1-01-01 Align radiation variables requirements

[fstuerzl]

Proposal Summary

Summary and Purpose

Provide consistent names and descriptions/definitions for radiation terminology; align terminology in use in OSCAR/Surface (based on the WMO codes registry) and OSCAR/Requirements.

Proposal

Add definitions and include the term "radiation" in the UV variables:

notation	path	name	description
267	\Atmosphere\Radiation\Background luminance	Background luminance	Luminous flux received from the background per unit solid angle and per unit area. Note: Luminous flux is a quantity derived from radiant flux by evaluating the radiation according to its action upon the International Commission on Illumination standard photometric observer.
270	\Atmosphere\Radiation\Sunshine duration	Sunshine duration	The total time in one day during which the direct solar irradiance is equal to or more than the threshold value for bright sunshine (the threshold being 120 W m–2 of direct solar irradiance).
271	\Atmosphere\Radiation\Surface albedo	Surface albedo	Hemispherically integrated reflectance of the Earth surface in the range 0.4-0.7 µm.
273	\Atmosphere\Radiation\UV\UV-B radiation	UV-B radiation	Ultraviolet radiation covering the range 280-315 nm.
356	\Atmosphere\Radiation\UV\UV Broadband radiation	UV Broadband radiation	Spectrally integrated UV radiation
357	\Atmosphere\Radiation\UV\UV Erythemal radiation	UV Erythemal radiation	Radiation effective in causing actinic erythema.
358	\Atmosphere\Radiation\UV\UV Multiband radiation	UV Multiband radiation	UV radiation measured across several discrete, narrow wavebands (e.g. 10 nm) within the UV range, providing coarse spectral information
359	\Atmosphere\Radiation\UV\UV Spectral radiation	UV Spectral radiation	UV radiation measured at continuous and higher spectral resolution (e.g. 1 nm) than multiband UV radiation.

Replace the name and add a description for the following variables:

notation	current name	new name	new description
566	Long-wave radiation (downwelling)	Downward long-wave irradiance	Flux density of downwelling long-wave radiation, at a specified level within the atmosphere, including the Earth surface.
567	Long-wave radiation (upwelling)	Upward long-wave irradiance	Flux density of upwelling long-wave radiation, at the top of the atmosphere or at a specified level within the atmosphere, including the Earth surface.
565	Long-wave radiation (direction unspecified)	Long-wave irradiance (direction unspecified)	Flux density of long-wave radiation (direction unspecified) at a specified level within the atmosphere, including the Earth surface. Long-wave radiation is the radiation at wavelengths greater than 3 μm (3000 nm), including most of the spectrum emitted by sources at terrestrial temperatures.
571	Diffuse solar radiation	Diffuse downward short-wave irradiance	Flux density of diffuse short-wave solar radiation, at a specified level within the atmosphere, including the Earth surface.
572	Direct solar radiation	Direct downward short-wave irradiance	Flux density of direct downwelling short-wave solar radiation, at the top of the atmosphere or at a specified level within the atmosphere, including the Earth surface.
573	Global solar radiation (downwelling)	Downward short-wave irradiance	Flux density of downwelling short-wave solar radiation, at the top of the atmosphere or a specified level within the atmosphere, including the Earth surface. [The term “global (downward) shprtwave irradiance’ is sometimes used to specify that this flux is the sum of direct and diffuse components.]
574	Global solar radiation (upwelling)	Upward short-wave irradiance	Flux density of upwelling short-wave radiation, at the top of the atmosphere or a specified level within the atmosphere, including the Earth surface.

