[CF-metadata] new standard names for surface aerosol optical properties

Cameron-smith, Philip cameronsmith1 at llnl.gov
Mon Mar 12 16:54:19 MDT 2012


Hi Markus,

Thank you for your comments. My responses to your comments are below.

Best wishes,

      Philip

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Dr Philip Cameron-Smith, pjc at llnl.gov, Lawrence Livermore National Lab.
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> -----Original Message-----
> From: Markus Fiebig [mailto:Markus.Fiebig at nilu.no]
> Sent: Monday, March 12, 2012 8:08 AM
> To: Cameron-smith, Philip; cf-metadata at cgd.ucar.edu
> Subject: RE: [CF-metadata] new standard names for surface aerosol
> optical properties
>
> Hi Philip,
>
> thanks for doing such a thorough job reviewing my proposal! Indeed, I
> should have seen a few of the points you mention even before I posted
> my proposed standard names. I therefore included, below my comments, an
> updated version of my proposed list of standard names and pertaining
> definitions.
>
> For the sake of ease, I put my comments inline below your items.
>
> Best regards,
> Markus
>
>
> Some general comments:
>
> 1) There is no mention of frequency in your definitions.  This is true
> for some of the existing std_name definitions too.  However, I note
> that the description of some of the existing std_names (eg
> atmosphere_absorption_optical_thickness_due_to_ambient_aerosol)
> contains the following, which I recommend that we include in all your
> definitions (CF is used for everything from UV to radio waves): "A
> coordinate variable of radiation_wavelength or radiation_frequency can
> be specified to indicate that the optical thickness applies at specific
> wavelengths or frequencies."
>
> MF: Yes, after I posted my proposal, I discovered these examples too. I
> will add this sentence to all spectral properties.
>
> 2) Do you really mean _due_to_dry_aerosol?  I interpret this to mean in
> a humid atmosphere you are quantifying the scattering that would occur
> if, hypothetically, the aerosols were dry rather than their actual
> ambient state.
>
> MF: Yes, this interpretation is correct. With few exceptions, the
> standard operating procedures (SOP) of ground-based aerosol optical
> property networks say that the sample is led into an instrument
> container or cabin that is climatised roughly to lab temperature. Other
> approaches would be too costly. That implies that the aerosol humidity
> state is altered anyway from the ambient state. To make measurements
> comparable and easier to interpret, it is thus specified that the
> sample should be dried to < 40% RH. I will add a corresponding sentence
> to the definitions. Do you think the standard name needs to be adapted
> as well, or is it sufficiently "self-explaining"?

[PJC] OK.  That makes sense (as a modeler I wasn't thinking of observational needs :-( ).   I would leave _due_to_dry_aerosol as you have proposed (but see responses to 3a and 3b).

> 3a) For the terms with _due_to_pm1(10)_dry_aerosol, is the pm1(10)
> selection on ambient aerosol of dry aerosol?
>
> MF: Good point, even though my answer won't satisfy. This isn't finally
> concluded in the current SOPs, and both practices are being used. This
> is where current observations are a little "unclean".

[PJC]  I would naturally expect X in _pmX_dry_aerosol to be the size of the dried aerosol.  If this is not what you need then we should probably change the std_name (I do not know of an analogous example in CF).   Either way, the definition should make clear what is intended.  FWIW, CF already has _pmX_ambient_aerosol.

> 3b) Dry aerosol is usually described for std_names as '"Dry aerosol"
> means aerosol without water'.  Your description is possibly better, but
> you might consider going with the former for consistency.
>
> MF: The wording I used describes what is used on the experimental side,
> so likely what the files will contain. I will have a look to make the
> wording more consistent.

[PJC] Your wording is probably what most people mean by 'dry'.  It is an explicit part of CF that the standard names are the physical quantity of interest, and instrument and models do their best to determine the quantity, even though they do so imperfectly.  IMHO, this is one of the great advantages of CF, although it is hard. How about you use something like the following, '"Dry aerosol" means aerosol without water, which in practice often means ...'?

