Setting up a new configuration

Starting from an existing configuration

There are three options to build a new configuration from an existing one.

Option 1: Duplicate an existing configuration

The NEMO so-called Reference Configurations cover a number of major features for NEMO setup (global, regional, 1D, using embedded zoom with AGRIF…)

One can create a new configuration by duplicating one of the reference configurations (ORCA2_ICE_PISCES in the following example)

$ ./makenemo –n 'ORCA2_ICE_PISCES_MINE' -r 'ORCA2_ICE_PISCES' -m 'my_arch'

Option 2: Duplicate with differences

Create and compile a new configuration based on a reference configuration (ORCA2_ICE_PISCES in the following example) but with different pre-processor options. For this either add add_key or del_key keys as required; e.g.

$ ./makenemo –n 'ORCA2_ICE_PISCES_MINE' -r 'ORCA2_ICE_PISCES' -m 'my_arch' del_key 'key_xios' add_key 'key_diahth'

Option 3: Use the SIREN tools to subset an existing model

Define a regional configuration which is a {sub,super}-set of an existing configuration.

This last option employs the SIREN software tools that are included in the standard distribution. The software is written in Fortran 95 and available in the ./tools/SIREN directory. SIREN allows you to create your own regional configuration embedded in a wider one.

SIREN is a set of programs to create all the input files you need to run a NEMO regional configuration.


Set of GLORYS files (GLObal ReanalYSis on the ORCA025 grid), as well as examples of namelists are available here.



Any questions or comments regarding the use of SIREN should be posted in the corresponding forum.

Option 4: Use the nesting tools to create embedded zooms or regional configurations from an existing grid


Creating a completely new configuration

From NEMO version 4.0 there are two ways to build configurations from scratch. The appropriate method to use depends largely on the target configuration. Method 1 is for more complex/realistic global or regional configurations and method 2 is intended for simpler, idealised configurations whose domains and characteristics can be described in simple geometries and formulae.

Option 1: Create and use a domain configuration file

This method is used by each of the reference configurations, so that downloading their input files linked to their description can help. Although starting from scratch, it is advisable to create the directory structure to house your new configuration by duplicating the closest reference configuration to your target application. For example, if your application requires both ocean ice and passive tracers, then use the ORCA2_ICE_PISCES as template, and execute following command to build your MY_NEW_CONFIG configuration:

$ ./makenemo –n 'MY_NEW_CONFIG' -r 'ORCA2_ICE_PISCES' -m 'my_arch'

where MY_NEW_CONFIG can be substituted with a suitably descriptive name for your new configuration.

The purpose of this step is simply to create and populate the appropriate WORK, MY_SRC and EXP00 subdirectories for your new configuration. Other choices for the base reference configuration might be


If your target application is ocean-only


If your target application is regional with open boundaries

All the domain information for your new configuration will be contained within a netcdf file called which you will need to create and place in the ./cfgs/MY_NEW_CONFIG/EXP00 sub-directory. Firstly though, ensure that your configuration is set to use such a file by checking that

ln_read_cfg = .true.

in ./cfgs/MY_NEW_CONFIG/EXP00/namelist_cfg

Create the file which must contain the following fields

/* configuration name, configuration resolution                 */
int    ORCA, ORCA_index
/* lateral global domain b.c.                                   */
int    Iperio, Jperio, NFoldT, NFoldF
/* flags for z-coord, z-coord with partial steps and s-coord    */
int    ln_zco, ln_zps, ln_sco
/* flag  for ice shelf cavities                                 */
int    ln_isfcav
/* geographic position                                          */
double glamt, glamu, glamv, glamf
/* geographic position                                          */
double gphit, gphiu, gphiv, gphif
/* Coriolis parameter (if not on the sphere)                    */
double iff, ff_f, ff_t
/* horizontal scale factors                                     */
double e1t, e1u, e1v, e1f
/* horizontal scale factors                                     */
double e2t, e2u, e2v, e2f
/* U and V surfaces (if grid size reduction in some straits)    */
double ie1e2u_v, e1e2u, e1e2v
/* reference vertical scale factors at T and W points           */
double e3t_1d, e3w_1d
/* vertical scale factors 3D coordinate at T,U,V,F and W points */
double e3t_0, e3u_0, e3v_0, e3f_0, e3w_0
/* vertical scale factors 3D coordinate at UW and VW points     */
double e3uw_0, e3vw_0
/* last wet T-points, 1st wet T-points (for ice shelf cavities) */
int    bottom_level, top_level

There are two options for creating a file:

  • Users can use tools of their own choice to build a with all mandatory fields.