Include the following new variables in table 1-01-01 (Atmosphere):

notation	name	description	tags	path
new id	Net short-wave irradiance	Flux density of short-wave solar radiation, as the difference between downwelling and upwelling radiation (positive for net downward), at the top of the atmosphere or at a specified level within the atmosphere, including the Earth surface.	Radiation	\Atmosphere\Radiation
new id	Short-wave irradiance (direction unspecified)	Flux density of short-wave radiation (direction unspecified) at a specified level within the atmosphere, including the Earth surface. Short-wave radiation includes most of the solar radiation spectrum in the atmosphere from 280 nm to 3000 nm (3 μm).	Radiation	\Atmosphere\Radiation
new id	Net long-wave irradiance	Flux density of upwelling long-wave radiation, as the difference between downwelling and upwelling radiation (positive for net downward) at a specified level within or at the top of the atmosphere, including the Earth surface.	Radiation	\Atmosphere\Radiation
new id	Upward spectral radiance	Upward radiant power per area unit, per solid angle, and per wavelength or wavenumber interval .	Radiation	\Atmosphere\Radiation
new id	Downward spectral radiance	Downward radiant power per area unit, per solid angle, and per wavelength or wavenumber interval .	Radiation	\Atmosphere\Radiation
new id	Solar spectral irradiance	Spectrally resolved flux density of radiation received from the sun at the top of the atmosphere	Radiation	\Atmosphere\Radiation
new id	Solar total irradiance	Spectrally integrated flux density of radiation received from the sun at the top of the atmosphere. Also called “Total solar irradiance (TSI)”.	Radiation	\Atmosphere\Radiation
new id	Earth surface short-wave bidirectional reflectance	Reflectance of the Earth surface as a function of the viewing angle and the illumination angle averaged over the range 0.4-0.7 µm.	Radiation	\Atmosphere\Radiation
new id	Earth surface spectral bidirectional reflectance	Reflectance of the Earth surface as a function of viewing angle, incidence angle and wavelength, spectrally resolved. [The distribution of this variable is represented by the Bidirectional Reflectance Distribution Function (BRDF) ]	Radiation	\Atmosphere\Radiation
new id	Earth surface emissivity	Emissivity of the Earth surface as a function of wavelength and viewing angle.	Radiation	\Atmosphere\Radiation
new id	Long-wave earth surface emissivity	Emissivity of the earth surface integrated over the thermal IR wavelength range and integrated hemispherically.	Radiation	\Atmosphere\Radiation

Reason

Some radiation variables in OSCAR/Requirements currently have no corresponding WMDR variable:

• https://space.oscar.wmo.int/variables/view/downward_short_wave_irradiance_at_earth_surface
• https://space.oscar.wmo.int/variables/view/downward_short_wave_irradiance_at_toa
• https://space.oscar.wmo.int/variables/view/upward_short_wave_irradiance_at_toa
• https://space.oscar.wmo.int/variables/view/upward_spectral_radiance_at_toa
• https://space.oscar.wmo.int/variables/view/solar_spectral_irradiance
• https://space.oscar.wmo.int/variables/view/earth_surface_short_wave_bidirectional_reflectance
• https://space.oscar.wmo.int/variables/view/long_wave_earth_surface_emissivity

Stakeholder(s)

@JohnEyre

Consultations

@laurentvuilleumier, @PeterBlattner, @AnnWebb

Context

[include references to manuals or guides that are reviewed to ensure alignment, if proposal differs then document how and why]

Expected Impact of Change

LOW for new codes and changes of names and descriptions

[fstuerzl]

In this context it also needs to be analysed, how the O/R variable "Radiation profile" can be expressed in WMDR terms.
It is defined as "Vertical profile of upward and downward LW and SW radiation components".
Does it make sense to introduce a collective term, such as "Radiation components"?
This would allow the following mapping:

O/R	WMDR
Variable: Radiation profile	Observed variable: "Radiation components" + geometry: "vertical profile"

[JohnEyre]

Hi Franziska, Re: radiation profile. I suggest we take this one back to WIGOS/RRR. Although “radiation profile” is a variable name in OSCAR/Requirements, no Application Area currently “owns” any requirements for it. We should draw this to their attention and ask them to reconsider both the variable name and its definition. John From: Franziska Stürzl ***@***.***> Sent: 27 April 2022 14:38 To: wmo-im/wmds ***@***.***> Cc: Eyre, John ***@***.***>; Assign ***@***.***> Subject: Re: [wmo-im/wmds] 1-01-01 Align radiation variables with OSCAR/Requirements and add definitions (Issue #389) This email was received from an external source. Always check sender details, links & attachments. In this context it also needs to be analysed, how the O/R variable "Radiation profile" can be expressed in WMDR terms. It is defined as "Vertical profile of upward and downward LW and SW radiation components". Does it make sense to introduce a collective term, such as "Radiation components"? This would allow the following mapping: O/R WMDR Variable: Radiation profile<https://space.oscar.wmo.int/variables/view/radiation_profile> Observed variable: "Radiation components" + geometry: "vertical profile" — Reply to this email directly, view it on GitHub<#389 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AWHTXRVWQ7D5FRSD7TKWZTTVHE7KJANCNFSM5UPDZJFA>. You are receiving this because you were assigned.Message ID: ***@***.******@***.***>>