> 4) I am not sure what you mean by surface_volume_ .
>
> MF: I was previously being told that the "volume" term needs to be part
> of a scattering_coefficient if the property refers to a volume of air
> as opposed to a surface. The "surface" preceding the "volume" on my
> proposed variable names is a qualification, and refers to the location
> , i.e. the lower bound of the atmosphere.

[PJC] I am still not sure I understand what you want.  Is this some sort of surface albedo?

> 5) You mention that _hemispheric_backscattering_ is scattering into the
> 'rearward hemisphere relative to the incident beam'.  This appears to
> be the same definition that is already given to both _backscattering_
> (eg, backscattering_ratio,
> histogram_of_backscattering_ratio_over_height_above_reference_ellipsoid
> ) and _backwards_scattering_ (eg,
> surface_backwards_scattering_coefficient_of_radar_wave,
> volume_attenuated_backwards_scattering_function_in_air_assuming_no_aero
> sol_or_cloud).  Hence, it appears that we already have duplicate terms
> in the std_name table.  We should probably pick a preferred option, and
> fix the other one with aliases.
>
> MF: This is going to be a tough one, especially since several
> communities with engrained customs are involved. I chose hemispheric in
> hemispheric_backscattering_coefficient, which is integrated over the
> rearward hemisphere as measured by some integrating nephelometers, from
> the backscattering_coefficient that isn't integrated over any solid
> angle (to be proposed soon), e.g. as measured by a lidar. In the
> examples you mention however, the term "backscattering" is already used
> for a property integrated over a solid angle. I've pondered over this
> one quite a while, but I keep coming back to the point that a
> standardised word for distinguishing angular integrated from non-
> angular integrated properties is needed - like "hemispheric". That
> wouldn't be a nice solution, since it implied aliases for 5 existing
> standard names. Any better ideas?

[PJC] I have a strong dislike for having multiple terms mean the same thing in CF.   When it does creep in I try to eliminate them.   I also like to try and keep terms short, because std_names can get very long.  CF already has _scattering_function_ for angle dependent scattering (eg, volume_scattering_function_of_radiative_flux_in_sea_water). On balance, I would use one of the existing terms (whichever CF decides is better).

> 6) We currently don't have any std_names with _due_to_aerosol.  They
> all specify either _ambient_ or _dry_.
>
> MF: The reason why I didn't write "ambient" or "dry" here is that some
> data is neither nor. As written above, the sample is usually fed into a
> lab container (implicit warming) and dried (by diffusion dryer). Some
> sites, e.g. (sub)-tropical ones in summer, don't manage to dry the
> sample to < 40% RH. For these sites, the humidity state needs to be
> given in the file since it varies (I will correct the corresponding
> sentence in the definition), but it is neither dry nor ambient.

[PJC] This is a challenge because it puts two CF goals in conflict, ie accurately describing a variable and enabling quantities from different instruments and models to intercompare.  To put it another way, the problem is that any data put into the std_name you suggest will be hard to use without additional information, since there is no way to know whether like quantities are being compared.   Is it normal to have observation temperature and humidity data along with the aerosol measurements?

