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Standard names: *Aerosol Lidar Variables by ACTRIS/EARLINET in ENVRI FAIR* #173

Closed claudio-dema closed 1 year ago

claudio-dema commented 2 years ago

Proposer's name Claudio Dema Date 2021-12-23.

- Term volume_backscattering_coefficient_of_radiative_flux_by_ranging_instrument_in_air_due_to_ambient_aerosol_particles - Description Volume backscattering coefficient by ranging instrument is the fraction of radiative flux, per unit path length and per unit solid angle, scattered at 180 degrees angle respect to the incident radiation and obtained through ranging techniques like lidar and radar. Backscattering coefficient is assumed to be related to the same wavelength of incident radiation. "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. The specification of a physical process by the phrase "dueto" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase. - Units m-1 sr-1

- Term lidar_ratio_by_ranging_instrument_in_air_due_to_ambient_aerosol_particles - Description The lidar ratio is the ratio of the “volume extinction coefficient” and the "volume backscattering coefficient of radiative flux by ranging instrument in air due to ambient aerosol particles". The ratio is assumed to be related to the same wavelength of incident radiation. "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. The specification of a physical process by the phrase "dueto" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase. - Units sr

- Term cloud_type - Description A variable with the standard name of cloud_type contains integers which indicate the absence of cloud or, in case of its presence, the character of the cloud itself. - Units 1

- Term volume_extinction_angstrom_exponent_in_air_due_to_ambient_aerosol_particles - Description The volume extinction Angstrom exponent is the Angstrom exponent obtained for the aerosol extinction instead that for the aerosol optical thickness and is alpha in the relating aerosol extinction at the wavelength lambda as a function of aerosol extinction at a different wavelength lambda0: ext(lambda) = ext(lambda0) * [lambda/lambda0] * (-1 alpha). "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. - Units 1

- Term volume_backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles - Description The volume backscattering Angstrom exponent is the Angstrom exponent obtained for the aerosol backscattering instead of aerosol optical thickness and is alpha in the relating aerosol backscatter at the wavelength lambda as a function of aerosol backscattering at a different wavelength lambda0: back(lambda) = back(lambda0) * [lambda/lambda0] * (-1 alpha). "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. - Units 1

- Term atmosphere_boundary_layer_top_height_defined_by_ambient_aerosol_particles_backscattering_by_ranging_instrument - Description The atmosphere boundary layer top height is the elevation above sea level of the top of the (atmosphere) planetary boundary layer. “defined_by” provides the information of the tracer used for identifying the atmospheric boundary layer top. "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. "By ranging instrument" means that the backscattering is obtained through ranging techniques like lidar and radar.
- Units m

- Term atmosphere_mixing_layer_top_height_defined_by_ambient_aerosol_particles_backscattering_by_ranging_instrument - Description The atmosphere mixing layer top height is the elevation above sea level of the top of the (atmosphere) mixing layer or convective boundary layer. “defined_by” provides the information of the tracer used for identifying the atmospheric boundary layer top. "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. "By ranging instrument" means that the volume backscattering coefficient is obtained through ranging techniques like lidar and radar. - Units m

- Term aerosol_type_in_atmosphere_layer - Description A variable with the standard name of aerosol_type contains integers which indicate the type of the aerosol determined following a certain aerosol typing schema. "Layer" means any layer with upper and lower boundaries that have constant values in some vertical coordinate. There must be a vertical coordinate variable indicating the extent of the layer(s). - Units 1

- Term volume_extinction_angstrom_exponent_in_air_due_to_ambient_aerosol_particles_in_atmosphere_layer - Description The volume extinction Angstrom exponent is the Angstrom exponent obtained for the aerosol extinction instead that for the aerosol optical thickness and is alpha in the relating aerosol extinction at the wavelength lambda as a function of aerosol extinction at a different wavelength lambda0: ext(lambda) = ext(lambda0) * [lambda/lambda0] * (-1 alpha). "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. "Layer" means any layer with upper and lower boundaries that have constant values in some vertical coordinate. There must be a vertical coordinate variable indicating the extent of the layer(s). - Units 1

