HI Nan and all,
I would suggest that for CF, the term albedo should be limited to its
stricter traditional definition of applying to the full spectrum of
solar radiation (in which case we would not suggest that a coordinate
variable be used to specify a radiation wavelength or frequency). If
you want to indicate the the fraction reflected radiation for a subset
of angles or wavelengths, there are in physics better terms to use (see
http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Reflectivity.html )
Here are some standard names that we might want to include in CF, which
would cover various cases described at the above URL:
reflectivity (a function of wavelength and angle)
hemispherical_reflectivity (a function of wavelength band)
hemispherical_spectral_reflectivity (a function of wavelength)
total_hemispherical_reflectivity (for our purposes this would be
redundant with albedo, I think)
I suggest "hemispherical_reflectivity" would be a good choice when
specifying a wavelength range.
best regards,
Karl
On 6/21/13 10:38 AM, Nan Galbraith wrote:
> Hi Karl -
>
> I've only used it the way you describe, but the definition that was
> originally
> proposed said 'A coordinate variable can be used to specify the
> radiation wavelength or frequency.'
>
> I checked some on line definitions, and that seems to be correct. E.g.
> wikipedia
> says 'When quoted unqualified, it usually refers to some appropriate
> average
> across the spectrum of visible light.' I didn't find any definitions
> that ruled
> out the use of a specific frequency range.
>
> My change was just to clarify that when a frequency is not specified,
> we mean
> the 'appropriate average'.
>
> Nan
>
>
> On 6/19/13 5:18 PM, Karl Taylor wrote:
>> Dear Nan,
>>
>> I'm not positive about this, but I thought albedo always referred to
>> the total fraction reflected (integrating over all shortwave
>> wavelengths and all angles). If you want to limit it to only a
>> portion of the shortwave spectrum, I think a different term
>> (reflectivity? reflectance?) should be used.
>>
>> Karl
>>
>> On 6/19/13 8:32 AM, Nan Galbraith wrote:
>>> Can we also update the definition for surface_albedo now? It's
>>> currently ' The surface
>>> called "surface" means the lower boundary of the atmosphere.' which
>>> doesn't quite
>>> capture the concept.
>>>
>>> We use a (constant, inferred) albedo in our surface flux
>>> calculations, and I'm
>>> planning to include it as a variable when I start publishing those
>>> files in CF.
>>>
>>> I'd like to see one detail added to the definitions below, that the
>>> outgoing energy in
>>> the ratio is reflected - not that I can think of another source of
>>> outgoing energy off
>>> the top of my head ...
>>>
>>> For example, the ESR glossary says: 'Albedo is the fraction of solar
>>> energy (shortwave
>>> radiation) reflected from the earth back into space. It is a measure
>>> of the reflectivity
>>> of the earth's surface' - so we could use something like
>>> 'Albedo is the ratio of outgoing reflected energy to incoming energy
>>> (irradiance).'
>>>
>>> Also, should we add the default frequency, for cases where no coordinate
>>> is supplied?
>>>
>>> 'A coordinate variable can be used to specify the radiation wavelength
>>> or frequency; the default frequency is anappropriateaverage across
>>> the spectrum of visible light.'
>>>
>>> (Note, the 'appropriate average' term is used in several descriptions)
>>>
>>> Thanks -
>>> Nan
>>>
>>>
>>>
>>>
>>> On 5/18/13 10:42 AM, Jonathan Gregory wrote:
>>>> Dear Ted
>>>>
>>>> That looks fine to me. It'd be interesting to know if anyone else has comments.
>>>>
>>>> Thanks
>>>>
>>>> Jonathan
>>>>
>>>> ----- Forwarded message from Ted Kennelly<ekennell at aer.com> -----
>>>>
>>>>> Date: Mon, 13 May 2013 18:01:03 -0400
>>>>> From: Ted Kennelly<ekennell at aer.com>
>>>>> To: Jonathan Gregory<j.m.gregory at reading.ac.uk>
>>>>> Subject: Re: [CF-metadata] New reflectance standard names
>>>>>
>>>>> Below are the updated descriptions for the two proposed standard
>>>>> names. Note CF does not seem to have a definition for albedo though
>>>>> it is used
>>>>> in a handful of standard_names.
>>>>>
>>>>> -ted
>>>>>
>>>>>
>>>>> toa_lambertian_equivalent_albedo
>>>>>
>>>>> Albedo is the ratio of the outgoing to the incoming power per unit
>>>>> area (irradiance). A coordinate variable can be used to specify the
>>>>> radiation wavelength or frequency. toa means top of atmosphere.
>>>>> lambertian_equivalent means the quantity is evaluated for a
>>>>> diffusely reflecting surface.
>>>>>
>>>>> canonical units = 1
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> toa_lambertian_equivalent_albedo_multiplied_by_cosine_solar_zenith_angle
>>>>>
>>>>> Albedo is the ratio of the outgoing to the incoming power per unit
>>>>> area (irradiance). A coordinate variable can be used to specify the
>>>>> radiation wavelength or frequency. toa means top of atmosphere.
>>>>> lambertian_equivalent means the quantity is evaluated for a
>>>>> diffusely reflecting surface.
>>>>> multiplied_by_cosine_solar_zenith_angle means that the quantity
>>>>> has normalized to remove the angular dependence of the incoming
>>>>> shortwave irradiance
>>>>>
>>>>> canonical units = 1
>>>>>
>>>>>
>>>>> Jonathan Gregory wrote:
>>>>>> Dear Ted
>>>>>>
>>>>>> Thanks for this explanation:
>>>>>>
>>>>>>> Reflectance is the ratio of outgoing radiance to incoming irradiance
>>>>>>> giving it units of per steradians.
>>>>>>> Albedo is the ratio of outgoing irradiance to incoming irradiance
>>>>>>> and is unitless.
>>>>>>> Because the proposed quantity is unitless, albedo is the better term.
>>>>>>> However, albedo is the integral over all angles of the bidirectional
>>>>>>> reflectance. In general, true
>>>>>>> planetary albedo would account for this angular dependence. To make clear the
>>>>>>> assumption regarding the Lambertian surface I still propose using the term
>>>>>> Lambertian equivalent in the name. Thus:
>>>>>>
>>>>>>> toa_lambertian_equivalent_albedo
>>>>>>> toa_lambertian_equivalent_albdeo_multiplied_by_cosine_solar_zenith_angle
>>>>>> OK, with small L in lambertian as you had before (because stdnames are all
>>>>>> in lower case). Thanks!
>>>>>>
>>>>>> Cheers
>>>>>>
>>>>>> Jonathan
>>>>>>
>
> --
> *******************************************************
> * Nan Galbraith Information Systems Specialist *
> * Upper Ocean Processes Group Mail Stop 29 *
> * Woods Hole Oceanographic Institution *
> * Woods Hole, MA 02543 (508) 289-2444 *
> *******************************************************
>
>
>
>
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