> -----------------------------------------------------------------------
> ------------------------------
> Updated list of proposed standard names (2012-03-12)
>
> 1) volume_scattering_coefficient_in_air_due_to_dry_aerosol
>
> The volume scattering coefficient is the fractional change of radiative
> flux per unit path length due to redirection of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). Aerosol takes
> up ambient water (a process known as hygroscopic growth) depending on
> the relative humidity and the composition of the aerosol. "Dry aerosol"
> means aerosol fully dehydrated from its ambient state, experimentally
> brought to a relative humidity less than 40%. The specification of a
> physical process by the phrase "due_to_" 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. A coordinate
> variable of radiation_wavelength or radiation_frequency needs to be
> specified to indicate that the property applies at specific wavelengths
> or frequencies.
>
>
> 2) volume_absorption_coefficient_in_air_due_to_dry_aerosol
>
> The volume absorption coefficient is the fractional change of radiative
> flux per unit path length due to absorption of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). Aerosol takes
> up ambient water (a process known as hygroscopic growth) depending on
> the relative humidity and the composition of the aerosol. "Dry aerosol"
> means aerosol fully dehydrated from its ambient state, experimentally
> brought to a relative humidity less than 40%. The specification of a
> physical process by the phrase "due_to_" 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. A coordinate
> variable of radiation_wavelength or radiation_frequency needs to be
> specified to indicate that the property applies at specific wavelengths
> or frequencies.
>
>
> 3)
> volume_hemispheric_backscattering_coefficient_in_air_due_to_dry_aerosol
>
> The volume hemispheric backscattering coefficient is the fractional
> change of radiative flux per unit path length due to redirection of an
> incident light beam into the rearward hemisphere relative to the
> incident beam by a component in the reference volume. It is distinct
> from the backscattering coefficient which isn't integrated over the
> rearward hemisphere. "Aerosol" means the suspended liquid or solid
> particles in air (except cloud droplets). Aerosol takes up ambient
> water (a process known as hygroscopic growth) depending on the relative
> humidity and the composition of the aerosol. "Dry aerosol" means
> aerosol fully dehydrated from its ambient state, experimentally brought
> to a relative humidity less than 40%. The specification of a physical
> process by the phrase "due_to_" 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. A coordinate variable of
> radiation_wavelength or radiation_frequency needs to be specified to
> indicate that the property applies at specific wavelengths or
> frequencies.
>
>
> 4)
> surface_volume_scattering_coefficient_at_stp_in_air_due_to_pm1_dry_aero
> sol
>
> The volume scattering coefficient is the fractional change of radiative
> flux per unit path length due to redirection of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). Aerosol takes
> up ambient water (a process known as hygroscopic growth) depending on
> the relative humidity and the composition of the aerosol. "Dry aerosol"
> means aerosol fully dehydrated from its ambient state, experimentally
> brought to a relative humidity less than 40%. The specification of a
> physical process by the phrase "due_to_" 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. "surface"
> means the lower boundary of the atmosphere. "at_stp" refers to a volume
> at standard conditions of temperature (273.15 K) and pressure (1013.25
> hPa). "pm1" refers to the fraction of aerosol particles with an
> aerodynamic diameter of less than or equal to 1 micrometer. A
> coordinate variable of radiation_wavelength or radiation_frequency
> needs to be specified to indicate that the property applies at specific
> wavelengths or frequencies.
>
>
> 5)
> surface_volume_scattering_coefficient_at_stp_in_air_due_to_pm10_dry_aer
> osol
>
> The volume scattering coefficient is the fractional change of radiative
> flux per unit path length due to redirection of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). Aerosol takes
> up ambient water (a process known as hygroscopic growth) depending on
> the relative humidity and the composition of the aerosol. "Dry aerosol"
> means aerosol fully dehydrated from its ambient state, experimentally
> brought to a relative humidity less than 40%. The specification of a
> physical process by the phrase "due_to_" 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. "surface"
> means the lower boundary of the atmosphere. "at_stp" refers to a volume
> at standard conditions of temperature (273.15 K) and pressure (1013.25
> hPa). "pm10" refers to the fraction of aerosol particles with an
> aerodynamic diameter of less than or equal to 10 micrometers. A
> coordinate variable of radiation_wavelength or radiation_frequency
> needs to be specified to indicate that the property applies at specific
> wavelengths or frequencies.
>
>
> 6)
> surface_volume_absorption_coefficient_at_stp_in_air_due_to_pm1_dry_aero