- Term volume_backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles_in_atmosphere_layer - Description The volume backscattering Angstrom exponent is the Angstrom exponent obtained for the aerosol backscatter instead that for the aerosol optical thickness and is alpha in the relating aerosol backscatter at the wavelength lambda as a function of aerosol backscatter at a different wavelength lambda0: back(lambda) = back(lambda0) * [lambda/lambda0] * (-1 alpha). "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. "Layer" means any layer with upper and lower boundaries that have constant values in some vertical coordinate. There must be a vertical coordinate variable indicating the extent of the layer(s). - Units 1

- Term atmosphere_optical_thickness_due_to_ambient_aerosol_particles_in_atmosphere_layer - Description The optical thickness is the integral along the path of radiation of a volume scattering/absorption/attenuation coefficient. The radiative flux is reduced by a factor exp(-optical_thickness) on traversing the path. A coordinate variable of radiation_wavelength or radiation_frequency can be specified to indicate that the optical thickness applies at specific wavelengths or frequencies. "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. "Layer" means any layer with upper and lower boundaries that have constant values in some vertical coordinate. There must be a vertical coordinate variable indicating the extent of the layer(s). - Units 1

JonathanGregory commented 2 years ago

Dear @claudio-dema

Thanks for these thoughtful proposals. I have a few comments.

volume_backscattering_coefficient_of_radiative_flux_by_ranging_instrument_in_air_due_to_ambient_aerosol_particles. (1) There are couple of existing names with the phrase backwards_scattering_coefficient, which I suppose means the same as backscattering_coefficient. If so, we should use the existing phrase, for consistency. (2) I think by_ranging_instrument should be omitted. We don't normally mention measurement techniques in standard names, because they aren't part of the description of the geophysical quantity. They are mentioned only if the measurement technique is essential to the nature of the quantity, or if the quantity refers to some property of the instrument. This comment applies to some others of your proposals. (3) You give the units as m-1 sr-1, but existing standard names of volume [backwards] scattering coefficient have units m-1. If it's m-1 sr-1 it's called "scattering function" according to existing standard names. Which one do you mean?

lidar_ratio_by_ranging_instrument_in_air_due_to_ambient_aerosol_particles. You explain that, "The lidar ratio is the ratio of the volume extinction coefficient and the volume backscattering coefficient." Although it's longer, it would make the standard name more self-explanatory, which is our aim with standard names, to incorporate this information rather than using the technical "jargon" phrase "lidar ratio" e.g. ratio_of_volume_extinction_coefficient_to_volume_backwards_scattering_coefficient_in_air_due_to_ambient_aerosol_particles. You can mention in the definition that it's often called "lidar ratio", which will help those who are familiar with it.

cloud_type. I am quite surprised that this hasn't been proposed before! (1) It would be more self-explanatory and consistent with other similar quantities, especially area_type and region, if cloud_type was defined as of string type. It could nonetheless be stored in an integer variable by using flag_values and flag_meaning attributes. (2) Should names of cloud types be standardised strings, as area_types are, by the area type table. (3) Rather than introducing a new standard name, I wonder whether area_type could be used for cloud types. Please could you give some examples of what you would use a cloud_type variable for? I see that convective_cloud is already included in the area_type table, and I think that's an argument in favour of adding more standardised cloud types to it, if that would meet your need. The table also includes cloud and clear_sky already, and we could also add shallow_convective_cloud and stratiform_cloud, which are phrases used in existing standard names.

volume_backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles. (1) You write that this is for "aerosol backscattering instead of aerosol optical thickness." Backscattering in included in optical thickness, so this is part of the unqualified Angstrom exponent - have I understood that correctly? (2) I think it would be easier to comprehend and more consistent with standard name style if it was called angstrom_exponent_in_air_due_to_backward_scattering_due_to_ambient_aerosol_particles. Would that be correct? (3) Is it really only backwards scattering, not scattering in general? I see that we already have standard names for both volume_backwards_scattering_coefficient_in_air_due_to_dried_aerosol_particles and volume_scattering_coefficient_of_radiative_flux_in_air_due_to_ambient_aerosol_particles.

