Running the observation operator code example

This section describes an example of running the observation operator code using profile data which can be freely downloaded. It shows how to adapt an existing run and build of NEMO to run the observation operator.

1. Compile NEMO with key_ diaobs set.
2. Download some ENSEMBLES EN3 data from http://www.hadobs.org. Choose observations which are valid for the period of your test run because the observation operator compares the model and observations for a matching date and time.
3. Add the following to the NEMO namelist to run the observation operator on this data. Set the enactfiles namelist variable to the observation file name:

/home/ntm/Workspace/NEMO/nemo_v3_6_STABLE/DOC/Namelists/namobs_example

Options are defined through the namobs namelist variables. The options ln_t3d and ln_s3d switch on the temperature and salinity profile observation operator code. The ln_ena switch turns on the reading of ENACT/ENSEMBLES type profile data. The filename or array of filenames are specified using the enactfiles variable. The model grid points for a particular observation latitude and longitude are found using the grid searching part of the code. This can be expensive, particularly for large numbers of observations, setting ln_grid_search_lookup allows the use of a lookup table which is saved into an “xypos“ file (or files). This will need to be generated the first time if it does not exist in the run directory. However, once produced it will significantly speed up future grid searches. Setting ln_grid_global means that the code distributes the observations evenly between processors. Alternatively each processor will work with observations located within the model subdomain (see section 12.3.3). A number of utilities are now provided to plot the feedback files, convert and recombine the files. These are explained in more detail in section 12.5.