> sol
>
> The volume absorption coefficient is the fractional change of radiative
> flux per unit path length due to absorption of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). Aerosol takes
> up ambient water (a process known as hygroscopic growth) depending on
> the relative humidity and the composition of the aerosol. "Dry aerosol"
> means aerosol fully dehydrated from its ambient state, experimentally
> brought to a relative humidity less than 40%. The specification of a
> physical process by the phrase "due_to_" 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. "surface"
> means the lower boundary of the atmosphere. "at_stp" refers to a volume
> at standard conditions of temperature (273.15 K) and pressure (1013.25
> hPa). "pm1" refers to the fraction of aerosol particles with an
> aerodynamic diameter of less than or equal to 1 micrometers. A
> coordinate variable of radiation_wavelength or radiation_frequency
> needs to be specified to indicate that the property applies at specific
> wavelengths or frequencies.
>
>
> 7)
> surface_volume_absorption_coefficient_at_stp_in_air_due_to_pm10_dry_aer
> osol
>
> The volume absorption coefficient is the fractional change of radiative
> flux per unit path length due to absorption of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). Aerosol takes
> up ambient water (a process known as hygroscopic growth) depending on
> the relative humidity and the composition of the aerosol. "Dry aerosol"
> means aerosol fully dehydrated from its ambient state, experimentally
> brought to a relative humidity less than 40%. The specification of a
> physical process by the phrase "due_to_" 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. "surface"
> means the lower boundary of the atmosphere. "at_stp" refers to a volume
> at standard conditions of temperature (273.15 K) and pressure (1013.25
> hPa). "pm10" refers to the fraction of aerosol particles with an
> aerodynamic diameter of less than or equal to 10 micrometers. A
> coordinate variable of radiation_wavelength or radiation_frequency
> needs to be specified to indicate that the property applies at specific
> wavelengths or frequencies.
>
>
> 8)
> surface_volume_hemispheric_backscattering_coefficient_at_stp_in_air_due
> _to_pm1_dry_aerosol
>
> The volume hemispheric backscattering coefficient is the fractional
> change of radiative flux per unit path length due to redirection of an
> incident light beam into the rearward hemisphere relative to the
> incident beam by a component in the reference volume. It is distinct
> from the backscattering coefficient which isn't integrated over the
> rearward hemisphere. "Aerosol" means the suspended liquid or solid
> particles in air (except cloud droplets). Aerosol takes up ambient
> water (a process known as hygroscopic growth) depending on the relative
> humidity and the composition of the aerosol. "Dry aerosol" means
> aerosol fully dehydrated from its ambient state, experimentally brought
> to a relative humidity less than 40%. The specification of a physical
> process by the phrase "due_to_" 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. "surface" means the lower
> boundary of the atmosphere. "at_stp" refers to a volume at standard
> conditions of temperature (273.15 K) and pressure (1013.25 hPa). "pm1"
> refers to the fraction of aerosol particles with an aerodynamic
> diameter of less than or equal to 1 micrometers. A coordinate variable
> of radiation_wavelength or radiation_frequency needs to be specified to
> indicate that the property applies at specific wavelengths or
> frequencies.
>
>
> 9)
> surface_volume_hemispheric_backscattering_coefficient_at_stp_in_air_due
> _to_pm10_dry_aerosol
>
> The volume hemispheric backscattering coefficient is the fractional
> change of radiative flux per unit path length due to redirection of an
> incident light beam into the rearward hemisphere relative to the
> incident beam by a component in the reference volume. It is distinct
> from the backscattering coefficient which isn't integrated over the
> rearward hemisphere. "Aerosol" means the suspended liquid or solid
> particles in air (except cloud droplets). Aerosol takes up ambient
> water (a process known as hygroscopic growth) depending on the relative
> humidity and the composition of the aerosol. "Dry aerosol" means
> aerosol fully dehydrated from its ambient state, experimentally brought
> to a relative humidity less than 40%. The specification of a physical
> process by the phrase "due_to_" 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. "surface" means the lower
> boundary of the atmosphere. "at_stp" refers to a volume at standard
> conditions of temperature (273.15 K) and pressure (1013.25 hPa). "pm10"
> refers to the fraction of aerosol particles with an aerodynamic
> diameter of less than or equal to 10 micrometers. A coordinate variable
> of radiation_wavelength or radiation_frequency needs to be specified to
> indicate that the property applies at specific wavelengths or
> frequencies.
>
>
> 10) volume_scattering_coefficient_in_air_due_to_aerosol
>
> The volume scattering coefficient is the fractional change of radiative
> flux per unit path length due to redirection of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). "Aerosol"