atmosphere_boundary|mixing_layer_top_height_defined_by_ambient_aerosol_particles_backscattering_by_ranging_instrument. (1) Elevation above sea level (strictly the geoid) is called altitude in standard names, whereas height is above the surface. (2) There is an existing standard name of model_level_number_at_top_of_atmosphere_boundary_layer; for consistency and readability I would prefer a similar phrase for altitude. (3) atmosphere_mixing_layer does not currently appear in standard names. The AMetSoc glossary says "mixing layer" is used in air-pollution meteorology, and it's the same as mixed layer, convective mixed layer and convective boundary layer. I suggest we say atmosphere_mixed_layer, because "mixed layer" is the main entry in the glossary, and analogous to the phrase ocean_mixed_layer. Putting (1-3) together, I would suggest altitude_at_top_of_atmosphere_boundary|mixed_layer_defined_by_backwards_scattering_by_ambient_aerosol_particles. (4) Your definitions don't say how you distinguish the boundary layer from the mixed layer. Since you are giving them different names, there must be some physical criterion you can include in the definitions.

aerosol_type_in_atmosphere_layer: (1) Could aerosol_type also be an area_type, like cloud_type, I wonder? Are there standardised names for types of aerosols? (2) I think it should be in_air because it's an intensive property of the local medium, not an extensive property of the layer. The vertical coordinate can have bounds for an intensive property as well; in fact it usually does e.g. for air_temperature.

volume_extinction|backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles_in_atmosphere_layer. What is the distinction between "in air" (in a previous one of the proposals) and "in atmosphere layer" here?

atmosphere_optical_thickness_due_to_ambient_aerosol_particles_in_atmosphere_layer. I think this is the same as the existing quantity atmosphere_optical_thickness_due_to_ambient_aerosol_particles, isn't it?

Best wishes

Jonathan

feggleton commented 2 years ago

I am also surprised we haven't had a conversation about cloud types before (perhaps before my time!). The following standard names do exist with reference to cloud but not what you are looking for:

high_type_cloud_area_fraction - referring to Cirrus, Cirrostratus, Cirrocumulus. low_type_cloud_area_fraction - referring to Stratus, Stratocumulus, Cumulus, Cumulonimbus medium_type_cloud_area_fraction - referring to Altostratus, Altocumulus, Nimbostratus scene_type_of_dvorak_tropical_cyclone_cloud_region - which uses flag_values or flag_meaning and refers to uniform_central_dense_overcast; embedded_center; irregular_central_dense_overcast; curved_band; shear

My suggestion would also be to incorporate this into the area_type table based on the fact that it already includes cloud, clear_sky and convective_cloud.

Fran

claudio-dema commented 2 years ago

Dear @JonathanGregory and @feggleton , thanks for your useful and constructive comments! Please, find my comments below:

volume_backscattering_coefficient_of_radiative_flux_by_ranging_instrument_in_air_due_to_ambient_aerosol_particles:

  1. Ok for adopting backwards_scattering_coefficient instead of backscattering_coefficient for consistency.
  2. by_ranging_instrument is relevant because the use of ranging instrument afftects the geophysical quantity measured: with backwards scattering coefficient by ranging instrument we mean “the fraction of radiative flux, per unit path length and per unit solid angle, scattered at 180 degrees angle respect to the incident radiation”. This is different, for example, from volume_backwards_scattering_coefficient_in_air_due_to_dried_aerosol_particles, in which backwards scattering refers to the sum of scattering “into all backward angles”
  3. “scattering function” is a complete different thing. We mean “scattering coefficient”. The reason for m-1 sr-1 is explained at point 2 above.

lidar_ratio_by_ranging_instrument_in_air_due_to_ambient_aerosol_particles: Ok for ratio_of_volume_extinction_coefficient_to_volume_backwards_scattering_coefficient_by_ranging_instrument_in_air_due_to_ambient_aerosol_particles. We will add in the definition that in the community this is often called “lidar ratio”, as you suggested.

cloud_type:

