[[PageOutline]]
Last edited [[Timestamp]]

[[BR]]

'''Author''' : Elisabeth REMY 

'''ticket''' : 944

'''Branch''' : [https://forge.ipsl.jussieu.fr/nemo/browser/branches/2012/dev_r3342_MERCATOR7_SST  2012/dev_r3342_MERCATOR7_SST ] 
----

=== Description ===

The SST model counterpart for a given date is computed as a nigth mean, by default, instead of instantaneous value as before. 
At high latitude, if there is no night, the 24h mean is taken. 
The logical to activate/desactivate this option is ln_sstnigth in the subroutine diaobs_init in the module diaobs.F90. 
It is set to true by default because daily satellite SSTs are postprocessed to give a "SST fundation" value which 
should not be affected by the diurnal cycle.  
Night mean SST values in numerical model are found to be closer to SST fundation than instantaneous or 24h mean values. 
The routine used to test night/day at a given time and grid point is sbcdcy.F90.  
This change made in the SST model counterpart will only affect diagnostics of the SST model equivalent stored in the feedback files 
when SST observation misfit diagnostics are activated. 
    
----
=== Testing ===
Testing could consider (where appropriate) other configurations in addition to NVTK].

||NVTK Tested||'''YES'''||
||Other model configurations||'''YES'''||
||Processor configurations tested||[ SETTE processor configs  ]||
||If adding new functionality please confirm that the [[BR]]New code doesn't change results when it is switched off [[BR]]and !''works!'' when switched on||'''YES'''||


=== Bit Comparability ===
||Does this change preserve answers in your tested standard configurations (to the last bit) ?||'''YES'''||
||Does this change bit compare across various processor configurations. (1xM, Nx1 and MxN are recommended)||'''YES'''||
||Is this change expected to preserve answers in all possible model configurations?||'''YES'''||
||Is this change expected to preserve all diagnostics? [[BR]]!,,!''Preserving answers in model runs does not necessarily imply preserved diagnostics. !''||'''NO'''||

The affected diagnostics are the values of the SST model counterpart stored in the feedback files. They are computed only if the SST data misfit 
diagnostic is activated in the namelist with ln_sst = .true. and a SST data type chosen (ln_reysst or ln_ghrsst).    
The difference comes from the new way of computing SST model counterpart with night mean instead of instanteneous mean in obs_oper.F90. 
The night mean is closer to the data value compared with in terms of physical content.   
The logical ld_sstnight is set to true by default in the subroutine dia_obs_init in the module diaobs.F90. This option can be switched back to 
recover the old option (ld_sstnight = .false. in diaobs.F90).  

----
=== System Changes ===
||Does your change alter namelists?||'''YES '''||
||Does your change require a change in compiler options?||'''NO '''||

We did not add any logical in the namelist but in the standard configuration, 
the sst dataset keyword was commented. 
We activated ln_sst and ln_reysst in the namelist in order to have our modification activated : 
- ln_sst = .true.
- ln_reysst  = .true. to specify the SST data set to read. 

----
=== Resources ===
!''Please !''summarize!'' any changes in runtime or memory use caused by this change......!''
to be done... We do not expect significant changes in the resources. 
----
=== IPR issues ===
||Has the code been wholly (100%) produced by NEMO developers staff working exclusively on NEMO?||'''YES'''||