  • Users can adapt and apply the supplied tool available in ./tools/DOMAINcfg. This tool is based on code extracted from NEMO version 3.6 and will allow similar choices for the horizontal and vertical grids that were available internally to that version. See DOMAINcfg tool for details.

Option 2: Adapt the usr_def configuration module of NEMO for you own purposes

This method is intended for configuring easily simple/idealised configurations which are often used as demonstrators or for process evaluation and comparison. This method can be used whenever the domain geometry has a simple mathematical description and the ocean initial state and boundary forcing is described analytically. As a start, consider the case of starting a completely new ocean-only test case based on the LOCK_EXCHANGE example.


We probably need an even more basic example than this with only one namelist and minimal changes to the usrdef modules

Firstly, construct the directory structure, starting in the cfgs directory:

$ ./makenemo -n 'MY_NEW_TEST' -t 'LOCK_EXCHANGE' -m 'my_arch'

where the -t option has been used to locate the new configuration in the tests subdirectory (it is recommended practice to keep full configurations and idealised cases clearly distinguishable). This command will create (amongst others) the following files and directories:


context_nemo.xml  domain_def_nemo.xml  field_def_nemo-oce.xml  file_def_nemo-oce.xml  iodef.xml
namelist_cfg      namelist_ref

usrdef_hgr.F90  usrdef_nam.F90  usrdef_zgr.F90  usrdef_istate.F90  usrdef_sbc.F90  zdfini.F90

The key to setting up an idealised configuration lies in adapting a small set of short Fortran 90 modules which should be dropped into the MY_SRC directory. Here the LOCK_EXCHANGE example is using 5 such routines but the full set that is available in the src/OCE/USR directory is:

usrdef_closea.F90  usrdef_fmask.F90  usrdef_hgr.F90  usrdef_istate.F90
usrdef_nam.F90     usrdef_sbc.F90    usrdef_zgr.F90

Before discussing these in more detail it is worth noting the various namelist controls that engage the different user-defined aspects. These controls are set using two new logical switches or are implied by the settings of existing ones. For example, the mandatory requirement for an idealised configuration is to provide routines which define the horizontal and vertical domains. Templates for these are provided in the usrdef_hgr.F90 and usrdef_zgr.F90 modules. The application of these modules is activated whenever:

ln_read_cfg = .false.

in any configuration’s namelist_cfg file. This setting also activates the reading of an optional &nam_usrdef namelist which can be used to supply configuration specific settings. These need to be declared and read in the usrdef_nam.F90 module.

Another explicit control is available in the &namsbc namelist which activates the use of analytical forcing. With

ln_usr = .true.

Other usrdef modules are activated by less explicit means. For example, code in usrdef_istate.F90 is used to define initial temperature and salinity fields if

ln_tsd_init   = .false.

in the &namtsd namelist. The remaining modules, namely usrdef_closea.F90 usrdef_fmask.F90 are specific to ORCA configurations and set local variations of some specific fields for the various resolutions of the global models. They do not need to be considered here in the context of idealised cases but it is worth noting that all configuration specific code has now been isolated in the usrdef modules. In the case of these last two modules, they are activated only if an ORCA configuration is detected. Currently, this requires a specific integer variable named ORCA to be set in a file.


This would be less confusing if the cn_cfg string is read directly as a character attribue from the

So, in most cases, the set up of idealised model configurations can be completed by copying the template routines from ./src/OCE/USR into your new ./cfgs/MY_NEW_TEST/MY_SRC directory and editing the appropriate modules as needed. The default set are those used for the GYRE reference configuration. The contents of MY_SRC directories from other idealised configurations may provide more convenient templates if they share common characteristics with your target application.

Whatever the starting point, it should not require too many changes or additional lines of code to produce routines in ./src/OCE/USR that define analytically the domain, the initial state and the surface boundary conditions for your new configuration.

To summarize, the base set of modules is:


Define horizontal grid


Define vertical grid


Provides at each time-step the surface boundary condition, i.e. the momentum, heat and freshwater fluxes


Defines initialization of the dynamics and tracers


Configuration-specific namelist processing to set any associated run-time parameters

with two specialised ORCA modules (not related to idealised configurations but used to isolate configuration specific code that is used in ORCA2 reference configurations and established global configurations using the ORCA tripolar grid):


only used in ORCA configurations for alteration of f-point land/ocean mask in some straits


only used in ORCA configurations for specific treatments associated with closed seas

From version 4.0, the NEMO release includes a tests subdirectory containing available and up to date test cases build by the community. These will not be fully supported as are NEMO reference configurations, but should provide a source of raw material.