[CF-metadata] Radiation standard names

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 3 Aug 2004 09:12:24 +0100

Dear Roy

Thanks for your question. Apologies for delay; I was away. Below is an attempt
at some answers. We could add these proposed standard names if you like.

A general comment is that all of them have _in_sea_water appended. I know you
aren't happy with that, pointing out that it seems to exclude lakes and
rivers. I agree, that's a problem. The reason for having "sea" and not just
_in_water was because water is also found in the air as clouds! So I am not
sure what to do about this. Suggestions welcome.

Best wishes

Jonathan


I hadn't come across the terms "vector irradiance" and "scalar irradiance"
before (I'm no expert either). Searching on the web I find that the former
means what is usually called just "irradiance" or "radiative flux density" in
physics, and "radiative flux" in CF (since most people found that to be a more
familiar term). An irradiance is just an incident flux normal to the surface.

Should we introduce "scalar irradiance" into standard names? It's evidently a
common term in radiative transfer theory in the ocean, and appears to be
sometimes used for the atmosphere. However it's not very obvious to me what it
means; if I was allowed to choose a term, I'd call it "spherical irradiance",
since it's the irradiance on a (hemi)spherical surface. I have found some
evidence for the existence of this term on the web! What do you think?


downwelling vector irradiance:

"This is the measurement made by a cosine-collector light meter either as
discrete wavelength values or as an integral value from 300-700 nm termed
'PAR'. Depending on instrument type it is expressed either as energy per unit
area or quanity of photons per unit area per unit time."

The standard names and canonical units are as follows. * indicates standard
names that I have constructed here; we already have the ones without *.

As a function of wavelength expressed in energy units
  downwelling_spectral_radiative_flux_in_sea_water (W m-2 m-1)
  surface_downwelling_spectral_radiative_flux_in_sea_water
PAR expressed in energy units
  *downwelling_photosynthetic_radiative_flux_in_sea_water (W m-2)
  *surface_downwelling_photosynthetic_radiative_flux_in_sea_water
As a function of wavelength expressed as a photon flux
  *downwelling_spectral_photon_flux_in_sea_water (mol m-2 s-1 m-1)
  *surface_downwelling_spectral_photon_flux_in_sea_water
PAR expressed as a photon flux
  downwelling_photosynthetic_photon_flux_in_sea_water (mol m-2 s-1)
  surface_downwelling_photosynthetic_photon_flux_in_sea_water

The spectral ones should have a coordinate of radiation_wavelength (m) to
state the wavelength. As a more precise alternative to the PAR quantities, you
could give them as
  *downwelling_radiative_flux_in_sea_water (W m-2)
  *surface_downwelling_radiative_flux_in_sea_water
  *downwelling_photon_flux_in_sea_water (mol m-2 s-1)
  *surface_downwelling_photon_flux_in_sea_water
with a radiation_wavelength coordinate having bounds of 300 and 700 nm.


downwelling scalar irradiance:

"This is the measurement made by a hemispherical collector (sometimes termed
2-pi). Again I have discrete wavelength, PAR and data as energy/photons."

The standard names for energy units would be as above with radiative_flux
replaced by spherical_irradiance (or scalar_irradiance).

On the web the only term I can find for the corresponding photon flux is
"photon fluence rate", but that doesn't appear common in oceanography. There
is no evidence for a term "scalar photon flux". Therefore I would propose
names as above with photon_flux replaced by photon_spherical_irradiance, since
the term "photon irradiance" does appear to exist.

None of these standard names currently exist.


downwelling radiance:

"This is the measurement made by a light sensor at the bottom of a tube. Again
I have discrete wavelength, PAR and data as energy/photons."

The difference between radiance and irradiance (i.e. flux) is that radiance
specifies a direction and is expressed per unit solid angle, so the units all
have sr-1 added. The standard names would be as in the first set with
radiative_flux replaced by radiance and photon_flux by photon_radiance, which
is a term that appears to exist on the web.

None of these standard names currently exist.
Received on Tue Aug 03 2004 - 02:12:24 BST