  1. cloud_type (and this applies also and even more for aerosol_type below) is not standardized: the classification of clouds in classes is not unique for all the algorithms and for the different detection techniques available. So that the more flexible and clean solutions could be having the cloud_type as numerical quantity (find below the discussion about integer or binary) and associate to it the flag_value and flag_meaning attributes. We propose the binary solution for this variable because this allows to handle hybrid situations: cloud_type is the results of algorithm for scene classification. There are situations in which algorithm can provide not a unique answer. Having a binary code for each possibility allows to combine all the possible results of the algorithm.
  2. See point 1
  3. We don’t think that area_type could be used for cloud types. The area_type is something related to observations from the top (satellite I would say) and to a column integrated information. Here the cloud_type is intended to be used for each point in the altitude resolved profile investigated with lidar measurements (however this can be applied also to other techniques like radar). So that using area_type would be misleading for the readers and data users.

volume_backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles: Ok. We will modify the name as angstrom_exponent_due_to_volume_backwards_scattering_in_air_due_to_ambient_aerosol_particles and the definition “The Angstrom exponent due to volume backwards scattering is the Angstrom exponent related only to the aerosol backwards scattering component”.

atmosphere_boundary|mixing_layer_top_height_defined_by_ambient_aerosol_particles_backscattering_by_ranging_instrument: Ok we would use “altitude” and so the term becomes altitude_at_top_of_atmosphere_boundary|mixed_layer_defined_by_ambient_aerosol_particles_backwards_scattering_by_ranging_instrument. About the physical criterion to distinguish between boundary and mixed layer, the mixing layer height is defined as the lowest layer where turbulent mixing processes establish an exchange between the surface layer and the atmosphere above. The atmospheric boundary layer height is defined as the lowest layer that generally contains most of the aerosol except special elevated layers, and it is considered as the mixing layer plus the residual layer, if that exists.

aerosol_type_in_atmosphere_layer:

  1. We don't think that area_type coud be used for aerosol types.
  2. Ok for adding in_air
  3. For aerosol_type the same approach we adopted for cloud_type would be the most suitable: adopting a byte solution in which all hybrid situations can be handled.

volume_extinction|backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles_in_atmosphere_layer: The physical quantity is the same, but the first is provided point by point in the vertical profile, while the second one is the representative value in a specific atmospheric layer.

atmosphere_optical_thickness_due_to_ambient_aerosol_particles_in_atmosphere_layer: The physical quantity is the same, but atmosphere_optical_thickness_due_to_ambient_aerosol_particles is the total atmospheric column, while atmosphere_optical_thickness_due_to_ambient_aerosol_particles_in_atmosphere_layer is obtained in a specific atmospheric layer.

All the best, Claudio

JonathanGregory commented 2 years ago

Dear @claudio-dema

Thanks for your careful replies. I think most of my questions are answered but I still have a few outstanding.

It seems that your volume backwards scattering coefficient (by ranging instrument) (m-1 sr-1) is not the same thing as the volume backwards scattering coefficient (m-1) in a couple of existing standard names. This is potentially confusing. I'm sorry to say that I don't yet understand what your quantity is, and "by ranging instrument" sounds wrong unless it means the ranging instrument itself is causing the scattering. You mention "radiative flux per unit solid angle". Does this mean you're considering radiative fluxes incident from many directions at once? You also mention "scattered at 180 degree angle", but I would think that the fraction scattered precisely at 180 degrees to the incident beam, or at any other precise angle, must be infinitesimal. I wonder if you could write some more, or show us a diagram, to explain?

For cloud_type and aerosol_type you mention hybrid types. I wonder therefore if you mean to use flag_masks, which can be combined bitwise, rather than flag_values, which are mutually exclusive?

The distinction for volume_extinction|backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles between the the value at a particular level and the representative value of a layer can be made with cell_methods; I don't think we need distinct standard names. For instance, if your vertical coordinate variable is altitude, they would be altitude: point and altitude: mean (or whatever statistic it is), with the layer defined by its bounds.

Existing standard names for extensive quantities of atmosphere layers have _in_atmosphere_layer but do not begin with atmosphere_, which refers to the whole atmosphere. You could define such a pair of names. However, it might be neater to use atmosphere_optical_thickness_due_to_ambient_aerosol_particles to refer to a layer by giving it a vertical coordinate with bounds to define the layer; wth no vertical coordinate, it would mean the whole atmosphere.

