1-01-01 Addition of definitions for cloud variables

[fstuerzl]

Initial request

Addition of missing definitions for cloud variables 181, 182, 183, 507, 508 and 533.
Name change for variable 508

Amendment details

updated 09 April 2024

Name:

notation	old name	new name
508	Optical depth within each cloud layer	Optical depth within a cloud layer

Descriptions:

notation	path	name	description
181	\Atmosphere\Clouds	Cloud drop effective radius	The area weighted mean radius of liquid water drops, assimilated to spheres of the same volume.
182	\Atmosphere\Clouds	Cloud hydrometeor concentration	Number of cloud hydrometeors (or cloud particles) per unit volume. Cloud hydrometeors refer to liquid and/or ice cloud particles.
183	\Atmosphere\Clouds	Effective radius of cloud hydrometeors	An ensemble mean size of cloud hydrometeors derived from size distribution. Cloud hydrometeors refer to liquid and/or ice cloud particles. For liquid phase cloud, the effective radius is defined as the area weighted mean radius of liquid water drops, assimilated to spheres of the same volume. The effective radius of cloud ice particles has been defined in literature in different ways. Clear and detailed definition should be provided as how this parameter is derived.
507	\Atmosphere\Clouds\Optical properties	Optical depth of fog	The degree to which fog prevents light from passing through it. The WMO Cloud Atlas refers to the term “fog” "when microscopic droplets reduce horizontal visibility at the Earth’s surface to less than 1 km" ([WMO, International Cloud Atlas, 2017 ed., https://cloudatlas.wmo.int/en/fog-compared-with-mist.html].
508	\Atmosphere\Clouds\Optical properties	Optical depth within a cloud layer	A part of cloud optical depth which results from a vertical integral of light extinction by cloud particles between the bottom and top of a cloud layer.
533	\Atmosphere\Clouds\Position	Height of inversion	Height of a stable layer of air with an increased temperature.

Comments

Every entry in the WMDR code lists should have a useful definition.

Requestor(s)

TT WIGOSMD

Stakeholder(s)

Enter list of stakeholder(s).

Publication(s)

Example: Manual on Codes (WMO-No. 306), Volume I.3, WMO Codes Registry, Code table 1-01-01

Expected impact of change

LOW
-> all changes can be implemented in OSCAR as soon as the codes approved.

Collaborators

Eliane Thürig, MeteoSwiss
@JREyre
@gaochen-larc
@meulenvd
@charlesabrock
@markusfiebig

References

WMO, International Cloud Atlas, 2017 ed.

Validation

No response

[fstuerzl]

See also discussion in issue #390

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.02.16-TT-WIGOSMD notes:

Franziska will review the status and update as needed.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2022.03.16-TT-WIGOSMD notes:
Franziska will look at.

[joergklausen]

507 defines 'fog' and 'mist', but not 'optical depth'
508 has no description at all
533 describes the phenomenon, but not the variable
@fstuerzl Please revise and update the issue.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023-03-17-TT-WIGOSMD notes:
Still a work in progress; we might deprecate optical depth in cloud layers; E-Profile network might have some feedback.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.05.04-TT-WIGOSMD notes:
still waiting for feedback from E-Profile on 508; @gaochen-larc said that lidar can measure this and will talk with a colleague;

[amilan17]

This doesn't help with the definition, but quick research in the Manual on Codes vI.2 shows that perhaps "optical depth within each cloud layer" was intended to represent the GRIB codes below that are now deprecated.

GRIB2
Code table 4.2
Product discipline 3 – Space products, parameter category 2: cloud properties
Number Parameter Units
0 Clear sky probability %
1 Cloud top temperature K
2 Cloud top pressure Pa
3 Cloud type Code table 4.218
4 Cloud phase Code table 4.218
5 Cloud optical depth Numeric
6 Cloud particle effective radius m
7 Cloud liquid water path kg m–2
8 Cloud ice water path kg m–2
9 Cloud albedo Numeric
10 Cloud emissivity Numeric
11 Effective absorption optical depth ratio Numeric
30 Measurement cost Numeric
31 Upper layer cloud optical depth (see Note) Numeric
32 Upper layer cloud top pressure (see Note) Pa
33 Upper layer cloud effective radius (see Note) m
34 Error in upper layer cloud optical depth (see Note) Numeric
35 Error in upper layer cloud top pressure (see Note) Pa
36 Error in upper layer cloud effective radius (see Note) m
37 Lower layer cloud optical depth (see Note) Numeric
38 Lower layer cloud top pressure (see Note) Pa
39 Error in lower layer cloud optical depth (see Note) Numeric
40 Error in lower layer cloud top pressure (see Note) Pa
Note: Numbers 31 to 40 are deprecated.

