Hello Jim,

the usage of solar_irradiance to refer to solar irradiance per unit horizontal 
area was suggested by the RFMIP team, and initially appeared to be supported by 
Roy .. but I'm happy to drop it.

Following the discussion here, which has been very helpful, I think that the 
standard name that RFMIP actually want is toa_incoming_shortwave_flux , and it 
might be helpful to indicate in the definition of this standard name that it is 
a synonym for Global Horizontal Irradiance at the top of the atmosphere. Do you 
agree with this interpretation? (toa_incoming_shortwave_flux carries the 
definition '"shortwave" means shortwave radiation. "toa" means top of 
atmosphere. The TOA incoming shortwave flux is the radiative flux from the sun 
i.e. the "downwelling" TOA shortwave flux. In accordance with common usage in 
geophysical disciplines, "flux" implies per unit area, called "flux density" in 
physics.' )

Your "Total Solar Irradiance" definition below refers to top-of-atmosphere .. 
shouldn't it at "one astronomical unit from the sun"?  The wikipedia page does 
suggest measuring it at the top of the atmosphere, but the current CF 
definition for solar_irradiance implies that it should be measured at a fixed 
distance from the sun.

regards,
Martin


________________________________

Hi.

I must respectfully disagree with any change to the solar irradiance
definition. The definition is accurate and correct, whether you are a
solar physicist or an oceanographer. The Wikipedia article on solar
irradiance gives good definitions for the various specific terms.

https://en.wikipedia.org/wiki/Solar_irradiance

Notice that none of the various terms ever treat the direct solar power
per unit area of a horizontal surface as a quantity unto itself. The
Vaisala definitions are essentially the same as the ones in the
Wikipedia article. The terms in use are:

  * Total Solar Irradiance - The power per unit area incident on a
    surface normal to the sun from direct solar radiation measured at
    top of atmosphere.
  * Direct Normal Irradiance - The power per unit area incident on a
    surface normal to the sun from direct solar radiation measured at
    the surface.
  * Diffuse Horizontal Irradiance - The power per unit area incident on
    a horizontal surface from all sources other than direct solar radiation.
  * Global Horizontal Irradiance - The power per unit area incident on a
    horizontal surface from all sources, including direct solar radiation.

I found one reference online to Direct Horizontal Irradiance. It does
not appear to be in common use in any community.

I can see defining standard names for the four terms above, but I think
that prior usage and general science usage call for solar_irradiance to
be associated only with TSI. So, we could have standard names

  * total_solar_irradiance
  * direct_normal_irradiance
  * direct_horizontal_irradiance
  * diffuse_horizontal_irradiance
  * global_horizontal_irradiance

with solar_irradiance as an alias for total_solar_irradiance, or vice
versa, but I think we shouldn't alter the definition of solar_irradiance.

Grace and peace,

Jim


On 1/26/18 11:52 AM, martin.juckes at 
stfc.ac.uk<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata> wrote:
> Dear Roy,
>
> If we were starting from scratch I would recommend the following:
>
> Solar irradiance is the power per unit area of the solar radiation received 
> from above at horizontal surface. It is distinct from the Direct Normal 
> Irradiance (which refers to radiation received by a surface perpendicular to 
> rays travelling from the sun) and the Total Solar Irradiance (which is the 
> mean value of Direct Normal Irradiance at a standard distance from the sun).
>
> I am, however, concerned that people may have already, on the basis of the 
> current definition, used the term for Total Solar Irradiance.
>
> regards,
> Martin
>
> ________________________________________
> From: Lowry, Roy K. [rkl at 
> bodc.ac.uk<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata>]
> Sent: 26 January 2018 16:03
> To: Juckes, Martin (STFC,RAL,RALSP); cf-metadata at 
> cgd.ucar.edu<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata>
> Subject: Re: Definiton of solar_irradiance
>
> Dear Martin,
>
>
> I agree that the current definition reflects more an astrophysics textbook 
> rather than common usage of the Standard Name and so I would support tweaking 
> the wording as you suggest. Care to come up with the replacement wording to 
> be used?
>
>
> Cheers, Roy.
>
>
> Please note that I partially retired on 01/11/2015. I am now only working 7.5 
> hours a week and can only guarantee e-mail response on Wednesdays, my day in 
> the office. All vocabulary queries should be sent to enquiries at 
> bodc.ac.uk.<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata> Please 
> also use this e-mail if your requirement is urgent.
>
>

________________________________
From: Lowry, Roy K. [r...@bodc.ac.uk]
Sent: 27 January 2018 07:54
To: Juckes, Martin (STFC,RAL,RALSP); cf-metadata@cgd.ucar.edu
Subject: Re: Definiton of solar_irradiance


For three, read four!