[JohnEyre]

See my comment on "radiation profile" above.
For all the other new variables, these are requests for OSCAR/Requirements for inclusion in WMDR. They appear to have acceptable names and definitions, but comments welcome from others.

[amilan17]

@amilan17 - send this issue to Heikki Pohjola (WMO Secretariat for Space) who may have feedback or be able to help identify others to help review this proposal.

[JohnEyre]

Following on from my comment above
"Although “radiation profile” is a variable name in OSCAR/Requirements, no Application Area currently “owns” any requirements for it. We should draw this to their attention and ask them to reconsider both the variable name and its definition.",
I am adding this to my list of issues for consideration by WIGOS RRR.

[amilan17]

subset of issue #190

[joergklausen]

I insist that the existing variables (terms) are scrutinized and properly updated if needed before we add more variables lightly. The terminology 'radiation' vs 'radiance' vs 'irradiance' refers vaguely to the same thing, but I know that some people would insist on one over the other. The existing terms were created and looked at by experts at the time, and I am requesting they be consulted (again). Names I suggest are Ann Webb (UK), Laurent Vuilleumier (MCH), and Julian Gröbner (PMOD/WRC).

[fierz]

I strongly support @joergklausen's request to have the named experts look at those terms/variables. I also feel there may be some duplications as well as a non-consistent use of terminology here. Ann Webb on board would be great as there would be a direct link to both the International Commission on Illumination (CIE) and the GIMO (WMO-No 8).

[JohnEyre]

@joergklausen @fierz @amilan17 @fstuerzl . I have checked these 6 variables in OSCAR/Requirements. There are 5 RRR Application Areas that have stated requirements for observations of some or all these variables: Global NWP, High-resolultion NWP, Nowcasting, Climate Monitoring (GCOS), Hydrology. I have also checked OSCAR/Space and, in most cases, there are tens of satellite instruments listed as contributing to the observation of these variables. However, I agree that the experts consulted on these issues (i.e. on the requirements, or on the space-based observing capabilities) have probably not given much thought to the exact names of the variables or their definitions. This task (of careful thought) was done by the WMO consultant tasked with proposing the variable names and definitions used in OSCAR/Requirements+Space, and these proposals were subsequently reviewed and accepted by the relevant CBS Expert Team (IPET-OSDE, which has now become JET-EOSDE). So I suggest that these variable names and definitions have already undergone considerable review, but only from the perspective of RRR. It would be beneficial to obtain a review by someone independent of the RRR process. However, as most of the relevant observations are from space, I suggest it should be someone familiar with these technologies. Comments welcome.

[fierz]

@JohnEyre Thanks for clarifying with OSCAR/Space and RRR. In my view there is quite a redundancy in the variables proposed in the initial lists above. For example, Long-wave earth surface emissivity is very close to Long-wave radiation (upwelling), etc. Thus I agree that experts of both surface and space measurements need to sort this out.
Note that one of the experts proposed by @joergklausen, Julian Gröbner, works in an institute also dealing with space measurements and he may act as the 'bridging expert' …

[JohnEyre]

@fierz . Emissivity is a dimensionless quantity, between 0 and 1. (Normally, emissivity = 1 - reflectivity.) In the thermal infra-red, which is where it matters for many observations and applications, emissivity is close to 1 for the ocean surface and for vegetated surfaces, but differs substantial from 1 for deserts and bare soils, and for some ice surfaces. It's also important to know the emissivity in the microwave, where it is substantially less than one for the ocean surface. Surface emissivity can be retrieved from some satellite measurements. Atlases of emissivity are also required for the processing of other types of satellite data.
So it differs substantially from measurements of radiation (radiances and irradiances, spectrally specific or spectrally integrated) at the surface (or the top of the atmosphere), as these are typical measured in units of W.m**-2 (integrated over wavelength) or spectrally specific quantities in W.m**-2.(cm**-1)-1 or W.m-2.(cm**-1)-1.sr-1.

