Hi All,
My apologies for not being so involved recently. I am catching up with this
thread.
I also vote "no" to changing 'flux' to 'flux_density'.
In addition to the points that Alison makes, I add the following:
+) If someone searches the descriptions for 'irradiance', the appropriate
std_names show up, so users will be guided to what they need.
At the risk of further inflaming the debate, I note that Wikipedia gives a good
summary of the two different definitions of flux, and the m^-2 description
comes first. Wikipedia also provides a quote from Maxwell that indicates the
m^-2 definition came first too:
In the case of fluxes, we have to take the integral, over a surface, of the
flux through every element of the surface. The result of this operation is
called the surface integral of the flux. It represents the quantity which
passes through the surface -James Clerk Maxwell
Best wishes,
Philip
-----------------------------------------------------------------------------------
Philip Cameron-Smith, [email protected], CliMA Group Leader, Lawrence Livermore
Nat. Lab.
-----------------------------------------------------------------------------------
-----Original Message-----
From: CF-metadata [mailto:[email protected]] On Behalf Of
[email protected]
Sent: Wednesday, June 03, 2015 10:44 AM
To: [email protected]
Subject: Re: [CF-metadata] flux
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: [email protected]
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
From: CF-metadata [mailto:[email protected]] On Behalf Of Nan
Galbraith
Sent: 20 May 2015 19:09
To: [email protected]
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 <[email protected]
Date: Thu, 14 May 2015 10:47:55 -0700
From: Karl Taylor <[email protected]>
To: [email protected]
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
--
*******************************************************
* Nan Galbraith Information Systems Specialist *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 (508) 289-2444 *
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