Computing filter increments

Note

This document is written as if your experiment was run with single_file_out = .true.. The potential permutations of filenames output by filter is enormous, so it isn’t feasible to write documentation for all possible cases.

After filter executes without error and produces an obs_seq.final file, a preassim.nc file, and an analysis.nc file, the first questions to ask are:

  1. Is the model state output from filter different from the input?

  2. Were any observations successfully assimilated?

You can check check if the output model state data was changed by the assimilation by using the ncdiff tool to create a file containing the difference of the preassim.nc and analysis.nc files. If you are running with single_file_in = .true. and single_file_out = .true. use ncdiff on the files output for the analysis and preassim stages:

$ ncdiff analysis.nc preassim.nc increments.nc

Otherwise, if you are running with single_file_in = .false. and single_file_out = .false., use ncdiff on the ensemble mean files for the analysis and preassim stages:

$ ncdiff analysis_mean.nc preassim_mean.nc increments.nc

ncdiff generates a file, increments.nc, that contains the increments, or innovations, created by filter. You can view the increments using ncview:

$ ncview increments.nc

to examine the ensemble mean variables. If all values are 0, then the assimilation changed nothing in the state.