[laurentvuilleumier]

Following @joergklausen request let me include a few comments below (I am Laurent Vuilleumier):

The distinction between radiance and irradiance is implicitly addressed by indicating the radiance is the radiant power per solid angle per unit area (we could add “crossing a surface perpendicular to the radiation beam”), while the irradiance is a flux density on a given surface (same surface for radiation from all directions, it is more instrument-related). Since the flux density is a radiation power through a surface (Wm-2), it follows that irradiance is the radiance “directionally”-integrated over a hemisphere. With this, I mean that the integration through the hemisphere includes a cos(theta) term to take into account the orientation of the incoming radiance with respect to the surface through which the irradiance is measured. More precisely, the integration is Integral( Radiance cos(theta) dOmega ), where dOmega is sin(theta) dphi dtheta, which gives Integral( Radiance cos(theta) sin(theta) dphi dtheta) where the integral runs for phi = 0,2pi and for theta = 0,pi.

I think this can be surmised from the definitions. Giving the difference between radiance and irradiance more precisely requires including formulas because describing it in writing as above is not optimal.

This said, I cannot distinguish “Downward short-wave irradiance” and “Global solar radiation (downwelling)” (573) or “Upward short-wave irradiance” and “Global solar radiation (upwelling)” (574), except if the new proposed definitions are meant for satellite-use and are related to TOA quantities. However, in name themselves (Downward short-wave irradiance, Upward short-wave irradiance or Upward spectral radiance), there is nothing to indicate these are TOA quantities. By the way, in the definition of upward spectral radiance, it is mentioned in the definition that it is measured at the TOA. But except if it is decided from now on from this definition that upward spectral radiance is measured at TOA, I see nothing in the name that says this.

Finally, I find confusing to use “Total Solar Irradiance” as the first definition item of the solar spectral irradiance, since I always thought “total” meant integrated over all wavelengths, which the solar spectral irradiance is precisely not. I would rather use a definition such as “spectral solar radiation density flux through a surface perpendicular to the solar beam in W m-2 nm-1. The Total Solar Irradiance results from the integration of the solar spectral irradiance over all wavelengths”.

[JohnEyre]

@laurentvuilleumier @joergklausen . In OSCAR/Requirements, "TOA" or "Surface" are included as part of the variable name. We have discussed this in TT-WIGOSMD and (tentatively) agreed that, for WMDR, it is better to keep "TOA" and "Surface" separate from the name of the geophysical variable, and to specify it through the "Layer" descriptor.
On "Total solar irradiance", I agree - "total" here means "integrated over wavelength" and so is not appropriate as part of the definition of solar spectral irradiance. (I think the reason for its inclusion is intended to be explanatory and not part of the definition - historically, satellite instruments have measured total irradiance but, more recently, there have also been instruments to measure the spectral irradiance.)

[fierz]

Thanks @laurentvuilleumier

Your remark

I cannot distinguish “Downward short-wave irradiance” and “Global solar radiation (downwelling)” (573) or “Upward short-wave irradiance” and “Global solar radiation (upwelling)” (574)

is very pertinent and I am truly sorry I chose the wrong variables above to make the case. BTW, I by far prefer “Upward short-wave irradiance” with its clear description than the much less appropriate “Global solar radiation (upwelling)” (574)_ .

@JohnEyre be assured I know the difference between emissivity, the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature, a variable on its own indeed, and other measurements of radiation.

