[CF-metadata] flux

alison.pamment at stfc.ac.uk alison.pamment at stfc.ac.uk
Wed Jun 3 11:43:36 MDT 2015


Dear All,

I vote no to the suggestion that we should change 'flux' to 'flux_density' in all the existing names.

I am always keen that we correct factual errors and remove ambiguities - the recent discussions on photosynthetic wavelengths and practical salinity units are two examples of that. However, in this instance we are not dealing with a factual error, but with a conscious choice that was made to use a particular convention. The thing about making choices is that you simply can't please all the people all the time (and whatever the final outcome of this discussion and the committee vote we still won't please everyone). While I accept that the wider physics community may more commonly use "flux density" I am firmly in the camp that says there is no real advantage for CF users in changing all these standard names.

The current wording of the definition, 'In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics,' is a clear, unambiguous statement and I think it accurately reflects the way the term is often used. Also, I have to say that I disagree with Charlie's comment:
> Fewer instances of "Why does CF call it something different than my textbook?".
Very few existing CF standard names look like the terms you would see in a text book because they are much more verbose (again a conscious choice!) and changing 'flux' to 'flux_density' wouldn't address that.

In the definitions of many existing names we include a statement to say that other communities call them something else. The recently agreed "FAPAR" name is an example:
fraction_of_surface_downwelling_photosynthetic_radiative_flux_absorbed_by_vegetation (canonical units: 1)
'The quantity with standard name fraction_of_surface_downwelling_photosynthetic_radiative_flux_absorbed_by_vegetation, often called Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), is the fraction of incoming solar radiation in the photosynthetically active radiation spectral region that is absorbed by a vegetation canopy.' 
The flux names are just another example in which we use the definitions to draw attention to different usage in different communities. I think this is a perfectly sensible and useful approach which helps people to find the term they need while allowing us to maintain a high degree of internal consistency between CF standard names. Internal consistency is a very important tool when it comes to managing the vocabulary content - it certainly makes my job easier :)

Best wishes,
Alison

------
Alison Pamment                                 Tel: +44 1235 778065
NCAS/Centre for Environmental Data Archival    Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory     
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.


From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Nan Galbraith
Sent: 20 May 2015 19:09
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] flux

Hi all - 

I'm finally catching up with this discussion, after being at sea for the past 
month. I have to agree with Seth McGinnis; I vote no.

We're happy with the CF terms and their definitions. The last sentence in the 
definition of surface_downwelling_shortwave_flux_in_air (and probably some of
the other flux terms) states:
In accordance with common usage in geophysical disciplines, "flux" implies per 
unit area, called "flux density" in physics.  
This seems perfectly clear. If flux_density is technically more correct, so be it; the 
canonical units and the definitions clarify the meaning. We could make *lots* of standard 
names longer and more technically complete; in reading through the threads on this 
request, I just haven't seen a clear explanation of why the existing flux terms now 
need to change.

Cheers - Nan

On 5/19/15 6:25 PM, Seth McGinnis wrote:
I vote no.

As mentioned, using "flux" to refer to what is technically "flux
density" is commonplace and normal in many geoscience fields, and it's
clear by the lack of questions about it that this does not cause
problems for users.

Deprecating names and replacing them with an alias creates the
opportunity for confusion, and given the number and popularity of the
names that would be affected, I think this change would create a great
deal of confusion.  A reasonable alternative solution
("integrated_flux") has been suggested for handling "proper" fluxes, and
as Karl says, we'd likely want to use that even if we did make the
change to avoid confusion with the old names.

So it seems to me that there's no real benefit to changing flux to
flux_density, and the potential for a very large downside.

Cheers,

--Seth McGinnis

On 5/19/15 3:23 PM, Jonathan Gregory wrote:
Dear all

Like Karl, I thought that many people would have opinions, but so far there
are none. Please express your views! Shall we rename flux quantities in units
of something per m2 to flux_density in all existing standard names? Here are
the kinds of flux [density] we name:

carbon energy evaporation graupel heat longwave mass melt mole momentum
photon precipitation radiative rainfall refreezing runoff salt shortwave
snowfall sublimation throughfall transpiration water_vapor water

With regard to your point, Karl, I think we would not use plain "flux" in the
area-integral sense. We would avoid using it altogether. We already have ways
of working round it e.g. northward heat transport in W and there is no need
to change those names.

Best wishes and thanks

Jonathan

----- Forwarded message from Karl Taylor <taylor13 at llnl.gov-----

Date: Thu, 14 May 2015 10:47:55 -0700
From: Karl Taylor <taylor13 at llnl.gov>
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] flux
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:31.0)
	Gecko/20100101 Thunderbird/31.6.0

Dear Jonathan,

My oh my, this is bound to generate lots of opinions.  I do recall
the original discussion conclusion that although "flux density" was
the proper name, we'd be lax in this case and go with common usage,
"flux". An argument against the common usage is that if we want to
define the flux density integrated over some surface, then we
couldn't call it "flux", which is what it is.  Perhaps to
distinguish this from "flux" (W m-2), we would call this
"integrated_flux" (W).  Do we have examples of having to do this
kind of thing in the current standard names?

Even if we rename "flux" "flux density", we probably wouldn't want
to refer to the integrated flux as simply "flux" because so many
fields have already been written named  "flux" when "flux density"
was meant.

It's not going to be easy.

Karl

On 5/14/15 9:37 AM, Jonathan Gregory wrote:
Dear all

In connection with the radiative flux from the sun, the question has come up
of whether we should use the phrase flux_density for a flux per unit area in
all the standard names which currently have the word "flux". This would be
correct in physical terminology, but years ago we chose to use "flux" because
it's the normal terminology in many geosciences. There are more than 200
standard names of "flux" - radiative fluxes, mass fluxes and mole fluxes. In
some of them I don't think "flux density" is ever used e.g. I have never
heard of an "ocean flux density adjustment", and Google finds one hit for
"snowfall flux density". However we could rename them all and establish aliases
to the present names, if that would be an advantage for users of standard
names. Should this be done?

Cheers

Jonathan



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