Best wishes

Jonathan

claudio-dema commented 2 years ago

Dear @JonathanGregory, thanks for your useful and constructive comments! Please, find my comments below:

It seems that your volume backwards scattering coefficient (by ranging instrument) (m-1 sr-1) is not the same thing as the volume backwards scattering coefficient (m-1) in a couple of existing standard names. This is potentially confusing. I'm sorry to say that I don't yet understand what your quantity is, and "by ranging instrument" sounds wrong unless it means the ranging instrument itself is causing the scattering. You mention "radiative flux per unit solid angle". Does this mean you're considering radiative fluxes incident from many directions at once? You also mention "scattered at 180 degree angle", but I would think that the fraction scattered precisely at 180 degrees to the incident beam, or at any other precise angle, must be infinitesimal. I wonder if you could write some more, or show us a diagram, to explain?

In our case we use lidar techniques, so I will here explain how backward scattering coefficient is measured by lidar but the same applies with small differences to other techniques like radar. Indeed lidar is a “radar” working in the optical range. A laser is used for sending a source of radiation into the atmosphere. Altitude by altitude, the light interacts with the atmospheric medium and therefore photons are partially scattered in different directions, partially absorbed, and partially transmitted at a larger distances. A receiver (typically a telescope) collects radiation scattered back (180°). The light is spectrally splitted and discriminated by special optics and so signals backscattered at different wavelengths are collected. Through specific equations the aerosol backward scattering coefficient is obtained. The following figure is a schema of the lidar measurement principle:

lidar_principle

The following reports a schematic concept of the scattering cross section, showing the scattering efficiency at each direction. Rayleigh scattering occurs when the dimensions of the scatter is much smaller than the wavelength of the incident electromagnetic radiation. Mie scattering occurs when the dimensions of the scattered is much larger than the wavelength of the incident electromagnetic radiation.

scattering_concept

As clearly reported in the schema, the volume backwards scattering coefficient at 180° is non infinitesimal neither null.

For cloud_type and aerosol_type you mention hybrid types. I wonder therefore if you mean to use flag_masks, which can be combined bitwise, rather than flag_values, which are mutually exclusive?

Yes, probably flag_masks is better in our case. Thank you.

The distinction for volume_extinction|backscattering_angstrom_exponent_in_air_due_to_ambient_aerosol_particles between the the value at a particular level and the representative value of a layer can be made with cell_methods; I don't think we need distinct standard names. For instance, if your vertical coordinate variable is altitude, they would be altitude: point and altitude: mean (or whatever statistic it is), with the layer defined by its bounds.

Ok, thanks for the useful comment.

Existing standard names for extensive quantities of atmosphere layers have _in_atmosphere_layer but do not begin with atmosphere_, which refers to the whole atmosphere. You could define such a pair of names. However, it might be neater to use atmosphere_optical_thickness_due_to_ambient_aerosol_particles to refer to a layer by giving it a vertical coordinate with bounds to define the layer; wth no vertical coordinate, it would mean the whole atmosphere.

Ok, fine for us

Best wishes Claudio

markusfiebig commented 2 years ago

Dear Jonathan and Claudio,

Claudio is certainly the expert on lidar terms here, but I would still have one comment:

You would now like to use for the backscattering Ångström coefficient

angstrom_exponent_due_to_volume_backwards_scattering_in_air_due_to_ambient_aerosol_particles

I need to admit that I find this wording misleading. The Ångström coefficient describes the wavelength dependence of a wavelength dependent property. The wording above suggests that the Ångström coefficient is caused by the concerned property, but that isn't the case. Rather, the Ångström coefficient describes a property of the wavelength dependent property. Wouldn't this version be better?

angstrom_exponent_of_volume_backwards_scattering_in_air_due_to_ambient_aerosol_particles

Regards, Markus

JonathanGregory commented 2 years ago

Dear @markusfiebig and @claudio-dema

Re

the Ångström coefficient describes a property of the wavelength dependent property

I am less expert than both of you. If this is so, I agree that Markus's wording angstrom_exponent_of_volume_backwards_scattering_in_air_due_to_ambient_aerosol_particles is fine, and better because it doesn't have due_to twice.