[laurentvuilleumier]

@JohnEyre, @fierz: OK, it seems the distinction between Downward short-wave irradiance and Global solar radiation (downwelling) as well as... is indeed their use as satellite-related (rather TOA) and ground-based measured (rather surface) quantities. The TOA vs. surface quality seems to be mentioned in another part of the description (layer) that was not apparent in this issue. It is fine with me and could help people making the connection between the variables and associated physical quantities, typically when validating satellite-derived quantities with surface measurements. In this case, I would really advocate including a note in the description mentioning that the variables Downward short-wave irradiance and Global solar radiation (downwelling) as well as... are referring to the same physical quantities in satellite vs. ground-based measured context, respectively.
I have to say that I agree with @fierz that the name Global solar radiation (upwelling) is not a very fitting choice. Upward short-wave irradiance is indeed better or eventually “upward reflected solar radiation”. However, I think the name of 574 has already been defined.
I also noticed I made a mistake in my description of the radiance integration into irradiance: the integration over theta runs from 0 to pi/2 and not 0 to pi.

[JohnEyre]

@laurentvuilleumier @fierz @joergklausen . This Issue (#389) arose out of the discussion on achieving consistency between WMDR and OSCAR/Requirements. So the variables under discussion here are those for which a requirement-to-observe has been stated by one or more WIGOS Applications Areas. Hence they are geophysical variables on which observational information (direct or indirect) has been requested. They don't necessarily correspond to variables than can be observed directly, either from surface or from space.
At Joerg's request, I have opened a new Issue (#414) to start to build a list of Level 1 variables (i.e. mainly engineering variables rather than geophysical variables) measured by satellite instruments (and in principle by other remote sensing systems). I can already see that there will be some overlaps between #389 and #414, but I hope we can resolve these as we proceed.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/Meeting-2022.09.22 notes:

• @amilan17 create new issue for table of codes that need definitions
• Joerg recommends that PMOD looks at this issue
• @gaochen-larc will ask Gregory to review

[gaochen-larc]

Contacted Schuster, Gregory L. (LARC-E302) gregory.l.schuster@nasa.gov. He will make comments directly or through me in a few weeks.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2022.10.06-TT-WIGOSMD meeting notes:

@gaochen-larc said that we may need another expert from NOAA or ESA to review

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2022.10.20-TT-WIGOSMD notes:

• @JohnEyre said that they agreed to delete "radiation profile" (but this isn't on the list above) at the workshop and are happy with the current definitions
• @fstuerzl @JohnEyre to add descriptions from OSCAR requirements
• @gaochen-larc will ping Greg to see if he can get a review
• reach out to Expert Team on Radiation References under SC-MINT

[gaochen-larc]

Would like to add a variable: Actinic Flux

Here is the AMS definition: The spherically integrated radiation flux in the earth's atmosphere that originates from the sun, including the direct beam and any scattered components.

[fstuerzl]

I've updated the proposal, by adding two definitions from OSCAR/Requirements to the variables "Background luminance" and "Surface albedo".
For the other radiation variables in WMDR, which are currently not defined, @JohnEyre and I propose to open another issue.

[JREyre]

@joergklausen @gaochen-larc
Some comments on the input from Joerg:

• Definitions for visible and UV look OK (consistent with WMDR usage). Adding these as notes could be helpful if this is consistent with WMDR practice.
• Definition of infrared is questionable at the longwave end (1 mm = 300 GHz). A satellite instrument (ICI - see OSCAR/Space) measuring in the range 183-664 GHz is described as "millimetre wave" and I am not aware of anyone who would consider it a far-IR instrument.
• Shortwave radiation. I see no reason for the limitation at the shortwave end ("longer than 280 nm"). This is more in the nature of a comment (because most solar radiation is in this range) than a definition.
• Longwave radiation. Again "typically originating from sources at terrestrial temperatures" is more in the nature of a comment than a definition.
• Bidirectional reflectance. The intended units here (for 0.4-0.7) are micrometres. However bidirectional reflectance is highly dependent on wavelength within this range. (That's what we mean by the colour of something!) So what was intended here? Is the wavelength dependence missing? Or is it intended as a spectrally averaged quantity?

[joergklausen]

@JREyre I was able to add the missing unit µm easily in 2 places. For the bidirectional reflectance, I tried to summarize your earlier comment in a shorter phrase. Please feel free to correct/expand. This is not my field of expertise ...

