NCO (NetCDF Operators)

You can merge netcdf files with the nco package. NCO is a set of linux command line utilities for performing common operations on netcdf files. This is useful for mergeing a set of files such as:

Concatenating files, creating a new dimension in the process

To concatenate files, use ncecat:

ncecat *nc -O

This will merge all the netcdf files in a folder, creating a new record dimension if one does not exist. The record dimension is often the time dimension, for example if you have a set of netCDF files, with each one representing some spatial field at a given timestep. If appropriate, you can rename this record dimension to something more useful using the ncrename utility (another utility in the NCO package).

Renaming dimensions

ncrename -d record,time

The -d flag specifies that we are going to rename the dimension in the netcdf file., from “record” to “time”. There are also flags to rename other attributes, see the ncrename manual page

Removing degenerate dimensions

To remove degenerate dimensions (by averaging over the dimension to be removed):

ncwa -a dim_name

ncwa (“Weighted average”) will average variables over the specified dimension. If our dimension is degenerate (dim = 1), then this is effectively a way to remove that dimension without changing any of the variable data (Since it is averaging the variable over 1).

Adding a new variable

If we need to add a new variable, this can be done with ncap2 (ncap is deprecated).

ncap2 -s'new_dim[$new_dim]=1234'

Note that this will add a single value of the time variable: 1234.

Changing a variable to vary over a newly added dimension

If we add a new dimension, the existing variables will not automatically be functions of this new dimension. So if we were to add a time dimension, we need to recreate our variables to remap over this new dimension (assuming this is correct and appropriate for that particular variable/dimension combination.)

ncap2 -s 'Var_new[$dim1, $dim2, $new_dim3]=Var_old'

Further nco utilities

The available utilities with nco are:

The NCO utilities are

  • ncap2 - arithmetic processor
  • ncatted - attribute editor
  • ncbo - binary operator
  • ncdiff - differencer
  • ncea - ensemble averager
  • ncecat - ensemble concatenator
  • ncflint - file interpolator
  • ncks - kitchen sink (extract, cut, paste, print data)
  • ncpdq - permute dimensions quickly
  • ncra - running averager
  • ncrcat - record concatenator
  • ncrename - renamer
  • ncwa - weighted averager

CDO (Climate Data Operators)

This is an equally capable set of netCDF tools written by the Max Planck Institute for Meteorology.

CDO tools page