Best wishes

Jonathan

JonathanGregory commented 2 years ago

Dear @claudio-dema

Thanks for your diagram and explanation of volume backwards scattering coefficient. Now I understand that per unit solid angle in your quantity refers to the direction of the scattered beam (given as an angle such as 180deg wrt the incident beam). Thus, it's a coefficient per unit length into unit solid angle of scattering. Is that correct?

If so, that makes sense, but it does not resolve the difficulty that we have two existing standard names volume_backwards_scattering_coefficient_in_air_due_to_dried_aerosol_particles and volume_backwards_scattering_coefficient_of_radiative_flux_in_sea_water whose canonical unit is m-1, not m-1 sr-1. (These also seem inconsistent because they should both specify radiative_flux, or neither of them should.) On the other hand, we have two standard names volume_attenuated_backwards_scattering_function_in_air[_assuming_no_aerosol_or_cloud], which are in m-1 sr-1. Do these describe the same kind of quantity as your proposed name with the same unit?

Best wishes and thanks

Jonathan

claudio-dema commented 2 years ago

Dear @JonathanGregory,

Thanks for your diagram and explanation of volume backwards scattering coefficient. Now I understand that per unit solid angle in your quantity refers to the direction of the scattered beam (given as an angle such as 180deg wrt the incident beam). Thus, it's a coefficient per unit length into unit solid angle of scattering. Is that correct? If so, that makes sense, but it does not resolve the difficulty that we have two existing standard names volume_backwards_scattering_coefficient_in_air_due_to_dried_aerosol_particles and volume_backwards_scattering_coefficient_of_radiative_flux_in_sea_water whose canonical unit is m-1, not m-1 sr-1. (These also seem inconsistent because they should both specify radiative_flux, or neither of them should.)

Exactly, these 2 parameters refer to different quantities respect to our measurements: the first one refers to the whole volume and not only the solid angle at 180°, the second is similar to the first but in the water). Both of them in our opinion should report radiative_flux for consistency.

On the other hand, we have two standard names volume_attenuated_backwards_scattering_function_in_air[_assuming_no_aerosol_or_cloud], which are in m-1 sr-1. Do these describe the same kind of quantity as your proposed name with the same unit?

No, this contains also the attenuation of the light while transmitting in the medium. In our case , the calibrated collected lidar signal is something similar and exactly it has the same unit m-1 sr-1. We will apply for name also for such quantity in the near future and will use this (volume_attenuated_backwards_scattering_function_in_air[_assuming_no_aerosol_or_cloud],) as starting point for our quantity name.

Best wishes and thanks.

Claudio

JonathanGregory commented 2 years ago

Dear @claudio-dema

Thanks. It looks like we have reached a common understanding! I think it would help if you have time to write down what we have so far agreed altogether, to make it easier for Alison @japamment or Fran @feggleton to consider.

Apparently we have some inconsistencies to resolve, though. I am not an expert in this (although I have learned more from this discussion). I observe that all the existing names volume...coefficient are in m-1 (there are several of them), and those with volume...function are in m-1 sr-1. I therefore supposed that this terminological distinction was being made consistently, between scattering in any direction (coefficient) or into unit solid angle (function). Does this make sense to you? I append a list of existing names.

Best wishes

Jonathan

volume_absorption_coefficient_in_air_due_to_dried_aerosol_particles:m-1
volume_absorption_coefficient_of_radiative_flux_in_sea_water:m-1
volume_absorption_coefficient_of_radiative_flux_in_sea_water_due_to_dissolved_organic_matter:m-1
volume_attenuation_coefficient_of_downwelling_radiative_flux_in_sea_water:m-1
volume_backwards_scattering_coefficient_in_air_due_to_dried_aerosol_particles:m-1
volume_backwards_scattering_coefficient_of_radiative_flux_in_sea_water:m-1
volume_beam_attenuation_coefficient_of_radiative_flux_in_sea_water:m-1
volume_beam_attenuation_coefficient_of_radiative_flux_in_sea_water_corrected_for_pure_water_attenuance:m-1
volume_extinction_coefficient_in_air_due_to_ambient_aerosol_particles:m-1
volume_extinction_coefficient_in_air_due_to_cloud_particles:m-1
volume_scattering_coefficient_of_radiative_flux_in_air_due_to_ambient_aerosol_particles:m-1
volume_scattering_coefficient_of_radiative_flux_in_air_due_to_dried_aerosol_particles:m-1
volume_scattering_coefficient_of_radiative_flux_in_sea_water:m-1