[PeterBlattner]

Dear colleagues,

@joergklausen invited me to comment on issue #389. I was involved in the revision of the CIE International Lighting Vocabulary (ILV, https://cie.co.at/e-ilv ) which is fully harmonized with the IEV part 845 (https://www.electropedia.org/iev/iev.nsf/index?openform&part=845 ). I am not really an expert on atmospheric radiation and some comments may therefore seem trivial or inappropriate.

In general, I would recommend that in addition to the quantity, the units are also described somewhere. For example at the end of the description " Downward short-wave irradiance" one could add "unit: W m^-2".

Some comments for the individual entries:

Downward short-wave irradiance :
According to the ILV the quantity "irradiance" https://cie.co.at/eilvterm/17-21-053 always refers to a point at a real or imaginary surface. Also I'm missing the "short-wave" in the description. Thus an alternative description could be:
Flux density of the downward short wave solar radiation with respect to area at a point on a real or imaginary surface. unit: W m^-2

Upward short-wave irradiance: similarly:
Flux density of short-wave solar radiation, reflected by the Earth surface and atmosphere with respect to area at a point on a real or imaginary surface. unit: W m^-2

Upward spectral radiance:
The description is inconsistent with the description of the upward short wave irradiance as all of a sudden "top of the atmosphere" is introduced. Either add it to the " Upward short-wave irradiance" or remove it here. In addition the unit could be added : unit: W m^-2 sr^-1

Solar spectral irradiance:
There is an inconsistency between the name and the description: According to my understanding "solar spectral irradiance" is a spectral quantity (https://cie.co.at/eilvterm/17-21-027 ) with unit : W m^-2 nm^-1 whereas the term total solar irradiance (TSI) is typically the solar spectral irradiance integrated over the full spectrum, unit: : W m^-2 .

Earth surface short-wave bidirectional reflectance
Again there is an inconsistency between the name and the description: The term "reflectance" relates to a dimensionless quantity (ratio of flux), see https://cie.co.at/eilvterm/17-24-064 , whereas the BRDF relates to a quantity with unit sr^-1 as it is a quotient between a radiance and a irradiance. Why the wavelength range is no limited to 0.4 um to 0.7 um? According to the ISO 9488 the wavelength range of "short-wave" is different.

Long-wave earth surface emissivity
There is an inconsistency between the name and the descirption: the description stipulates that it is a spectral quantity, hence "spectral" should be added in the name. Alternatively, the description can be revised ("Spectral emissivity of the earth surface integrated over the thermal IR wavelength range").

Hope you find these comments useful.

[laurentvuilleumier]

@joergklausen @JREyre

About John's comments to ISO definitions communicated by Jörg:

• I agree that 1mm is very far as upper limit for IR radiation (Definition of infrared is questionable at the longwave end).
• Shortwave radiation (I see no reason for the limitation at the shortwave end): my feeling is that the short-end limit (280nm) is instrument-related. At the other end, the cutoff between shortwave and longwave is instrument-related, since they are clearly separated, while there is an overlap between the IR part of the solar spectrum and the lower end of the terrestrial spectrum. By analogy, I think that the short-end limit was set at 280nm because all pyranometers I know of have their spectral range starting at about 280nm.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.01.12-TT-WIGOSMD notes:

• not ready for FT2023-1, it has had good feedback, but it's opened up new questions
• Proposal summary is updated with latest information

[AnnWebb]

This is now a very long chain of comments, so rather than try and pick out previous comments and provide a response, I will simply give my views.

1. I agree with Peter that units should be provided with all definitions - this can also help to ensure consistency. Also that anything 'spectral' should have units of 'per nm', albeit that we understand the measurement must be made across a small but finite waveband.