> without further qualification such as dry or ambient means that the
> aerosol humidity state is neither dry nor ambient and stated in the
> data. To specify the relative humidity at which the property applies,
> provide scalar coordinate variable with the standard name of
> "relative_humidity". The specification of a physical process by the
> phrase "due_to_" 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. A coordinate variable of radiation_wavelength or
> radiation_frequency needs to be specified to indicate that the property
> applies at specific wavelengths or frequencies.
>
>
> 11) volume_absorption_coefficient_in_air_due_to_aerosol
>
> The volume absorption coefficient is the fractional change of radiative
> flux per unit path length due to absorption of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). "Aerosol"
> without further qualification such as dry or ambient means that the
> aerosol humidity state is neither dry nor ambient and stated in the
> data. To specify the relative humidity at which the property applies,
> provide scalar coordinate variable with the standard name of
> "relative_humidity". The specification of a physical process by the
> phrase "due_to_" 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. A coordinate variable of radiation_wavelength or
> radiation_frequency needs to be specified to indicate that the property
> applies at specific wavelengths or frequencies.
>
>
> 12) volume_hemispheric_backscattering_coefficient_in_air_due_to_aerosol
>
> The volume hemispheric backscattering coefficient is the fractional
> change of radiative flux per unit path length due to redirection of an
> incident light beam into the rearward hemisphere relative to the
> incident beam by a component in the reference volume. It is distinct
> from the backscattering coefficient which isn't integrated over the
> rearward hemisphere. "Aerosol" without further qualification such as
> dry or ambient means that the aerosol humidity state is neither dry nor
> ambient and stated in the data. To specify the relative humidity at
> which the property applies, provide scalar coordinate variable with the
> standard name of "relative_humidity". The specification of a physical
> process by the phrase "due_to_" 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. A coordinate variable of
> radiation_wavelength or radiation_frequency needs to be specified to
> indicate that the property applies at specific wavelengths or
> frequencies.
>
>
> 13)
> surface_volume_scattering_coefficient_at_stp_in_air_due_to_pm1_aerosol
>
> The volume scattering coefficient is the fractional change of radiative
> flux per unit path length due to redirection of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). "Aerosol"
> without further qualification such as dry or ambient means that the
> aerosol humidity state is neither dry nor ambient and stated in the
> data. To specify the relative humidity at which the property applies,
> provide scalar coordinate variable with the standard name of
> "relative_humidity". The specification of a physical process by the
> phrase "due_to_" 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. "surface" means the lower boundary of the
> atmosphere. "at_stp" refers to a volume at standard conditions of
> temperature (273.15 K) and pressure (1013.25 hPa). "pm1" refers to the
> fraction of aerosol particles with an aerodynamic diameter of less than
> or equal to 1 micrometer. A coordinate variable of radiation_wavelength
> or radiation_frequency needs to be specified to indicate that the
> property applies at specific wavelengths or frequencies.
>
>
> 14)
> surface_volume_scattering_coefficient_at_stp_in_air_due_to_pm10_aerosol
>
> The volume scattering coefficient is the fractional change of radiative
> flux per unit path length due to redirection of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). "Aerosol"
> without further qualification such as dry or ambient means that the
> aerosol humidity state is neither dry nor ambient and stated in the
> data. To specify the relative humidity at which the property applies,
> provide scalar coordinate variable with the standard name of
> "relative_humidity". The specification of a physical process by the
> phrase "due_to_" 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. "surface" means the lower boundary of the
> atmosphere. "at_stp" refers to a volume at standard conditions of
> temperature (273.15 K) and pressure (1013.25 hPa). "pm10" refers to the
> fraction of aerosol particles with an aerodynamic diameter of less than
> or equal to 10 micrometers. A coordinate variable of
> radiation_wavelength or radiation_frequency needs to be specified to
> indicate that the property applies at specific wavelengths or
> frequencies.
>
>
> 15)
> surface_volume_absorption_coefficient_at_stp_in_air_due_to_pm1_aerosol
>
> The volume absorption coefficient is the fractional change of radiative
> flux per unit path length due to absorption of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). "Aerosol"
> without further qualification such as dry or ambient means that the
> aerosol humidity state is neither dry nor ambient and stated in the
