7. Initial conditions ‐ pseudo‐restart file - NOC-MSM/SEAsia GitHub Wiki

The most usual way to initialise a regional model is by imposing Temperature and Salinity from a reanalysis product (like copernicus), while the model starts from rest (i.e., current velocities set to zero). This way the model will need a more thorough spin up (in the deeper ocean spin up is of order of centuries while in shelf seas that are shallower the spin up is of order of years to decade).

An alternative approach is to use a soft-restart to initialise your model, where a pseudo-restart file containing Temperature, Salinity Sea Surface Height, and current velocities (U and V) is constructed from a previous model run (e.g., a coarser resolution model) or a reanalysis product.

Example of creating an pseudo-restart file from Copernicus reanalysis

(the example script uses matlab)

1. Download your reanalysis product for the date you want to start your simulation; here we are using GLORYS12 that can be downloaded here: https://resources.marine.copernicus.eu/product-detail/GLOBAL_MULTIYEAR_PHY_001_030

2. Within the matlab script Restart_int_reanalysis.m define your domain configuration name, mask of your domain and the dataset that you want to generate the restart from:

file='domain_cfg.nc';
...
file='mesh_mask.nc';
....
file_data='global-reanalysis-phy-001-030-daily_1637057175950.nc';

3 Run the matlab script Restart_int_reanalysis.m This script will

(i) Use function Int_RESTART_2D_reanalysis.m to conduct a linear horizontal interpolation on the 2D fields (i.e. sea surface height), and also apply flooding and re-masking to "deal" with any inconsistencies in the coastline between the model domain and the reanalysis.

(ii) Use function Int_RESTART_3D_reanalysis.m to conduct a linear horizontal interpolation, and a vertical interpolation that is suitable for interpolating from data on z-level to sigma or hybrid sigma-z level model. Flooding is imposed to temperature and salinity, but there is no flooding for currents to avoid generating any unrealistic currents along the coast and the bottom that can lead to instabilities causing the model to "blow up" before it can slowly adjust to them.

(iii) Save a netcdf file with a "pseudo-restart" compatible to be used with NEMO.

Additional Wisdom / Advice

To run your NEMO model with this restart in the namelist set you starting date here:

nn_date0    =  20000101

and use these options to have a soft restart (i.e., with using only T, S, U, V, SSH rather other fields that your actual model restart will and should contain):

ln_rstart   = .true.
nn_euler    =    1 
nn_rstctl   =   0
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