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] [--cmp
10 cmp_sng] [--cnk_byt sz_byt][--cnk_csh sz_byt][--cnk_dmn nm,sz_lmn]
11 [--cnk_map map] [--cnk_min sz_byt] [--cnk_plc plc] [--cnk_scl
12 sz_lmn][-D dbg_lvl] [-d dim,[ min][,[ max]]] [-F] [--fl_fmt=fmt] [-G
13 gpe_dsc] [-g grp[,...]] [--glb att_name= att_val]] [-h] [--hdf]
14 [--hdr_pad sz_byt][--hpss_try] [-L dfl_lvl] [-l path] [--msa]
15 [--no_cll_msr] [--no_frm_trm] [--no_tmp_fl] [-O] [-p path] [-R] [-r]
16 [--ram_all] [-t thr_nbr] [--uio] [--unn] [-v var[,...]] [-X box] [-x]
17 file_1 file_2 file_3
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20 ncbo subtracts variables in file_2 from the corresponding variables
21 (those with the same name) in file_1 and stores the results in file_3.
22 Variables in file_2 are broadcast to conform to the corresponding vari‐
23 able in file_1 if necessary. Broadcasting a variable means creating
24 data in non-existing dimensions from the data in existing dimensions.
25 For example, a two dimensional variable in file_2 can be subtracted
26 from a four, three, or two (but not one or zero) dimensional variable
27 (of the same name) in file_1. This functionality allows the user to
28 compute anomalies from the mean. Note that variables in file_1 are not
29 broadcast to conform to the dimensions in file_2. Thus, ncbo, the num‐
30 ber of dimensions, or rank, of any processed variable in file_1 must be
31 greater than or equal to the rank of the same variable in file_2. Fur‐
32 thermore, the size of all dimensions common to both file_1 and file_2
33 must be equal.
34 When computing anomalies from the mean it is often the case that file_2
36 was created by applying an averaging operator to a file with the same
37 dimensions as file_1, if not file_1 itself. In these cases, creating
38 file_2 with ncra rather than ncwa will cause the ncbo operation to
39 fail. For concreteness say the record dimension in file_1 is time. If
40 file_2 were created by averaging file_1 over the time dimension with
41 the ncra operator rather than with the ncwa operator, then file_2 will
42 have a time dimension of size 1 rather than having no time dimension at
43 all In this case the input files to ncbo, file_1 and file_2, will have
44 unequally sized time dimensions which causes ncbo to fail. To prevent
45 this from occurring, use ncwa to remove the time dimension from file_2.
46 An example is given below.
47 ncbo will never difference coordinate variables or variables of type
49 NC_CHAR or NC_BYTE. This ensures that coordinates like (e.g., latitude
50 and longitude) are physically meaningful in the output file, file_3.
51 This behavior is hardcoded. ncbo applies special rules to some NCAR
52 CSM fields (e.g., ORO). See NCAR CSM Conventions for a complete de‐
53 scription. Finally, we note that ncflint (ncflint netCDF File Interpo‐
54 lator) can be also perform file subtraction (as well as addition, mul‐
55 tiplication and interpolation).
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58 Say files 85_0112.nc and 86_0112.nc each contain 12 months of data.
59 Compute the change in the monthly averages from 1985 to 1986:
60 ncbo 86_0112.nc 85_0112.nc 86m85_0112.nc
61 The following examples demonstrate the broadcasting feature of ncbo.
63 Say we wish to compute the monthly anomalies of T from the yearly aver‐
64 age of T for the year 1985. First we create the 1985 average from the
65 monthly data, which is stored with the record dimension time.
66 ncra 85_0112.nc 85.nc
67 ncwa -O -a time 85.nc 85.nc
68 The second command, ncwa, gets rid of the time dimension of size 1 that
69 ncra left in 85.nc. Now none of the variables in 85.nc has a time di‐
70 mension. A quicker way to accomplish this is to use ncwa from the be‐
71 ginning:
72 ncwa -a time 85_0112.nc 85.nc
73 We are now ready to use ncbo to compute the anomalies for 1985:
74 ncbo -v T 85_0112.nc 85.nc t_anm_85_0112.nc
75 Each of the 12 records in t_anm_85_0112.nc now contains the monthly de‐
76 viation of T from the annual mean of T for each gridpoint.
77 Say we wish to compute the monthly gridpoint anomalies from the zonal
79 annual mean. A zonal mean is a quantity that has been averaged over
80 the longitudinal (or x) direction. First we use ncwa to average over
81 longitudinal direction lon, creating xavg_85.nc, the zonal mean of
82 85.nc. Then we use ncbo to subtract the zonal annual means from the
83 monthly gridpoint data:
84 ncwa -a lon 85.nc xavg_85.nc
85 ncbo 85_0112.nc xavg_85.nc tx_anm_85_0112.nc
86 Assuming 85_0112.nc has dimensions time and lon, this example only
87 works if xavg_85.nc has no time or lon dimension.
88 As a final example, say we have five years of monthly data (i.e., 60
90 months) stored in 8501_8912.nc and we wish to create a file which con‐
91 tains the twelve month seasonal cycle of the average monthly anomaly
92 from the five-year mean of this data. The following method is just one
93 permutation of many which will accomplish the same result. First use
94 ncwa to create the file containing the five-year mean:
95 ncwa -a time 8501_8912.nc 8589.nc
96 Next use ncbo to create a file containing the difference of each
97 month's data from the five-year mean:
98 ncbo 8501_8912.nc 8589.nc t_anm_8501_8912.nc
99 Now use ncks to group the five January anomalies together in one file,
100 and use ncra to create the average anomaly for all five Januarys.
101 These commands are embedded in a shell loop so they are repeated for
102 all twelve months:
103 foreach idx (01 02 03 04 05 06 07 08 09 10 11 12)
104 ncks -F -d time,,,12 t_anm_8501_8912.nc foo.
105 ncra foo. t_anm_8589_.nc
106 end
107 Note that ncra understands the stride argument so the two commands in‐
108 side the loop may be combined into the single command
109 ncra -F -d time,,,12 t_anm_8501_8912.nc foo.
110 Finally, use ncrcat to concatenate the 12 average monthly anomaly files
111 into one twelve-record file which contains the entire seasonal cycle of
112 the monthly anomalies:
113 ncrcat t_anm_8589_??.nc t_anm_8589_0112.nc
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117 NCO manual pages written by Charlie Zender and originally formatted by
118 Brian Mays.
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122 Report bugs to <http://sf.net/bugs/?group_id=3331>.
123
126 Copyright © 1995-present Charlie Zender
127 This is free software; see the source for copying conditions. There is
128 NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
129 PURPOSE.
130
133 The full documentation for NCO is maintained as a Texinfo manual called
134 the NCO Users Guide. Because NCO is mathematical in nature, the docu‐
135 mentation includes TeX-intensive portions not viewable on character-
136 based displays. Hence the only complete and authoritative versions of
137 the NCO Users Guide are the PDF (recommended), DVI, and Postscript ver‐
138 sions at <http://nco.sf.net/nco.pdf>, <http://nco.sf.net/nco.dvi>, and
139 <http://nco.sf.net/nco.ps>, respectively. HTML and XML versions are
140 available at <http://nco.sf.net/nco.html> and
141 <http://nco.sf.net/nco.xml>, respectively.
142 If the info and NCO programs are properly installed at your site, the
144 command
145 info nco
147 should give you access to the complete manual, except for the TeX-in‐
149 tensive portions.
150
153 The NCO homepage at <http://nco.sf.net> contains more information.
154 NCBO(1)