ncwa - Online in the Cloud

PROGRAM:

NAME

ncwa - netCDF Weighted Averager

SYNTAX

ncwa [-3] [-4] [-6] [-7] [-A] [-a dim[,...]] [-B mask_cond] [-b] [--bfr
sz][-C][-c][--cnk_byt sz][--cnk_dmn nm,sz] [--cnk_map map] [--cnk_min sz] [--cnk_plc plc]
[--cnk_scl sz][-D dbg_lvl] [-d dim,[ min][,[ max]]] [--dbl|flt] [-F] [-G gpe_dsc] [-g
grp[,...]] [--glb att_name= att_val]] [-h] [--hdf] [--hdr_pad sz] [-I] [-L dfl_lvl][-l
path] [-M val] [-m mask] [-N] [--no_cll_mth] [--no_tmp_fl] [-O] [-o output-file] [-p path]
[--ppc var1[, var2[,...]]= prc]] [-R] [-r] [--ram_all] [-T mask_comp] [-t thr_nbr] [--unn]
[-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.

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