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

From: Schultz, Martin <m.schultz>
Date: Wed, 11 Jul 2007 17:33:45 +0200

Dear Karl et al.,

     in my view this difference is at best philosophical. If time doesn't matter then you would not need the information "over_time" nor the time interval over which the change occured. As soon as you define the time interval you end up with a rate of change, no matter what. What use can you make of the information "the pressure change is 10 hPa."? The only sensible way I can think of to express this would be "The pressure change over a 3-hour interval is 10 hPa." Everything else must imply a certain amount of time, which may be known to a specific community by long-standing agreements, but is not suitable for defining a standard in my humble view.

     OK - there is a difference, which is probably what you have in mind: with "rate of change" one typically labels an "instantaneous rate of change", i.e. the derivative of a quantity. In contrast, the "change" you are alluding to is the integral over this derivative, or integral{dQ/dt*dt'}. But then the name should reflect this as "integrated_change..." -- perhaps this is better defined in the cell_method as proposed earlier, but I think it is important to include the term "integrated change" somewhere.

Best regards,

Martin

< Dr. Martin G. Schultz, ICG-II, Forschungszentrum J?lich >
< D-52425 J?lich, Germany >
< ph: +49 (0)2461 61 2831, fax: +49 (0)2461 61 8131 >
< email: m.schultz at fz-juelich.de >
< web: http://www.fz-juelich.de/icg/icg-2/m_schultz >





-----Original Message-----
From: Karl Taylor [mailto:taylor13 at llnl.gov]
Sent: Wednesday, July 11, 2007 5:14 PM
To: Schultz, Martin
Cc: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] CF-metadata Digest - cell method change...

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 Forschungszentrum Juelich 
> GmbH
> 52425 Juelich
> 
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> 
> 
> 
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Forschungszentrum J?lich GmbH
52425 J?lich
Sitz der Gesellschaft: J?lich
Eingetragen im Handelsregister des Amtsgerichts D?ren Nr. HR B 3498
Vorsitzende des Aufsichtsrats: MinDirig'in B?rbel Brumme-Bothe
Vorstand: Prof. Dr. Achim Bachem (Vorsitzender), Dr. Ulrich Krafft (stellv. 
Vorsitzender)
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Received on Wed Jul 11 2007 - 09:33:45 BST