NAME
ncwa - netCDF Weighted Averager
SYNTAX
ncwa [-3] [-4] [-6] [-A] [-a dim[,...]] [-B mask_cond] [-b] [-C] [-c]
[-D dbg] [-d dim,[ min][,[ max]]] [-F] [-h] [-I] [-L dfl_lvl][-l path]
[-M val] [-m mask] [-N] [-O] [-o output-file] [-p path] [-R] [-r] [-T
mask_comp] [-t thr_nbr] [-v var[,...]] [-w weight] [-x] [-y op_typ]
input-file output-file
DESCRIPTION
ncwa averages variables in a single file over arbitrary dimensions,
with options to specify weights, masks, and normalization. The default
behavior of ncwa is to arithmetically average every numerical variable
over all dimensions and produce a scalar result. To average variables
over only a subset of their dimensions, specify these dimensions in a
comma-separated list following -a, e.g., -a time,lat,lon. As with all
arithmetic operators, the operation may be restricted to an arbitrary
hypserslab by employing the -d option ncwa also handles values matching
the variable’s _FillValue attribute correctly. Moreover, ncwa
understands how to manipulate user-specified weights, masks, and
normalization options. With these options, ncwa can compute
sophisticated averages (and integrals) from the command line.
mask and weight, if specified, are broadcast to conform to the
variables being averaged. The rank of variables is reduced by the
number of dimensions which they are averaged over. Thus arrays which
are one dimensional in the input-file and are averaged by ncwa appear
in the output-file as scalars. This allows the user to infer which
dimensions may have been averaged. Note that that it is impossible for
ncwa to make make a weight or mask of rank W conform to a var of rank V
if W > V. This situation often arises when coordinate variables
(which, by definition, are one dimensional) are weighted and averaged.
ncwa assumes you know this is impossible and so ncwa does not attempt
to broadcast weight or mask to conform to var in this case, nor does
ncwa print a warning message telling you this, because it is so common.
Specifying dbg > 2 does cause ncwa to emit warnings in these
situations, however.
Non-coordinate variables are always masked and weighted if specified.
Coordinate variables, however, may be treated specially. By default,
an averaged coordinate variable, e.g., latitude, appears in output-file
averaged the same way as any other variable containing an averaged
dimension. In other words, by default ncwa weights and masks
coordinate variables like all other variables. This design decision
was intended to be helpful but for some applications it may be
preferable not to weight or mask coordinate variables just like all
other variables. Consider the following arguments to ncwa: “-a
latitude -w lat_wgt -d latitude,0.,90.” where lat_wgt is a weight in
the latitude dimension. Since, by default ncwa weights coordinate
variables, the value of latitude in the output-file depends on the
weights in lat_wgt and is not likely to be 45.---the midpoint latitude
of the hyperslab. Option -I overrides this default behavior and causes
ncwa not to weight or mask coordinate variables. In the above case,
this causes the value of latitude in the output-file to be 45.---which
is a somewhat appealing result. Thus, -I specifies simple arithmetic
averages for the coordinate variables. In the case of latitude, -I
specifies that you prefer to archive the central latitude of the
hyperslab over which variables were averaged rather than the area
weighted centroid of the hyperslab. Note that the default behavior of
( -I) changed on 1998/12/01---before this date the default was not to
weight or mask coordinate variables. The mathematical definition of
operations involving rank reduction is given above.
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.