1NCBO(1) General Commands Manual NCBO(1)
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6 ncbo - netCDF Binary Operator
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9 ncbo [-3] [-4] [-5] [-6] [-7] [-A] [--bfr sz_byt][-C][-c] [--cnk_byt
10 sz_byt][--cnk_csh sz_byt][--cnk_dmn nm,sz_lmn] [--cnk_map map]
11 [--cnk_min sz_byt] [--cnk_plc plc] [--cnk_scl sz_lmn][-D dbg_lvl] [-d
12 dim,[ min][,[ max]]] [-F] [--fl_fmt=fmt] [-G gpe_dsc] [-g grp[,...]]
13 [--glb att_name= att_val]] [-h] [--hdf] [--hdr_pad sz_byt][--hpss_try]
14 [-L dfl_lvl] [-l path] [--msa] [--no_cll_msr] [--no_frm_trm]
15 [--no_tmp_fl] [-O] [-p path] [-R] [-r] [--ram_all] [-t thr_nbr] [--unn]
16 [-v var[,...]] [-X box] [-x] file_1 file_2 file_3
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19 ncbo subtracts variables in file_2 from the corresponding variables
20 (those with the same name) in file_1 and stores the results in file_3.
21 Variables in file_2 are broadcast to conform to the corresponding vari‐
22 able in file_1 if necessary. Broadcasting a variable means creating
23 data in non-existing dimensions from the data in existing dimensions.
24 For example, a two dimensional variable in file_2 can be subtracted
25 from a four, three, or two (but not one or zero) dimensional variable
26 (of the same name) in file_1. This functionality allows the user to
27 compute anomalies from the mean. Note that variables in file_1 are not
28 broadcast to conform to the dimensions in file_2. Thus, ncbo, the num‐
29 ber of dimensions, or rank, of any processed variable in file_1 must be
30 greater than or equal to the rank of the same variable in file_2. Fur‐
31 thermore, the size of all dimensions common to both file_1 and file_2
32 must be equal.
33 When computing anomalies from the mean it is often the case that file_2
35 was created by applying an averaging operator to a file with the same
36 dimensions as file_1, if not file_1 itself. In these cases, creating
37 file_2 with ncra rather than ncwa will cause the ncbo operation to
38 fail. For concreteness say the record dimension in file_1 is time. If
39 file_2 were created by averaging file_1 over the time dimension with
40 the ncra operator rather than with the ncwa operator, then file_2 will
41 have a time dimension of size 1 rather than having no time dimension at
42 all In this case the input files to ncbo, file_1 and file_2, will have
43 unequally sized time dimensions which causes ncbo to fail. To prevent
44 this from occurring, use ncwa to remove the time dimension from file_2.
45 An example is given below.
46 ncbo will never difference coordinate variables or variables of type
48 NC_CHAR or NC_BYTE. This ensures that coordinates like (e.g., latitude
49 and longitude) are physically meaningful in the output file, file_3.
50 This behavior is hardcoded. ncbo applies special rules to some NCAR
51 CSM fields (e.g., ORO). See NCAR CSM Conventions for a complete
52 description. Finally, we note that ncflint (ncflint netCDF File Inter‐
53 polator) can be also perform file subtraction (as well as addition,
54 multiplication and interpolation).
55
57 Say files 85_0112.nc and 86_0112.nc each contain 12 months of data.
58 Compute the change in the monthly averages from 1985 to 1986:
59 ncbo 86_0112.nc 85_0112.nc 86m85_0112.nc
60 The following examples demonstrate the broadcasting feature of ncbo.
62 Say we wish to compute the monthly anomalies of T from the yearly aver‐
63 age of T for the year 1985. First we create the 1985 average from the
64 monthly data, which is stored with the record dimension time.
