[MITgcm-devel] missing_value in mds diags

[Martin Losch]

Hi Gael,

long ago I tried to implement some reasonable "missing_value" attribute for mnc output. This attribute needs to be turned on explicitly (define DIAGNOSTICS_MISSING_VALUE) and then the missing value defaults to UNSET_RL/RS/I etc. The implementation is a little incomplete and not entirely general, so I think that the way this is done is the best I could do at that time.
The D*_FRACT_FILL is a bit different, and I do not even remember, if the missing_value stuff works with mnc and this routine.

For MDS, on the one hand having missing_value=0 seems sensible, as this is what we have for land points already.  On the other hand, you cannot decide between land points and points where there is no ice and you might want to have value indicating this? 12345.6 is a very unlikely number that you not get by chance, while 0 can happen, especially when there is no ice cover. I am not sure. 

May I ask that the default missing_value for mnc remains unchanged (UNSET_RL/RS/I)?

Martin

On Jun 15, 2011, at 4:59 PM, Gael Forget wrote:

> 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
> 
> 
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