[CF-metadata] CF-metadata Digest - cell method change...

From: Karl Taylor <taylor13>
Date: Wed, 11 Jul 2007 08:13:49 -0700

Martin and all,

I think the quantity under discussion is delta-p, *not* delta-p/delta-t;
one is a simple difference, the other a rate of change. I think Seth
McGinnis (email copied directly below) provided the correct approach for
specifying standard name, units and cell-methods for delta-p.

Do you agree?

Thanks for contributing to the discussion.

Karl

It looks like a case where you apply one of the transformational rules:

RULE: change_over_time_in_X

UNITS: [X]

MEANING: change in a quantity X over a time-interval, which should
be defined by the bounds of the time coordinate.

(From "Guidelines for Construction of CF Standard Names, at:
http://cf-pcmdi.llnl.gov/documents/cf-standard-names/guidelines )

So the standard name would be "change_over_time_in_air_pressure", I
think, with an associated timebnds variable as described in section 7.1.

--Seth

----
Seth McGinnis
Associate Scientist
ISSE/NCAR
----
Schultz, Martin wrote:
> Dear all,
> 
>      I am loosely following the CF discussions. But here I stumbled: if
> you have a variable changin over time, then its physical unit can't be
> "Pa" any longer but should be "Pa s-1" in SI units or something like "Pa
> (3h)-1" if the seconds are inconvenient. Looks like one of those
> problems where habits have generated their own rules which makes it
> difficult to formulate them in precise physically correct rules.
> 
> Best regards,
> 
> Martin
> 
>     
> 
> Message: 2
> Date: Fri, 29 Jun 2007 09:42:33 -0700
> From: "Godin, Michael" <godin at mbari.org>
> Subject: [CF-metadata] Proposed cell_methods method
> CF Community,
> 
> In another community, a member is trying to correctly represent a
> variable with a long name of "3 hour pressure change".  The variable has
> units of Pa, so I believe it's standard name should be "air_pressure",
> not "tendency_of_air_pressure".  However, the variable should probably
> have a cell_methods attribute that indicates this is a change over time,
> not an absolute measurement.  But how to do so?
> 
> Perhaps something like cell_methods="time: change (interval: 3 hour)" 
> 
> But there is no cell_method in appendix E like "change".  
> 
> The alternative is to propose a new standard name
> "change_over_time_in_air_pressure", but doing so seems inelegant. 
> 
> Mike
> 
> _____________________________________________
> Michael A. Godin
> Software Engineer
> Monterey Bay Aquarium Research Institute
> Phone: 831-775-2063  http://www.mbari.org
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Received on Wed Jul 11 2007 - 09:13:49 BST