65 ncra 85_0112.nc 85.nc
66 ncwa -O -a time 85.nc 85.nc
67 The second command, ncwa, gets rid of the time dimension of size 1 that
68 ncra left in 85.nc. Now none of the variables in 85.nc has a time
69 dimension. A quicker way to accomplish this is to use ncwa from the
70 beginning:
71 ncwa -a time 85_0112.nc 85.nc
72 We are now ready to use ncbo to compute the anomalies for 1985:
73 ncbo -v T 85_0112.nc 85.nc t_anm_85_0112.nc
74 Each of the 12 records in t_anm_85_0112.nc now contains the monthly
75 deviation of T from the annual mean of T for each gridpoint.
76 Say we wish to compute the monthly gridpoint anomalies from the zonal
78 annual mean. A zonal mean is a quantity that has been averaged over
79 the longitudinal (or x) direction. First we use ncwa to average over
80 longitudinal direction lon, creating xavg_85.nc, the zonal mean of
81 85.nc. Then we use ncbo to subtract the zonal annual means from the
82 monthly gridpoint data:
83 ncwa -a lon 85.nc xavg_85.nc
84 ncbo 85_0112.nc xavg_85.nc tx_anm_85_0112.nc
85 Assuming 85_0112.nc has dimensions time and lon, this example only
86 works if xavg_85.nc has no time or lon dimension.
87 As a final example, say we have five years of monthly data (i.e., 60
89 months) stored in 8501_8912.nc and we wish to create a file which con‐
90 tains the twelve month seasonal cycle of the average monthly anomaly
91 from the five-year mean of this data. The following method is just one
92 permutation of many which will accomplish the same result. First use
93 ncwa to create the file containing the five-year mean:
94 ncwa -a time 8501_8912.nc 8589.nc
95 Next use ncbo to create a file containing the difference of each
96 month's data from the five-year mean:
97 ncbo 8501_8912.nc 8589.nc t_anm_8501_8912.nc
98 Now use ncks to group the five January anomalies together in one file,
99 and use ncra to create the average anomaly for all five Januarys.
100 These commands are embedded in a shell loop so they are repeated for
101 all twelve months:
102 foreach idx (01 02 03 04 05 06 07 08 09 10 11 12)
103 ncks -F -d time,,,12 t_anm_8501_8912.nc foo.
104 ncra foo. t_anm_8589_.nc
105 end
106 Note that ncra understands the stride argument so the two commands
107 inside the loop may be combined into the single command
108 ncra -F -d time,,,12 t_anm_8501_8912.nc foo.
109 Finally, use ncrcat to concatenate the 12 average monthly anomaly files
110 into one twelve-record file which contains the entire seasonal cycle of
111 the monthly anomalies:
112 ncrcat t_anm_8589_??.nc t_anm_8589_0112.nc
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116 NCO manual pages written by Charlie Zender and originally formatted by
117 Brian Mays.
118
121 Report bugs to <http://sf.net/bugs/?group_id=3331>.
122
125 Copyright © 1995-2018 Charlie Zender
126 This is free software; see the source for copying conditions. There is
127 NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
128 PURPOSE.
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132 The full documentation for NCO is maintained as a Texinfo manual called
133 the NCO Users Guide. Because NCO is mathematical in nature, the docu‐
134 mentation includes TeX-intensive portions not viewable on character-
135 based displays. Hence the only complete and authoritative versions of
136 the NCO Users Guide are the PDF (recommended), DVI, and Postscript ver‐
137 sions at <http://nco.sf.net/nco.pdf>, <http://nco.sf.net/nco.dvi>, and
138 <http://nco.sf.net/nco.ps>, respectively. HTML and XML versions are
139 available at <http://nco.sf.net/nco.html> and
140 <http://nco.sf.net/nco.xml>, respectively.
141 If the info and NCO programs are properly installed at your site, the
143 command
144 info nco
146 should give you access to the complete manual, except for the TeX-
148 intensive portions.
149
152 The NCO homepage at <http://nco.sf.net> contains more information.
153 NCBO(1)