[fstuerzl]

I've updated the descriptions for variables 507 and 533.

Optical depth of fog: The degree to which fog prevents light from passing through it. The WMO Cloud Atlas refers to the term “fog” "when microscopic droplets reduce horizontal visibility at the Earth’s surface to less than 1 km" ([WMO, International Cloud Atlas, 2017 ed., https://cloudatlas.wmo.int/en/fog-compared-with-mist.html].

Height of inversion: Height of a stable layer of air with an increased temperature.

Variable 508 "Optical depth within each cloud layer" is still unlear.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2023.06.01-TT-WIGOSMD:
Franziska updated two variables; Gao is still trying to get feedback from a colleague on 508.

[joergklausen]

Please consult: Alexander Haefele

[gaochen-larc]

Optical depth within each cloud layer - a component of cloud optical depth which results from a vertical integral of light extinction by cloud particles between the observed bottom and top of a cloud layer.

Comments and changes are welcome.

[amilan17]

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

• code 508, change name to "Optical depth within a cloud layer"; Joerg updated the issue summary; 
• @amilan17 will update the branch

[amilan17]

@wmo-im/tt-wigosmd I updated the branch for 507, 508 and 533, but I have a couple questions:

• why are we superseding 182 and 183? Is it truly an outdated vocabulary term? Are hydrometeors being replaced by "cloud drops"? This link has a definition of hydrometeors for reference: https://space.oscar.wmo.int/variables/view/hydrometeor_type. 
• There are other hydrometeors in this table "522, Hydrometeor radius" and "523,Hydrometeor type"...

[joergklausen]

@meulenvd Can you please comment on the term "hydrometeor"?

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2024.01.18-TT-WIGOSMD notes:
@gaochen-larc and @fstuerzl will check to see if this proposal is ready

[gaochen-larc]

To me, this is proposal is not ready. @fstuerzl, do you agree?

After looking into "hydrometeors" more, we may want to leave it and add descriptions.

[charlesabrock]

“hydrometeors” includes rain, hail, snow, etc. which are not necessarily “cloud particles”. cb ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Charles Brock NOAA Chemical Sciences Laboratory 325 Broadway R/CSD2 Boulder, CO 80305 303-887-2523 (mobile) ***@***.***

…

On Jan 18, 2024, at 2:41 PM, gaochen-larc ***@***.***> wrote: To me, this is proposal is not ready. @fstuerzl <https://github.com/fstuerzl>, do you agree? I think "hydrometeors" may be replaced by "cloud particles". — Reply to this email directly, view it on GitHub <#443 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ATIXPDLJ7WB5MUS23HHUBLLYPGJG5AVCNFSM6AAAAAAT2SOVKGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTQOJZGI2DKMBXGY>. You are receiving this because you are subscribed to this thread.

[gaochen-larc]

“hydrometeors” includes rain, hail, snow, etc. which are not necessarily “cloud particles”. cb ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Charles Brock NOAA Chemical Sciences Laboratory 325 Broadway R/CSD2 Boulder, CO 80305 303-887-2523 (mobile) @.***
…
On Jan 18, 2024, at 2:41 PM, gaochen-larc @.***> wrote: To me, this is proposal is not ready. @fstuerzl https://github.com/fstuerzl, do you agree? I think "hydrometeors" may be replaced by "cloud particles". — Reply to this email directly, view it on GitHub <#443 (comment)>, or unsubscribe https://github.com/notifications/unsubscribe-auth/ATIXPDLJ7WB5MUS23HHUBLLYPGJG5AVCNFSM6AAAAAAT2SOVKGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTQOJZGI2DKMBXGY. You are receiving this because you are subscribed to this thread.

Agree!

[markusfiebig]

2 cents here:

The ACTRIS vocabulary still uses hydrometeor, mainly because it relates so closely to radar observations:
https://vocabulary.actris.nilu.no/actris_vocab/hydrometeor

However, a hydrometeor isn't the same as a cloud particle, we list hydrometeor as cloud related.

[gaochen-larc]

Propose to adopt or modify the AMS hydrometeor definition: "Any product of condensation or deposition of atmospheric water vapor, whether formed in the free atmosphere or at the earth's surface, any water particle blown by the wind from the earth's surface."

