Dear all,
a (late, sorry!) input on the definition of sea water reflectance, as an
ocean color product.
I have some doubts for "surface_bidirectional_reflectance".
For the sake of completeness, in marine optics, 'reflectance' is used mainly
in two ways:
the irradiance reflectance, Eu/Ed (upwelling irradiance / downwelling
irradiance), usually measured at 0- (just below water surface).
& the radiance reflectance, Lu/Ed (upwelling radiance / downwelling
irradiance), at 0- or 0+ (just above water).
Often, the radiance reflectance is given at 0+ and then noted Lw/Ed (water
leaving radiance / downwelling irradiance), and is also termed remote
sensing reflectance, because it is a standard satellite product (Eu/Ed is
not).
This product has a bidirectional dependence (on geometry of illumination and
observation).
Actually, looking at the existing CF list, some quantities relevant to the
discussion are there, and might be helpful.
I would say that the ratio of
surface_upwelling_spectral_radiative_flux_in_sea_water
and
surface_downwelling_spectral_radiative_flux_in_sea_water
is Eu/Ed at 0- (irradiance reflectance).
and the ratio of:
surface_upwelling_spectral_radiance_in_air_emerging_from_sea_water
and
surface_downwelling_spectral_radiative_flux_in_air
is Lw/Ed at 0+ , i.e. (remote sensing) radiance reflectance (the term
'emerging_from_sea_water' is important).
So following the CF definition of a ratio, the remote sensing reflectance
would be something like:
ratio_of_upwelling_radiance_emerging_from_sea_water_to_
downwelling_radiative_flux_in_air , in sr^-1.
If the term "surface_bidirectional_reflectance" is fully equivalent to this
(long!) definition, it is fine. Part of my doubts come from the fact that I
weren't able to find 'reflectance' in the CF list.
"surface_bidirectional_reflectance" implies that it is a radiance
reflectance (bidirectional). On the other hand, it should specify the
vertical coordinate ('in_air' or 'in_sea_water'). For 0+ (Lw), it should say
something about 'emerging_from_sea_water' because this is the satellite
product of interest.
Being bidirectional, it should contain the value of 'angle_of_incidence' and
other angles if possible (in the present case the term 'angle_of_reflection'
is not appropriate because we are not describing a reflection - the geometry
of observation should be defined with a viewing angle and a relative
azimuth, but I don't know the names for these). These angles are 0
(vertical) if the remote sensing reflectance is 'normalized'. In optical
remote sensing, 'normalized' usually means corrected for bidirectional
effects (as if sun and observer were directly ahead). The satellite product
then becomes an intrinsic property of the water, independent of conditions
of observation and illumination. This is at least valid for the NASA
products. This might be too complicated to be included in the name, and
anyway the actual result depends on the models used for these corrections. I
guess that these aspects should be described in the product description.
I hope this helps,
Best,
frederic
-----Original Message-----
From: cf-metadata-boun...@cgd.ucar.edu
[mailto:cf-metadata-boun...@cgd.ucar.edu] On Behalf Of Jonathan Gregory
Sent: Monday, 19 October, 2009 19:04
To: olivier lauret
Cc: Laurence Crosnier; cf-metadata@cgd.ucar.edu; Andrei LINTU;
cristina.tronc...@artov.isac.cnr.it; Philippe Garnesson; Ludovic BOURG; Yann
BARZIC
Subject: [CF-metadata] Sea water transparency and reflectance.
Dear Olivier
> *About reflectance, the quantity that is usually provided in ocean
> colour products is the bidirectional reflectance; albedo is of the
> same nature than reflectance, except that it is integrated on all
> directions.
I see, thanks.
> In CF there is `toa_bidirectional_reflectance', which is quite closed
> to the quantity we are looking for. [Philippe and ocean colour people,
> of course please tell me if I'm wrong]
>
> Then I would suggest to turn my first proposal into
> `surface_bidirectional_spectral_reflectance': with "surface" instead
> of "toa", and adding "spectral" because this field is provided at
> several wavelengths. Do you agree?
I don't think you need "spectral", which is a word we use to indicate that
the quantity is per unit wavelength or frequency. The TOA bidirectional
reflectance also depends on wavelength (see its definition) and you specify
the wavelength with a coordinate variable. So I would suggest simply
surface_bidirectional_reflectance.
Do you also need standard names of angle_of_incidence and
angle_of_reflection
to use for those independent (coordinate) variables on which the reflectance
depends?
> **About Secchi disk
>
> Yes the intention is primary to provide the depth in meters. So I'd
> prefer something closed to your second suggestion, like
> "secchi_depth_of_sea_water".
Fine. It is not self-explanatory, but that is not always attainable, and at
least it indicates that it's a property of sea water, which should be useful
to the user of the data.
Best wishes
Jonathan
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