volume_attenuated_backwards_scattering_function_in_air:m-1 sr-1
volume_attenuated_backwards_scattering_function_in_air_assuming_no_aerosol_or_cloud:m-1 sr-1
volume_scattering_function_of_radiative_flux_in_air_due_to_ambient_aerosol_particles:m-1 sr-1
volume_scattering_function_of_radiative_flux_in_sea_water:m-1 sr-1
claudio-dema commented 2 years ago

Dear @JonathanGregory ,

Thanks. It looks like we have reached a common understanding! I think it would help if you have time to write down what we have so far agreed altogether, to make it easier for Alison @japamment or Fran @feggleton to consider.

Yes, I'll send you a summary of what we have agreed altogether very soon.

Apparently we have some inconsistencies to resolve, though. I am not an expert in this (although I have learned more from this discussion). I observe that all the existing names volume...coefficient are in m-1 (there are several of them), and those with volume...function are in m-1 sr-1. I therefore supposed that this terminological distinction was being made consistently, between scattering in any direction (coefficient) or into unit solid angle (function). Does this make sense to you? I append a list of existing names.

In the meanwhile, we had a look at the list of existing names you sent us. Reading the definitions of these variables, we suppose that terminological distinction between 'coefficient' and 'function' is related to how radar measurements are performed (for example, volume_attenuated_backwards_scattering_function_in_air:m-1 sr-1 is almost the raw signal measured at different angles, while volume_backwards_scattering_coefficient_in_air_due_to_dried_aerosol_particles:m-1 is the retrieved property obtained integrating the different raw signals). In lidar measurements, instead, we typically work at a unique angle (180 degrees). So the term 'coefficient' commonly refers to properties of atmospheric components. That's why we would keep the term 'coefficient' even if the unit is m-1 sr-1 in our variables.

Best,

Claudio

JonathanGregory commented 2 years ago

Dear @claudio-dema

Thanks for agreeing to summarise what we have agreed.

Thanks also for considering the question of functions versus coefficient. This distinction seems consistent with these collections of articles on scattering coefficient and scattering function.

Looking at their definitions, I wonder whether the existing standard standard names volume_attenuated_backwards_scattering_function_in_air[_assuming_no_aerosol_or_cloud] should actually have coefficient, not function and units of m-1, not m-1 sr-1. The definition says "The volume scattering function is the fraction of incident radiative flux scattered into unit solid angle per unit path length," which is consistent with the unit m-1 sr-1. But then it says, "Backwards scattering refers to the sum of scattering into all backward angles i.e. scattering_angle exceeding pi/2 radians. A scattering_angle should not be specified with this quantity." To me that suggests an integral over scattering angles, which should remove sr-1 from the unit, and turn it into a coefficient. What do you - or anyone else - think of that?

Your backwards scattering coefficient is the same sort of quantity as the other existing quantities for volume_scattering_function_of_radiative_flux. You call it a coefficient, not a function, because pi is the only angle at which you view, but it's still sr-1, whereas the other coefficient names do not have sr-1 because they are angular integrals. Is that right? If so, it still seems less than ideal to me that we should use the phrase "scattering coefficient" to mean things with different canonical units.