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2024.02.15-TT-WIGOSMD notes:

• team discussed 182; Jitze noted that hydrometeors are in the Technical Regulations;
• DECISION: keep "hydrometeor" in 182 and 183 name; still needs definition; no need to supersede;
• proposal summary (first comment) needs to be updated with the most recent proposal;

181: used definition from -> https://en.wikipedia.org/wiki/Cloud_drop_effective_radius#cite_note-1
182: needs a definition
183: needs definition

@gaochen-larc -- please propose some definitions and update proposal summary (if possible)

[gaochen-larc]

@charlesabrock and @markusfiebig: how would you define "cloud hydrometeor"? This term has been used in code 182 (Cloud hydrometeor concentration) and code 183 (Effective radius of cloud hydrometeors), which started this issue.

[charlesabrock]

I would support using the AMS definition. I don’t see that the WMO has one. Chuck ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Charles Brock NOAA Chemical Sciences Laboratory 325 Broadway R/CSD2 Boulder, CO 80305 303-887-2523 (mobile) ***@***.***

…

On Feb 15, 2024, at 5:39 AM, gaochen-larc ***@***.***> wrote: @charlesabrock <https://github.com/charlesabrock> and @markusfiebig <https://github.com/markusfiebig>: how would you define "cloud hydrometeor"? This term has been used in code 182 (Cloud hydrometeor concentration) and code 183 (Effective radius of cloud hydrometeors), which started this issue. — Reply to this email directly, view it on GitHub <#443 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ATIXPDNJUKR534K6U4R34CTYTX6X7AVCNFSM6AAAAAAT2SOVKGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTSNBWGAYTSNZSGY>. You are receiving this because you were mentioned.

[JREyre]

@gaochen-larc @joergklausen @amilan17 @meulenvd
A few comments:

• 182, 183. I support the need to retain the "hydrometeor" variables.
• 182. Definition should include that this is a number density.
• 181, 183. Effective radius can be defined at the level of a single particle. However, is is usually intended, both in measurement and in modelling, to mean the mean-average over an ensemble of particles. Should we include "mean" in the name of the variable?
• 507, 508. Optical depth is a dimensionless quantity (not a length). It is exp(-x), where x is the ratio of (some) emerging radiative quantity and the equivalent incident radiative quantity. Definitions should make this clear. Or is it sufficient to refer to a Manual in which this is explained?
• 533. (a) This is not a cloud variable. (b) An inversion is a layer is which some variable (e.g. temperature) has a gradient "inverted" (of the opposite sign) from its usual gradient. Normally "inversion height" is used to mean the TOP of the inversion layer (e.g. the height of a capping inversion is the height of its top). Not sure how to deal with this. cf. Planetary boundary layer (PBL) and PBL top.

[gaochen-larc]

proposed description for Cloud hydrometeor concentration (182) and Effective radius of cloud hydrometeors (183):

Cloud hydrometeor concentration: number of cloud hydrometeors (or cloud particles) per unit volume. Cloud hydrometeors refer to one type of hydrometeor, i.e., liquid and/or ice cloud particles.

Effective radius of cloud hydrometeors: The area weighted mean radius of cloud hydrometeors (cloud particles) as derived from the size distribution.

@JREyre: The effective radius has been widely used to refer the area weighted mean radius in literature, especially dealing with remote sensing. It was first published, however, in 1974 (as shown in wiki page provided by Anna). Interestingly I could not find this term in the classical textbook "The Physics of Clouds". If we add "mean" to the name, it may be inconsistent with many publications...

[charlesabrock]

I concur with these proposed definitions. cb ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Charles Brock NOAA Chemical Sciences Laboratory 325 Broadway R/CSD2 Boulder, CO 80305 303-887-2523 (mobile) ***@***.***

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On Feb 19, 2024, at 10:13 AM, gaochen-larc ***@***.***> wrote: proposed description for Cloud hydrometeor concentration (182) and Effective radius of cloud hydrometeors (183): Cloud hydrometeor concentration: number of cloud hydrometeors (or cloud particles) per unit volume. Cloud hydrometeors refer to one type of hydrometeor, i.e., liquid and/or ice cloud particles. Effective radius of cloud hydrometeors: The area weighted mean radius of cloud hydrometeors (cloud particles) as derived from the size distribution. @JREyre <https://github.com/JREyre>: The effective radius has been widely used to refer the area weighted mean radius in literature, especially dealing with remote sensing. It was first published, however, in 1974 (as shown in wiki page provided by Anna). Interestingly I could not find this term in the classical textbook "The Physics of Clouds". If we add "mean" to the name, it may be inconsistent with many publications... — Reply to this email directly, view it on GitHub <#443 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ATIXPDPDYYBEWSPEGBLARPTYUOB35AVCNFSM6AAAAAAT2SOVKGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTSNJSHEYDIMZTGM>. You are receiving this because you were mentioned.

