. Often one wishes to generate a file which is the
gridpoint average of many separate files. This may be to reduce
statistical noise by combining the results of a large number of
experiments, or it may simply be a step in a procedure whose goal is to
compute anomalies from a mean state. In any case, when one desires to
generate a file whose properties are the mean of all the input files,
then ncea is the operator to use. ncea assumes coordinate variable are
properties common to all of the experiments and so does not average
them across files. Instead, ncea copies the values of the coordinate
variables from the first input file to the output file.
EXAMPLES
Consider a model experiment which generated five realizations of one
year of data, say 1985. You can imagine that the experimenter slightly
perturbs the initial conditions of the problem before generating each
new solution. Assume each file contains all twelve months (a seasonal
cycle) of data and we want to produce a single file containing the
ensemble average (mean) seasonal cycle. Here the numeric filename
suffix denotes the experiment number (not the month):
ncea 85_01.nc 85_02.nc 85_03.nc 85_04.nc 85_05.nc 85.nc
ncea 85_0[1-5].nc 85.nc
ncea -n 5,2,1 85_01.nc 85.nc
These three commands produce identical answers. The output file,
85.nc, is the same size as the inputs files. It contains 12 months of
data (which might or might not be stored in the record dimension,
depending on the input files), but each value in the output file is the
average of the five values in the input files.
In the previous example, the user could have obtained the ensemble
average values in a particular spatio-temporal region by adding a
hyperslab argument to the command, e.g.,
ncea -d time,0,2 -d lat,-23.5,23.5 85_??.nc 85.nc
In this case the output file would contain only three slices of data in
the time dimension. These three slices are the average of the first
three slices from the input files. Additionally, only data inside the
tropics is included.
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 © 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.