[MITgcm-devel] missing_value in mds diags

[Gael Forget]

Dear all,

while preparing the diags for seaice tracers, which are done with DIAGNOSTICS_FRACT_FILL, 
I had a hard time figuring out why the fields were 12345.6 (i.e. the value of UNSET_RL) where
there is no ice. My initial reaction was obvisouly that I coded something terribly wrong.
Then I noticed the missing_value run time parameter and set it to 0. It did not do me 
any good because it is only used with mnc. After discussing this with Jean Michel, 
I would propose we make this capability effective with mds, and possibly set the 
missing_value default to 0 for mds diags.

I understand that the missing value is not presently written in the 
meta file, so the average user cannot what the missing value is
(I did not until two days ago). This is true regardless of whether
the missing value is 12345.6 or 0. Also, assuming a user decides to 
specify it, then it implies that at least he knows what the value is. So 
I think he should be allowed to specify and use missing_value with mds.

Furthermore, with the 12345.6 value, the first reaction when plotting a field
is not 'this is the missing value' but 'this diagnostic file' is completely wrong. 
Typically, I suspect some users will shy away from the generic seaice tracer
stuff, or send emails to support, just because the diags look ugly at first glance. 
So I would prefer a zero default for the diags missing_value with mds.

I was wondering whether someone disagrees or has suggestions?

Cheers,
Gael