[meulenvd]

I would support using the AMS definition. I don’t see that the WMO has one. Chuck

@charlesabrock @amilan17 @joergklausen @JREyre @gaochen-larc @markusfiebig @fstuerzl

Maybe it's good to refer to the WMO definitions, which can be found in WMO-No. 182 (International Meteorological Vocabulary) and WMO-No. 407 (International Cloud Atlas, Manual on the Observation of Clouds and Other Meteors). Both are WMO Basic documents and WMO-No. 407 is part of the WMO Regulatory Material. Definitions in both documents are interlinked.

Definitions provided are:

WMO-No. 182 (International Meteorological Vocabulary):

C1450 cloud
A hydrometeor consisting of minute particles of liquid water or ice, or of both, suspended in the free air and usually not touching the ground. It may also include larger particles of liquid water or ice and non-aqueous liquid or solid particles such as those present in fumes, smoke and dust.

C1580 cloud drop (C1590)
C1590 cloud droplet
Drop, supercooled or not, belonging to a cloud and having a diameter of about 4 to 100 µm.

H0970 hydrometeor
(1) Meteor consisting of an ensemble of liquid or solid water particles falling through or suspended in the atmosphere, blown by the wind from the Earth's surface or deposited on objects on the ground or in the free air.
(2) Liquid or solid water particles falling or suspended in the atmosphere.

WMO-No. 407 (International Cloud Atlas, Manual on the Observation of Clouds and Other Meteors) [mandatory]

Definition of a cloud
A cloud is a hydrometeor consisting of minute particles of liquid water or ice, or of both, suspended in the atmosphere and usually not touching the ground. It may also include larger particles of liquid water or ice, as well as non-aqueous liquid or solid particles such as those present in fumes, smoke or dust.

Meteorological definition of a meteor
In meteorology, a phenomenon observed in the atmosphere or on the surface of the Earth is known as a meteor. It may be a form of precipitation, a suspension or a deposit of liquid or solid particles. It may also be a form of optical or electrical phenomena. Meteors are generally visible to human observers, but in the case of thunder, it is audible.
Sometimes, names of meteors are also applied to related concepts. For instance, the word “snow” may refer to a type of hydrometeor (ensemble of falling particles), to snow cover (ensemble of particles lying on the ground) or to the substance snow (as in “snow blown from mountains” or a snowball). The constituent particles of snow in these three cases are either snow crystals or snowflakes.
Also, certain meteors are named based on their constituent particles; for example, the hydrometeor “snow grains" is an ensemble of falling snow grains.

General classification of meteors
Meteors present a great diversity of character. However, by considering their constituent particles or the physical processes surrounding their occurrence, meteors can be classified into four groups: hydrometeors, lithometeors, photometeors and electrometeors.

Hydrometeors
Hydrometeors consist of liquid or solid water particles. They may be suspended in the atmosphere, fall through the atmosphere, be blown by the wind from the Earth’s surface or be deposited on other objects. Snow or water on the ground is, by convention, not considered a hydrometeor.
We describe the following five types of hydrometeors:
• Suspended particles;
• Falling particles (precipitation);
• Particles raised by the wind from the Earth’s surface;
• Deposits of particles;
• Spouts.

[JREyre]

@charlesabrock @amilan17 @joergklausen @JREyre @gaochen-larc @markusfiebig @fstuerzl @meulenvd
I am concerned that "effective radius" is ambiguous. It has two plausible meanings, and I think both are used by some communities: (a) the radius that a non-spherical particle would have if it were spherical (with the same mass and density), (b) a weighted-average radius that would lead to the same radiative properties as found for a ensemble of particles (with different radii). (b) may assume that the particles are all spherical but with a size distribution n(r). For an ensemble of non-spherical particles, then a combination of (a) and (b) may be needed.
I plan to consult 2 Met Office (UK) experts in these matters: Pete Francis, an expert in remote sensing of cloud from space, and Antony Baran, an expert in the parametrisation and modelling (NWP modelling and radiative modelling) of ice cloud.