2. There are inconsistencies/potential confusions with the terms 'surface' and 'top of atmosphere' (TOA). The radiation definition (e.g. radiance / irradiance) is the same whatever the surface through which the flux passes or is incident upon: the earth's surface, TOA, or some level in between (e.g. cloud top). Thus downward short-wave irradiance is a general (any surface) description while Total solar irradiance (the same radiation quantity) is specified only for TOA, as is solar spectral irradiance. This is confusing, and as someone who makes ground-based measurements I would say it is incorrect and TSI can also be used for the surface of the Earth. It is also known as Global solar irradiance, but I note this is to be superseded anyway. There should be a single definition for radiation quantity (irrespective of surface) and a comment to indicate this is (usually) the earth surface or TOA depending on measurement system/application. In the same vein, short-wave and solar are both used to mean the same thing (wavelengths of radiation received from the sun): should a single term be used throughout? Personally I prefer 'solar' as short-wave is subjective and certainly may mean other things in different disciplines, but I suspect I may lose that argument.

3. There is no downward spectral radiance. Regarding the request to include actinic flux, actinic flux is understood as the spectral radiance integrated over 4pi, at least when referring to atmospheric chemistry. Actinic just means 'having a photochemical effect' and the spectral radiance is weighted for whichever reaction is under consideration. More generally 'actinic flux' could be the flux through any surface, so some care is needed in defining the term for use in this community.

4. My understanding of albedo is the reflectance of downwelling short-wave radiation (or solar radiation) by a surface (e.g. Earth or TOA, the latter giving planetary albedo). While 'spectral albedo' or albedo for a particular waveband is sometimes specified (better to use spectral reflection), the general term refers to all wavelengths of solar radiation, not just the visible. The general term BRDF is also not confined to the visible, though there may be measurement techniques that only measure it in the visible. In any case, it is very wavelength dependent, even in the visible which we identify as colour.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.02.16-TT-WIGOSMD notes:
combined with #426; Franziska and John are working on the revised proposal based on comments/reviews and will look at with Joerg. They will present results at a future meeting.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023-03-17-TT-WIGOSMD notes:
work in progress (combining with #426)

[fstuerzl]

The revised proposal (combining issue #389 and #426) is now published in the proposal summary above. Thanks to @JohnEyre and @joergklausen for their contributions!

Experts advise is still needed for the following variables/definitions:

• Diffuse short-wave irradiance: Is a definition of the net flux direction needed? The direction would normally be downward, but it is theoretically possible that it could be upward.
• Background luminance: Is the note included in the definition needed? (Note: Luminous flux is a quantity derived from radiant flux by evaluating the radiation according to its action upon the International Commission on Illumination standard photometric observer.)

[amilan17]

@fstuerzl I'm going to create a new branch from master, because it will be too difficult to rebase.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.04.21-TT-WIGOSMD:
Franziska will work on branch

[JREyre]

@fstuerzl @gaochen-larc @joergklausen @amilan17

For the record, I am copying here the comments from Sebastian Schmidt on my comments (above), together with my replies. I reviewed all the comments again, and so some of my replies are modified from previous emails:

1. John: Diffuse short-wave irradiance: Is a definition of the net flux direction needed? The direction would normally be downward, but it is theoretically possible that it could be upward.
   Sebastian: Upward flux is always diffuse. When talking about diffuse irradiance, I can confirm that we typically mean downward, but I also agree that we could be more specific if that's preferable.
   John: I don’t think I agree that the upward flux is always diffuse. Over cloud it will often be close to diffuse. But, for example, over a calm (mirror-like) ocean with a cloud-free sky, it is close to specular. (I am taking “diffuse” to mean no variation with nadir angle but, from a measurement perspective, it will usually mean that the measurement integrates over all nadir angles, over the whole hemisphere. However, these are definitions of geophysical variables, not measurements.) However, Sebastian’s comment does not affect the definition.
   Sebastian: I think there's a confusion here between net irradiance (difference between downwelling and upwelling), and diffuse irradiance (component of the downwelling shortwave irradiance). Am I mis-reading something there?
   John: Net flux v upward and downward fluxes. The “requirement” is (usually) to observe the net, but measurements are usually of upward or downward. So, I think there is good reason to have both on the list, but we need to check that this is done consistently. I think they currently are consistent (if, perhaps, not complete). Does everyone agree?