Please note that I partially retired on 01/11/2015. I am now only working 7.5 
hours a week and can only guarantee e-mail response on Wednesdays, my day in 
the office. All vocabulary queries should be sent to enquir...@bodc.ac.uk. 
Please also use this e-mail if your requirement is urgent.


________________________________
From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of Lowry, Roy K. 
<r...@bodc.ac.uk>
Sent: 27 January 2018 07:51
To: martin.juc...@stfc.ac.uk; cf-metadata@cgd.ucar.edu
Subject: Re: [CF-metadata] Definiton of solar_irradiance


Dear Martin,


I think that there has been some misunderstanding on my part, pointed out by 
Jim's e-mail and there is currently no Standard Name for what is often termed 
'solar irradiance' in observation data sets.


Should we proceed with the three new Standard Names suggested by Jim?


Cheers, Roy.


Please note that I partially retired on 01/11/2015. I am now only working 7.5 
hours a week and can only guarantee e-mail response on Wednesdays, my day in 
the office. All vocabulary queries should be sent to enquir...@bodc.ac.uk. 
Please also use this e-mail if your requirement is urgent.


________________________________
From: martin.juc...@stfc.ac.uk <martin.juc...@stfc.ac.uk>
Sent: 27 January 2018 00:05
To: Lowry, Roy K.; cf-metadata@cgd.ucar.edu
Subject: RE: Definiton of solar_irradiance

Hello Roy,

maybe .. but I was talking about the current definition of the CF standard name 
"solar_irradiance", which, currently, has nothing to do with the vertical,

regards,
Martin

________________________________
From: Lowry, Roy K. [r...@bodc.ac.uk]
Sent: 26 January 2018 20:35
To: Juckes, Martin (STFC,RAL,RALSP); cf-metadata@cgd.ucar.edu
Subject: Re: Definiton of solar_irradiance


Hello Martin,


Isn't that was what cosine collectors on radiometers are for? I thought they 
resolved the vertical component of the radiation within the hemisphere sampled.


Cheers, Roy.


Please note that I partially retired on 01/11/2015. I am now only working 7.5 
hours a week and can only guarantee e-mail response on Wednesdays, my day in 
the office. All vocabulary queries should be sent to enquir...@bodc.ac.uk. 
Please also use this e-mail if your requirement is urgent.


________________________________
From: martin.juc...@stfc.ac.uk <martin.juc...@stfc.ac.uk>
Sent: 26 January 2018 19:15
To: Lowry, Roy K.; cf-metadata@cgd.ucar.edu
Subject: RE: Definiton of solar_irradiance

Hello Roy,

but the flow of radiant energy isn't necessarily perpendicular to the surface 
of the Earth. At the top of the atmosphere the flow of energy will generally be 
along a ray from the Sun. If you want it to be the irradiance on a horizontal 
surface you really need to take out the bit about being perpendicular to the 
flow of energy.

You can't make the name work for horizontal surfaces without contradicting the 
assertion in the current definition that it applies to TSI,

regards,
Martin

________________________________
From: Lowry, Roy K. [r...@bodc.ac.uk]
Sent: 26 January 2018 17:46
To: Juckes, Martin (STFC,RAL,RALSP); cf-metadata@cgd.ucar.edu
Subject: Re: Definiton of solar_irradiance


Hello again,


What we map to the 'solar_irradiance' Standard name are parameters such as 
'Downwelling vector irradiance as energy of electromagnetic radiation (solar 
wavelengths) in the atmosphere by pyranometer'. So in the use case I was 
thinking of it's both a horizontal surface and a surface perpendicular to the 
direction of flow of the radiant energy i.e. we have made an assumption the the 
direction of flow of the radiant energy is perpendicular to the Earth's surface.


Consequently, I would amend the definition as below rather than redefining from 
scratch with the risk you pointed out of changing the semantics of existing 
data sets.


"Irradiance" means the power per unit area (called radiative flux in other 
standard names), the area being normal to the direction of flow of the radiant 
energy.  The area is a horizontal surface for radiant energy flowing 
perpendicular to the Earth's surface.


Cheers, Roy.


Please note that I partially retired on 01/11/2015. I am now only working 7.5 
hours a week and can only guarantee e-mail response on Wednesdays, my day in 
the office. All vocabulary queries should be sent to enquir...@bodc.ac.uk. 
Please also use this e-mail if your requirement is urgent.