Best wishes

Jonathan

claudio-dema commented 2 years ago

Dear @JonathanGregory,

Thanks also for considering the question of functions versus coefficient. This distinction seems consistent with these collections of articles on scattering coefficient and scattering function. Looking at their definitions, I wonder whether the existing standard standard names volume_attenuated_backwards_scattering_function_in_air[_assuming_no_aerosol_or_cloud] should actually have coefficient, not function and units of m-1, not m-1 sr-1. The definition says "The volume scattering function is the fraction of incident radiative flux scattered into unit solid angle per unit path length," which is consistent with the unit m-1 sr-1. But then it says, "Backwards scattering refers to the sum of scattering into all backward angles i.e. scattering_angle exceeding pi/2 radians. A scattering_angle should not be specified with this quantity." To me that suggests an integral over scattering angles, which should remove sr-1 from the unit, and turn it into a coefficient. What do you - or anyone else - think of that?

Sorry, this is not of my competence, even if I could speculate that they refer to an integral over the angles exceeding pi/2 but then the obtained quantity is reported to unit angle dividing for the total angle considered. In this case m-1 sr-1 could apply. But it is better to ask to the person who inserted the definition .

Your backwards scattering coefficient is the same sort of quantity as the other existing quantities for volume_scattering_function_of_radiative_flux. You call it a coefficient, not a function, because pi is the only angle at which you view, but it's still sr-1, whereas the other coefficient names do not have sr-1 because they are angular integrals. Is that right? If so, it still seems less than ideal to me that we should use the phrase "scattering coefficient" to mean things with different canonical units.

I do not think that such strict correspondence sr-1 to function is correct in general. Function means that the reported quantity is a function of a varying parameter. In the case of our backwards scattering coefficient, there is no varying angle because the technique of measurements (by_ranging_instrument) dictates the angle to be 180° (as reported in our proposed definition). So that what is measured varies only if the aerosol particles change and it is a characteristic of observed aerosol particles, i.e. a coefficient.

What follows is a summary of what we agreed.

Best wishes, Claudio

JonathanGregory commented 2 years ago

Dear @claudio-dema

Thanks very much for the summary. This all seems clear and I'm grateful for your patience in clarifying these quantities, which I think should be added to the standard name table. I hope you'll excuse my commenting again on two of them, the cloud_type and aerosol_type_in_atmosphere_layer_in_air. I think these quantities should not be defined as numerical byte values, although that's how you store them in practice. I suggest that their definitions should use the same phrasing as area_type and region:

A variable with the standard_name of cloud_type contains either strings which indicate the cloud type, or flags which can be translated to strings using flag_values and flag_meanings attributes.

Would that be suitable? If so, they have no canonical unit.

I agree with your remark

Function means that the reported quantity is a function of a varying parameter

and I think that's why volume_attenuated_backwards_scattering_function_in_air should be a coefficient, given its definition. But of course this isn't part of the present issue.

Best wishes

Jonathan

claudio-dema commented 2 years ago

Dear @JonathanGregory ,

Thanks very much for the summary. This all seems clear and I'm grateful for your patience in clarifying these quantities, which I think should be added to the standard name table. I hope you'll excuse my commenting again on two of them, the cloud_type and aerosol_type_in_atmosphere_layer_in_air. I think these quantities should not be defined as numerical byte values, although that's how you store them in practice. I suggest that their definitions should use the same phrasing as area_type and region:

A variable with the standard_name of cloud_type contains either strings which indicate the cloud type, or flags which can be translated to strings using flag_values and flag_meanings attributes. Would that be suitable? If so, they have no canonical unit.

Thanks for this comment, it's perfectly suitable. Adapting the two definitions of cloud_type and aerosol_type_in_atmosphere_layer_in_air we obtain:

How do you suggest to proceed for the publication of these new variable names?

Regards, Claudio

JonathanGregory commented 2 years ago

Dear @claudio-dema

How do you suggest to proceed for the publication of these new variable names?

This issue is in the list for inclusion in the next version of the standard name table, which Alison @japamment is working on preparing. You don't need to do anything. Thanks for your patience.

Best wishes

Jonathan

feggleton commented 2 years ago

Thanks everyone for this proposal and the detail discussion to get to this point and for providing the summary of agreed terms. These looks fine to be accepted. Thanks

JonathanGregory commented 1 year ago

I believe these are included now, @claudio-dema, thanks @feggleton @japamment. I see them in version 81.