2. John: Background luminance: Is the note included in the definition needed? (Note: "Luminous flux is a quantity derived from radiant flux by evaluating the radiation according to its action upon the International Commission on Illumination standard photometric observer.")
   Sebastian: I would say an explanation would be helpful because it's not a typical quantity that we come across very often. That said, I am not sure I understand the note 🙂 What is meant by "background" in this context? Sorry to not have a straight answer for you here.....
   John: I agree with Sebastian - I too do not understand the comment. Can we identify an expert who is more familiar with this area? Alternatively, we could drop "Background luminance" from the list for the time being, in order to move this Issue along.

3. Long-wave irradiance (direction unspecified).
   Sebastian: I think that's fine, but irradiance is often just specified w.r.t. a horizontal reference. (There are exceptions to this, for example, when quantifying horizontal photon flux density, but that is only rarely measured – almost never in the longwave).
   John: I agree with the comment, but I don't this affects the definition. noting that this is a geophysical variable, not a measurement.

4. Earth surface short-wave bidirectional reflectance.
   Sebastian: This is of course correct, but the specification of the wavelength band seems somewhat arbitrary. In practice, most BRDF measurements are done spectrally and not averaged over the visible band.
   John: Spectrally integrated v spectrally resolved. It is true that most new missions measure spectrally resolved quantities, but some old missions measured spectrally integrated, and so there is good reason to include both.

5. Albedo
   Sebastian: While we are on the topic: If BRDF is included, it might be useful to include albedo also since it is one of the primary modulators of Earth's radiation budget. It can be derived from upwelling and downwelling shortwave spectral or broadband irradiance, and therefore is not a primary parameter – maybe have a discussion amongst your team whether it's appropriate to include?
   John: We have an observed variable called "surface albedo" under "atmosphere". It currently lacks a definition. Franziska: why have we not addressed this before?

To summarise, Sebastian has given us some good comments, but I don't think they affect our proposed variables or their definitions, with the following exceptions:

• Background luminance. What to do about this? I suggest 2 options above.
• Albedo. What to do about the lack of definition?

[gaochen-larc]

AMS definition for Surface Albedo: The ratio, expressed as a percentage, of the amount of electromagnetic radiation reflected by the earth's surface to the amount incident upon it. Value varies with wavelength and with the surface composition.

I wonder if the definition should also indicate surface albedo is also a function of incident angle.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.05.04-TT-WIGOSMD notes:
There has been a lot of feedback recently and John is still following through.

[AnnWebb]

Add the concept of planetary albedo? Includes reflection from the atmosphere including clouds – as measured from satellites. Ann From: gaochen-larc ***@***.***> Sent: 04 May 2023 00:05 To: wmo-im/wmds ***@***.***> Cc: Ann Webb ***@***.***>; Mention ***@***.***> Subject: Re: [wmo-im/wmds] 1-01-01 Align radiation variables requirements (Issue #389) AMS definition for Surface Albedo: The ratio, expressed as a percentage, of the amount of electromagnetic radiation reflected by the earth's surface to the amount incident upon it. Value varies with wavelength and with the surface composition. I wonder if the definition should also indicate surface albedo is also a function of incident angle. — Reply to this email directly, view it on GitHub<#389 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/A45BQQZVBYWQ7MIUSAYXW6DXELQCJANCNFSM5UPDZJFA>. You are receiving this because you were mentioned.Message ID: ***@***.***>

[JREyre]

@AnnWebb @gaochen-larc @joergklausen @fstuerzl
Hi Ann,
That's a very good thought. However, I have checked the OSCAR/Requirements and OSCAR/Space databases, and "planetary albedo" does not appear in either. This means that no Application Area (not even GCOS, for Climate Monitoring) has stated a requirement to observe it, and no Space Agency has registered a capability to observe it.
I will feed this back to the managers of the WIGOS RRR process.
John

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.06.01-TT-WIGOSMD notes:
@JohnEyre will add relevant content based on recent feedback; @fstuerzl will update branch; @joergklausen will validate PR

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.06.15-TT-WIGOSMD notes:

@joergklausen will review PR

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.06.30-TT-WIGOSMD notes:

@joergklausen will review PR 497

[joergklausen]

@amilan17 Done. Pls merge PR