Hi Frederic, Thank you for your input.
Yes there is already a reflectance quantity in CF, but it is the TOA one. Anyway to me the definition of reflectance in general is probably broader than in your needs, then I think introducing "ratio_of_upwelling_radiance_emerging_from_sea_water_to_downwelling_radiative_flux_in_air (sr^-1)" as you suggested, is a good idea because usually CF standard names have to match exactly the geophysical content of one variable. And, moreover, I think it's better to build a new standard name from existing ones. Concerning the geometry of observation, we could introduce "angle_of_incidence" as suggested by Jonathan, plus probably something like "azimuth_angle"? And in CF there is already quantities such as "solar_azimuth_angle" and "solar_elevation_angle" to describe the solar part in the geometry of observation in ocean colour products, if necessary. Best regards, Olivier. -----Message d'origine----- De : Frederic MELIN [mailto:frederic.me...@jrc.ec.europa.eu] Envoyé : jeudi 22 octobre 2009 10:16 À : 'Jonathan Gregory'; olivier lauret Cc : 'Laurence Crosnier'; cf-metadata@cgd.ucar.edu; 'Andrei LINTU'; cristina.tronc...@artov.isac.cnr.it; 'Philippe Garnesson'; 'Ludovic BOURG'; 'Yann BARZIC' Objet : RE: [CF-metadata] Sea water transparency and reflectance. 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 _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata Cliquez sur l'url suivante https://www.mailcontrol.com/sr/RCjMXLML3IDTndxI!oX7UsS4EM7lwkAjyFLYg!GrdgFF6yBa0rdJ+xw5y7JcjAyIV0j1AIkJyVMd5rLFZ!o!pA== si ce message est indésirable (pourriel). _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata