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Author : John Siddorn

ticket : 985

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The current stretched sigma coordinate options mean the size of the surface box varies greatly, leading to inconsistent flux inputs. A terrain following coordinate system which still provides a constant surface box height has been developed.

This has been tested in a series of test configurations based upon a shelf model the AMM7 and shown to improve air-sea exchange and reduce pressure gradient errors.

Testing has been done in NEMO3.2 using non-linear free surface and comparing the presently used Song and Haidvogel 1994 stretched with this new formulation. This has been written up in a paper to be submitted shortly (attached).


The change has meant updates to the following files:

  • changed CONFIG/XXXXX/EXP00/namelist → to include the various options for the coordinate
  • changed NEMO/OPA_SRC/par_AMM_12km.h90 to make it an L50 model required for the stretching
  • changed NEMO/OPA_SRC/DOM/domzgr.F90 → to include the new stretching function
  • changed NEMO/OPA_SRC/DOM/domwri.F90 → correct a bug in the _sco write that makes the gdept output 3D, and include hydrostatic consistency parameter
  • changed NEMO/OPA_SRC/DOM/domain.F90 → added hydrostatic consistency calculation
  • NEMO/OPA_SRC/DOM/om_oce.F90 → add new parameter rx1 (hydro)

The domzgr has been reordered to make it more readable, with the different stretching functions included as subroutines. The checks on whether the coordinate is sensible has been updated to include looping over mbathy rather than jpk to ensure that when disappearing coordinates are used it doesn't fail when it shouldn't.


Scientific testing has been done on a combination of the AMM7km configuration and a seamount test case, and is detailed in the attached draft paper.


This testing was done in NEMO3.2, so the code has been updated to NEMO3.4 and testing has been done on the AMM12 standard configuration. The upgrade to 3.4, code tidying and namelist changes for standard configurations are detailed here. All testing has been done on the IBM P7 with the following compile flags.

-qrealsize=8 -qextname -qsuffix=f=f90 -qarch=pwr7 -qtune=pwr7 -NS32768 -I/data/nwp/ofrd/frpk/netcdf-3.6.0-p1_ec/include -qxflag=p6div:p6divmsg -g -O3 -qstrict

Steps 1-3 were done with the AMM12 standard configuration but updated to L50. To prevent having to rebuild all boundaries this was done with key_bdy off. The simulations were run for one day (1st Jan 2007) with no lateral boundary forcing but with atmospheric forcing. A surge event meant SSHs of around 1m were seen in the Southern NS. The surface waters cool during the day.

Step 1: Test the standard 3.4 code with 50 levels using the Song and Haidvogel stretch. Compare with the updated code using the same stretch. This should give near identical results.

Note the test on the coordinates fails with 50 levels in the standard code (although not for 33 levels) - this is a bug, and has been fixed.

The 3.4 standard AMM12 compared with the new code, but the same stretched coordinates gives identical outputs for tracers (T,S,SSH) after one day to the precision of the output netcdf files (ncdiff -v votemper,sossheig … gives zeros).

Step 2: Compare the grids

Step 3: Compare the simulations between Song and Haidvogel and new coordinate stretch to ensure sensible answers.

Difference in SSH after one day between teh L50 Song and Haidvogel and the new coordinates: no boundary forcing used, but atmospheric pressure so surge events are seen SST difference between new adn SH94 coordinates after 1 day. 0.1 deg range

The images show the difference between the new simulation and the standard. After one day the two simulations give similar results, but with variations of ±1cm in SSH (left image) around steep slopes and shallow waters. The constant and (in deep) waters shallower surface box in the new simulation result in cooler midnight surface waters (±0.1oC) than in the song and haidvogel simulation. The better resolution at the surface results in quicker response to atm forcing in the new simulation presumably.


The AMM12 configuration has been updated to include a restart file at 3.4 (spunup from rest for two days) and ten days of boundary and surface forcing

Last modified 8 years ago Last modified on 2012-10-17T12:22:57+02:00

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