Bottom grid points with unrealistically cold temperature in ORCA2_LIM

[fabien.roquet]

I just realized that ORCA2 features a few bottom grid points with unrealistically cold properties (<-2degC). I checked outputs from nemo trunk (v3_5) revision 3856 (in ​http://dodsp.idris.fr/rfry154/reference_trunk_v35_Output/) to verify that the problem is not due to my own setup, and I indeed found bottom grid point colder than -2degC in several year-mean output files:

• ORL2PISV35_00000101_00001231_1Y_grid_T.nc : no problem
• ORL2PISV35_01000101_01001231_1Y_grid_T.nc : no problem
• ORL2PISV35_02000101_02001231_1Y_grid_T.nc : -2.03845
• ORL2PISV35_03000101_03001231_1Y_grid_T.nc : -2.57319
• ORL2PISV35_04000101_04001231_1Y_grid_T.nc : -2.7857
• ORL2PISV35_05000101_05001231_1Y_grid_T.nc : -2.88635

From my limited analysis, it seems that this situation always happens only in partial cell grid points, and can happen at around any depth. There is a clear tendency of cold drift through time, + spatial spreading of the phenomenon.
I have checked quickly on ORCA1, and didn't find any such anomalous cold grid points, but that doesn't mean that the problem does not exist for other configurations than ORCA2.
I did a run without bbl (namelist nn_bbl_ldf and nn_bbl_adv set to 0), and the problem still persists so it seems bbl is not the source of this problem.
Does anyone know what is going on?

[fabien.roquet]

The bug seems to be related to partial cell settings, allowing for bottom grid cells that are too thin.
If the following parameters are used in the &namdom section of the namelist:

rn_e3zps_min= 100. ! partial step thickness is set larger than the minimum of
rn_e3zps_rat= 0.2 ! rn_e3zps_min and rn_e3zps_rat*e3t, with 0<rn_e3zps_rat<1

the run ORCA2 does not produce any more cold temperatures.
I would recommend to use these values in the standard ORCA2 configuration.