________________________________
From: martin.juc...@stfc.ac.uk <martin.juc...@stfc.ac.uk>
Sent: 26 January 2018 16:52
To: Lowry, Roy K.; cf-metadata@cgd.ucar.edu
Subject: RE: Definiton of solar_irradiance

Dear Roy,

If we were starting from scratch I would recommend the following:

Solar irradiance is the power per unit area of the solar radiation received 
from above at horizontal surface. It is distinct from the Direct Normal 
Irradiance (which refers to radiation received by a surface perpendicular to 
rays travelling from the sun) and the Total Solar Irradiance (which is the mean 
value of Direct Normal Irradiance at a standard distance from the sun).

I am, however, concerned that people may have already, on the basis of the 
current definition, used the term for Total Solar Irradiance.

regards,
Martin

________________________________________
From: Lowry, Roy K. [r...@bodc.ac.uk]
Sent: 26 January 2018 16:03
To: Juckes, Martin (STFC,RAL,RALSP); cf-metadata@cgd.ucar.edu
Subject: Re: Definiton of solar_irradiance

Dear Martin,


I agree that the current definition reflects more an astrophysics textbook 
rather than common usage of the Standard Name and so I would support tweaking 
the wording as you suggest. Care to come up with the replacement wording to be 
used?


Cheers, Roy.


Please note that I partially retired on 01/11/2015. I am now only working 7.5 
hours a week and can only guarantee e-mail response on Wednesdays, my day in 
the office. All vocabulary queries should be sent to enquir...@bodc.ac.uk. 
Please also use this e-mail if your requirement is urgent.


________________________________
From: martin.juc...@stfc.ac.uk <martin.juc...@stfc.ac.uk>
Sent: 26 January 2018 15:57
To: Lowry, Roy K.; cf-metadata@cgd.ucar.edu
Subject: RE: Definiton of solar_irradiance

Hello Roy,

I suspected that there might be such a usage ... but don't you agree that the 
current CF definition, which I've quoted below, is inconsistent with this? But 
it could be adapted with a small change of wording,

regards,
Martin

________________________________
From: Lowry, Roy K. [r...@bodc.ac.uk]
Sent: 26 January 2018 15:51
To: Juckes, Martin (STFC,RAL,RALSP); cf-metadata@cgd.ucar.edu
Subject: Re: Definiton of solar_irradiance


Hello Martin,


>From an oceanographic perspective I had always thought of solar (and other 
>waveband) irradiance as the energy incident on the sea surface, which is a 
>horizontal surface. So, I would like to think of solar_irradiance as being 
>horizontal_solar_irradiance and if any new Standard Name is required to cover 
>the current Vaisala instrumentation use case then that should be 
>normal_standard_irradiance.


Cheers, Roy.


Please note that I partially retired on 01/11/2015. I am now only working 7.5 
hours a week and can only guarantee e-mail response on Wednesdays, my day in 
the office. All vocabulary queries should be sent to enquir...@bodc.ac.uk. 
Please also use this e-mail if your requirement is urgent.


________________________________
From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of 
martin.juc...@stfc.ac.uk <martin.juc...@stfc.ac.uk>
Sent: 26 January 2018 13:37
To: cf-metadata@cgd.ucar.edu
Subject: [CF-metadata] Definiton of solar_irradiance

Hello,

the cf standard name has a definition:

The quantity with standard name solar_irradiance, often called Total Solar 
Irradiance (TSI), is the radiation from the sun integrated over the whole 
electromagnetic spectrum and over the entire solar disk. The quantity applies 
outside the atmosphere, by default at a distance of one astronomical unit from 
the sun, but a coordinate or scalar coordinate variable of distance_from_sun 
can be used to specify a value other than the default. "Irradiance" means the 
power per unit area (called radiative flux in other standard names), the area 
being normal to the direction of flow of the radiant energy.

My question is about the last phrase, which I have highlighted. The flow of 
radiant energy from the sun at the top of the atmosphere is directed away from 
the sun .. so this definition would imply that the irradiance is defined 
relative to a fixed plane in the solar coordinate system. This is OK for solar 
physicists, but atmospheric scientists are sometime interested in irradiance 
relative to a horizontal surface.

Vaisalla distinguish between the two by defining "horizontal solar irradiance" 
to be the irradiance on a horizontal surface and "normal solar irradiance" to 
be irradiance on a surface perpendicular to a line to the sun 
(<UrlBlockedError.aspx>http://www.3tier.com/en/support/solar-online-tools/what-solar-values-are-shown-map/
 ).

Should "solar_irradiance" apply to both usages, or do we need a new standard 
name, e.g. "horizonatl_solar_irradiance"?

regards,
Martin

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