--- name: "ncbo" section: "1" platform: "linux" package: "nco" version: "5.3.3-1" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/ncbo" --- # ncbo(1) netCDF Binary Operator ## NAME ncbo - netCDF Binary Operator ## SYNTAX ncbo \[-3\] \[-4\] \[-5\] \[-6\] \[-7\] \[-A\] \[--bfr *sz\_byt*\]*\[-C\]*\[-c\] \[--cmp *cmp\_sng*\] \[--cnk\_byt *sz\_byt*\]*\[--cnk\_csh* *sz\_byt*\]*\[--cnk\_dmn* *nm,sz\_lmn*\] \[--cnk\_map *map*\] \[--cnk\_min *sz\_byt*\] \[--cnk\_plc *plc*\] \[--cnk\_scl *sz\_lmn*\]*\[-D* *dbg\_lvl*\] \[-d *dim*,\[ *min*\]\[,\[ *max*\]\]\] \[-F\] \[--fl\_fmt=fmt\] \[-G *gpe\_dsc*\] \[-g *grp*\[,...\]\] \[--glb *att\_name=* *att\_val*\]\] \[-H\] \[-h\] \[--hdf\] \[--hdr\_pad *sz\_byt*\]*\[--hpss\_try\]* \[-L *dfl\_lvl*\] \[-l *path*\] \[--msa\] \[--no\_cll\_msr\] \[--no\_frm\_trm\] \[--no\_tmp\_fl\] \[-O\] \[-p *path*\] \[--qnt *var1*\[, *var2*\[,...\]\]= *prc*\]\] \[--qnt\_alg *alg\_nm\]* \[-R\] \[-r\] \[--ram\_all\] \[-t *thr\_nbr*\] \[--uio\] \[--unn\] \[-v *var*\[,...\]\] \[-X *box*\] \[-x\] *file\_1* *file\_2* *file\_3* ## DESCRIPTION **ncbo** subtracts variables in *file\_2* from the corresponding variables (those with the same name) in *file\_1* and stores the results in *file\_3*. Variables in *file\_2* are *broadcast* to conform to the corresponding variable in *file\_1* if necessary. Broadcasting a variable means creating data in non-existing dimensions from the data in existing dimensions. For example, a two dimensional variable in *file\_2* can be subtracted from a four, three, or two (but not one or zero) dimensional variable (of the same name) in **file\_1**. This functionality allows the user to compute anomalies from the mean. Note that variables in *file\_1* are *not* broadcast to conform to the dimensions in *file\_2*. Thus, **ncbo**, the number of dimensions, or *rank*, of any processed variable in *file\_1* must be greater than or equal to the rank of the same variable in *file\_2*. Furthermore, the size of all dimensions common to both *file\_1* and *file\_2* must be equal. When computing anomalies from the mean it is often the case that *file\_2* was created by applying an averaging operator to a file with the same dimensions as *file\_1*, if not *file\_1* itself. In these cases, creating *file\_2* with **ncra** rather than **ncwa** will cause the **ncbo** operation to fail. For concreteness say the record dimension in **file\_1** is **time**. If *file\_2* were created by averaging *file\_1* over the **time** dimension with the **ncra** operator rather than with the **ncwa** operator, then *file\_2* will have a **time** dimension of size 1 rather than having no **time** dimension at all In this case the input files to **ncbo**, *file\_1* and *file\_2*, will have unequally sized **time** dimensions which causes **ncbo** to fail. To prevent this from occurring, use **ncwa** to remove the **time** dimension from *file\_2*. An example is given below. **ncbo** will never difference coordinate variables or variables of type **NC\_CHAR** or **NC\_BYTE**. This ensures that coordinates like (e.g., latitude and longitude) are physically meaningful in the output file, *file\_3*. This behavior is hardcoded. **ncbo** applies special rules to some NCAR CSM fields (e.g., **ORO**). See *NCAR CSM Conventions* for a complete description. Finally, we note that **ncflint** (ncflint netCDF File Interpolator) can be also perform file subtraction (as well as addition, multiplication and interpolation). ## EXAMPLES Say files **85\_0112.nc** and **86\_0112.nc** each contain 12 months of data. Compute the change in the monthly averages from 1985 to 1986: ncbo 86\_0112.nc 85\_0112.nc 86m85\_0112.nc The following examples demonstrate the broadcasting feature of **ncbo**. Say we wish to compute the monthly anomalies of **T** from the yearly average of **T** for the year 1985. First we create the 1985 average from the monthly data, which is stored with the record dimension **time**. ncra 85\_0112.nc 85.nc\ ncwa -O -a time 85.nc 85.nc The second command, **ncwa**, gets rid of the **time** dimension of size 1 that **ncra** left in **85.nc**. Now none of the variables in **85.nc** has a **time** dimension. A quicker way to accomplish this is to use **ncwa** from the beginning: ncwa -a time 85\_0112.nc 85.nc We are now ready to use **ncbo** to compute the anomalies for 1985: ncbo -v T 85\_0112.nc 85.nc t\_anm\_85\_0112.nc Each of the 12 records in **t\_anm\_85\_0112.nc** now contains the monthly deviation of **T** from the annual mean of **T** for each gridpoint. Say we wish to compute the monthly gridpoint anomalies from the zonal annual mean. A *zonal mean* is a quantity that has been averaged over the longitudinal (or *x*) direction. First we use **ncwa** to average over longitudinal direction **lon**, creating **xavg\_85.nc**, the zonal mean of **85.nc**. Then we use **ncbo** to subtract the zonal annual means from the monthly gridpoint data: ncwa -a lon 85.nc xavg\_85.nc\ ncbo 85\_0112.nc xavg\_85.nc tx\_anm\_85\_0112.nc Assuming **85\_0112.nc** has dimensions **time** and **lon**, this example only works if **xavg\_85.nc** has no **time** or **lon** dimension. As a final example, say we have five years of monthly data (i.e., 60 months) stored in **8501\_8912.nc** and we wish to create a file which contains the twelve month seasonal cycle of the average monthly anomaly from the five-year mean of this data. The following method is just one permutation of many which will accomplish the same result. First use **ncwa** to create the file containing the five-year mean: ncwa -a time 8501\_8912.nc 8589.nc Next use **ncbo** to create a file containing the difference of each month's data from the five-year mean: ncbo 8501\_8912.nc 8589.nc t\_anm\_8501\_8912.nc Now use **ncks** to group the five January anomalies together in one file, and use **ncra** to create the average anomaly for all five Januarys. These commands are embedded in a shell loop so they are repeated for all twelve months: foreach idx (01 02 03 04 05 06 07 08 09 10 11 12) \ ncks -F -d time,,,12 t\_anm\_8501\_8912.nc foo.\ ncra foo. t\_anm\_8589\_.nc\ end Note that **ncra** understands the **stride** argument so the two commands inside the loop may be combined into the single command ncra -F -d time,,,12 t\_anm\_8501\_8912.nc foo. Finally, use **ncrcat** to concatenate the 12 average monthly anomaly files into one twelve-record file which contains the entire seasonal cycle of the monthly anomalies: ncrcat t\_anm\_8589\_??.nc t\_anm\_8589\_0112.nc ## AUTHOR **NCO** manual pages written by Charlie Zender and originally formatted by Brian Mays. ## REPORTING BUGS Report bugs to \. ## COPYRIGHT Copyright © 1995-present 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 Users 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 Users Guide** are the PDF (recommended), DVI, and Postscript versions at \, \, and \, respectively. HTML and XML versions are available at \ and \, 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 \ contains more information.