> data. To specify the relative humidity at which the property applies,
> provide scalar coordinate variable with the standard name of
> "relative_humidity". The specification of a physical process by the
> phrase "due_to_" 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. "surface" means the lower boundary of the
> atmosphere. "at_stp" refers to a volume at standard conditions of
> temperature (273.15 K) and pressure (1013.25 hPa). "pm1" refers to the
> fraction of aerosol particles with an aerodynamic diameter of less than
> or equal to 1 micrometers. A coordinate variable of
> radiation_wavelength or radiation_frequency needs to be specified to
> indicate that the property applies at specific wavelengths or
> frequencies.
>
>
> 16)
> surface_volume_absorption_coefficient_at_stp_in_air_due_to_pm10_aerosol
>
> The volume absorption coefficient is the fractional change of radiative
> flux per unit path length due to absorption of an incident light beam
> by a component in the reference volume. "Aerosol" means the suspended
> liquid or solid particles in air (except cloud droplets). "Aerosol"
> without further qualification such as dry or ambient means that the
> aerosol humidity state is neither dry nor ambient and stated in the
> data. To specify the relative humidity at which the property applies,
> provide scalar coordinate variable with the standard name of
> "relative_humidity". The specification of a physical process by the
> phrase "due_to_" 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. "surface" means the lower boundary of the
> atmosphere. "at_stp" refers to a volume at standard conditions of
> temperature (273.15 K) and pressure (1013.25 hPa). "pm10" refers to the
> fraction of aerosol particles with an aerodynamic diameter of less than
> or equal to 10 micrometers. A coordinate variable of
> radiation_wavelength or radiation_frequency needs to be specified to
> indicate that the property applies at specific wavelengths or
> frequencies.
>
>
> 17)
> surface_volume_hemispheric_backscattering_coefficient_at_stp_in_air_due
> _to_pm1_aerosol
>
> The volume hemispheric backscattering coefficient is the fractional
> change of radiative flux per unit path length due to redirection of an
> incident light beam into the rearward hemisphere relative to the
> incident beam by a component in the reference volume. It is distinct
> from the backscattering coefficient which isn't integrated over the
> rearward hemisphere. "Aerosol" means the suspended liquid or solid
> particles in air (except cloud droplets). "Aerosol" without further
> qualification such as dry or ambient means that the aerosol humidity
> state is neither dry nor ambient and stated in the data. To specify the
> relative humidity at which the property applies, provide scalar
> coordinate variable with the standard name of "relative_humidity". The
> specification of a physical process by the phrase "due_to_" 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.
> "surface" means the lower boundary of the atmosphere. "at_stp" refers
> to a volume at standard conditions of temperature (273.15 K) and
> pressure (1013.25 hPa). "pm1" refers to the fraction of aerosol
> particles with an aerodynamic diameter of less than or equal to 1
> micrometers. A coordinate variable of radiation_wavelength or
> radiation_frequency needs to be specified to indicate that the property
> applies at specific wavelengths or frequencies.
>
>
> 18)
> surface_volume_hemispheric_backscattering_coefficient_at_stp_in_air_due
> _to_pm10_aerosol
>
> The volume hemispheric backscattering coefficient is the fractional
> change of radiative flux per unit path length due to redirection of an
> incident light beam into the rearward hemisphere relative to the
> incident beam by a component in the reference volume. It is distinct
> from the backscattering coefficient which isn't integrated over the
> rearward hemisphere. "Aerosol" means the suspended liquid or solid
> particles in air (except cloud droplets). "Aerosol" without further
> qualification such as dry or ambient means that the aerosol humidity
> state is neither dry nor ambient and stated in the data. To specify the
> relative humidity at which the property applies, provide scalar
> coordinate variable with the standard name of "relative_humidity". The
> specification of a physical process by the phrase "due_to_" 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.
> "surface" means the lower boundary of the atmosphere. "at_stp" refers
> to a volume at standard conditions of temperature (273.15 K) and
> pressure (1013.25 hPa). "pm10" refers to the fraction of aerosol
> particles with an aerodynamic diameter of less than or equal to 10
> micrometers. A coordinate variable of radiation_wavelength or
> radiation_frequency needs to be specified to indicate that the property
> applies at specific wavelengths or frequencies.
>
>
>  _______________________________________
>  Dr. Markus Fiebig
>
>  Dept. Atmospheric and Climate Research (ATMOS) Norwegian Institute for
>  Air Research (NILU) P.O. Box 100
>  N-2027 Kjeller
>  Norway
>
>  Tel.: +47 6389-8235
>  Fax : +47 6389-8050
>  e-mail: Markus.Fiebig at nilu.no
>  skype: markus.fiebig



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