[gaochen-larc]

I consulted a cirrus cloud expert myself and learned "effective radius" have had more than one definition. However, they are all some kind of weighted mean.

https://journals.ametsoc.org/view/journals/clim/11/7/1520-0442_1998_011_1793_teriic_2.0.co_2.xml

[JREyre]

@charlesabrock @amilan17 @joergklausen @JREyre @gaochen-larc @markusfiebig @fstuerzl @meulenvd
I have now had replies from my former colleagues, Pete Francis and Anthony Baran. (Note that a paper by Pete in 1994 was one of the references in the paper by Wyser.)
They offer two other references:

• https://journals.ametsoc.org/view/journals/atsc/55/11/1520-0469_1998_055_2039_tdasoa_2.0.co_2.xml
• https://acp.copernicus.org/articles/11/3417/2011/
  The first one (McFarquhar and Heymsfield) summarises the problem we have for our work (section 3, 1st para):
  "The definition and meaning of an effective dimension for a cloud composed of nonspherical ice crystals is ambiguous since it does not represent a physical or measurable property of the ice crystal size distribution. It could be defined either as proportional to the ratio of volume to projected area [Eq. (4)] or as a mean particle size for scattering; however, unlike the case of water clouds, it cannot be both simultaneously because the right-hand side of [Eq.(3)] does not equal the right-hand side of [Eq. (1)] for particles without circular cross sections. A number of different definitions of re are summarized in [Table 1]. The dependence of re on crystal habit and distribution breadth is described by these equations. All definitions of re are equivalent for spherical particles. It is also noted that some authors originally formulated their definitions as an effective diameter, which is defined to be twice the value of re."
  So, it is surprising that other terms have not been invented, each with an unambiguous definition. If they have not, then I suggest we will have to adopt a catch-all definition for effective radius and highlight the inherent ambiguity.

[gaochen-larc]

@JREyre: Thank you for the thoughtful comments and the reference!

Here is another proposal:
Effective radius of cloud hydrometeors: an indicator of the ensemble mean size of cloud hydrometeors derived from size distribution. Cloud hydrometeors refer to one type of hydrometeor, i.e., liquid and/or ice cloud particles. For liquid phase cloud, the effective radius is defined as the area weighted mean radius. The effective radius of cloud ice particles has been defined in literature in different ways. The data producer should provide clear definition how this parameter is derived.

please feel free to comment or edit this proposed definition. Thanks!

@joergklausen and @amilan17 since this measurand cannot be unambiguously defined, would this be a problem?

[amilan17]

@JREyre @gaochen-larc @meulenvd Thank you very much for working on this. However, It looks like it is still in the final stages of discussion and the branch has not been updated.

[gaochen-larc]

@JREyre @gaochen-larc @meulenvd Thank you very much for working on this. However, It looks like it is still in the final stages of discussion and the branch has not been updated.

@amilan17 are you ok with the proposed 183's description being ambiguous?

[JREyre]

@gaochen-larc @amilan17 @joergklausen @meulenvd
Concerning the proposed definition:

• 1st sentence. It's more than an indicator - it's a length (a radius).
• 3rd sentence (liquid phase). I suggest to add "cross-sectional" before "area", for consistency with the definitions given by Wyser and by McFarquhar and Heymsfield.
• 5th sentence (data producer). I agree with the spirit of this, but not sure "data producer is quite right for the definition of a geophysical variable (rather than an observation or data product of that variable).

[gaochen-larc]

@JREyre : Thank you!

3rd try:
an ensemble mean size of cloud hydrometeors derived from size distribution. Cloud hydrometeors refer to one type of hydrometeor, i.e., liquid and/or ice cloud particles. For liquid phase cloud, the effective radius is defined as the cross-sectional area weighted mean radius. The effective radius of cloud ice particles has been defined in literature in different ways. Clear and detailed definition should be provided as how this parameter is derived.

Please feel free to make edits

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2024.03.21%E2%80%90TT%E2%80%90WIGOSMD notes:
@gaochen-larc - please provide a comment below with a summary of the complete proposal to review

[gaochen-larc]

notation	name	description
182	Cloud hydrometeor concentration	number of cloud hydrometeors (or cloud particles) per unit volume. Cloud hydrometeors refer to liquid and/or ice cloud particles.
183	Effective radius of cloud hydrometeors	an ensemble mean size of cloud hydrometeors derived from size distribution. Cloud hydrometeors refer to liquid and/or ice cloud particles. For liquid phase cloud, the effective radius is defined as the cross-sectional area weighted mean radius. The effective radius of cloud ice particles has been defined in literature in different ways. Clear and detailed definition should be provided as how this parameter is derived.
181	Cloud drop effective radius	The area weighted mean radius of liquid water drops, assimilated to spheres of the same volume.

[amilan17]

https://github.com/wmo-im/tt-wigosmd/wiki/2024.04.09%E2%80%90TT%E2%80%90WIGOSMD notes:
Gao provided a report on the status of the issue; DECISION: Use the same description from 181 into 183. The team approves of the proposal and branch should be updated for @joergklausen to validate. @fstuerzl will update branch

[fstuerzl]

Branch is ready for validation, @joergklausen