Dear Philip,
Thanks for the message and your perspective on equivalence of standard
names. In reviewing the surface_temperature definition again and
thinking about the observational data product motivating my original
submission, I do agree with you that the intent of the two quantities is
the same and, thus, the surface_temperature standard name will suffice
for the datal product that I have submitted. Namely, they both refer to
the interface or skin temperature of the land surface. For a model
surface temperature, this is the temperature of the surface exactly at
the interface of the land and atmosphere, while the land surface
temperature inferred from an infrared radiometer, for instance, is the
effective temperature of the land interface/skin and other possible
terrestrial features (e.g. snow, vegetation) at the surface. While both
the model and observed surface temperature will have there own
associated uncertainties, these two quantities are compared directly in
practice in the community. In other words, I think they would pass the
test proposed in your post...
If I want to validate one dataset against another (model-model, model-obs, or obs-obs), would I be comfortable doing a simple subtraction between the variables in the two datasets (ie, without any sort of correction)?" If the answer is "yes", then I consider the quantities to be comparable, and they should have the same std_name. If the answer is "no" then they should have different std_names.
Related to this is another point...for the case of a remotely sensed
temperature over land, the temperature will be sensitive to the
penetration depth of the radiation used for the measurement. The
penetration depth of the infrared measurements, for example, varies with
surface type (e.g., soil, inland water, snow, ice), therefore is is not
practical to assign a particular depth to the remotely sensed surface
skin temperature measurements. This lack of preciseness argues for not
creating a unique standard name for the observable land surface
temperature. On the other hand, it is possible to be more
radiometrically precise with the sea surface skin temperature inferred
from an observation and this bears it out in the
sea_surface_skin_temperature definition.
So, I would like to retract my proposal for the
land_surface_skin_temperature. I believe that the current standard name
of surface_temperature will work fine.
Sincerely,
Jonathan Wrotny
On 10/4/2013 11:36 PM, Cameron-smith, Philip wrote:
> Hi John,
>
> I apologize for not being clear :-(, so thank you for asking :-).
>
> I think you are raising a couple of issues:
>
> 1) Is it appropriate for an observation and a model variable to have the same std_name?
>
> This wasn't the point I was trying to discuss. So hopefully I can clarify my thoughts a bit.
>
> Consider the (hypothetical) case where there is a theoretical quantity (e.g. skin temperature), which is calculated as part of a PERFECT model, and is also measured by a PERFECT instrument. In that case, both the model and observational data should have the same std_name.
>
> Now consider the case where the model and/or instrument are NOT perfect (e.g., the model equations actually use the average temperature of the top 20m of soil, or the instrument actually measures the air temperature 10m above the surface).
>
> The test I apply is: "If I want to validate one dataset against another (model-model, model-obs, or obs-obs), would I be comfortable doing a simple subtraction between the variables in the two datasets (ie, without any sort of correction)?" If the answer is "yes", then I consider the quantities to be comparable, and they should have the same std_name. If the answer is "no" then they should have different std_names. Hence, the question of whether the data is from models or observations is not relevant.
>
> For my imperfect example above I would say it fails the test. If I was faced with that situation I would want to apply corrections to account for the difference between an air temperature and a soil temperature below the surface.
>
> If we improve the quantities (modeled or measured), then at some point they become 'close enough' to pass the test.
>
> Now for the specific case at hand, would I directly compare the skin temperature in my model against a spectroscopic measurement of the skin temperature from space (which presumably includes assumptions about fitting a black body and accounting for atmospheric absorption)? My answer would be "yes", and hence my objection to the proposed std_name.
>
> As a side note, I believe the decision of which std_name is appropriate for a quantity is generally best made by the person who creates the data, because they understand their own model or instrument, and they can also make the decision at the time the CF file is being created. But this is getting into a separate issue.
>
> 2) Why am I objecting to land_surface_skin_temperature when sea_surface_skin_temperature has already been accepted?
>
> There are many quirks and inconsistencies in CF, as you know ;-), mostly because we discuss std_names one at a time (so we don't always see the big picture). I don't recall discussing sea_surface_skin_temperature. Perhaps it was before I joined the mailing list, or I wasn't paying attention. It is also possible that someone at the time made the argument that the temperature 10-20 microns below the surface of the ocean is sufficiently physically distinct from the theoretical skin temperature. [Do we want to reopen this discussion?]
>
> If the experts on this mailing list speak up and say that the distinction between 10-20 microns depth and the infinitesimal skin is physically important in practice, then I shall drop my objections. I just note that the proposal description for land_surface_skin_temperature states that the distinction is not very important.
>
> Best wishes :-),
>
> Philip
>
> -----------------------------------------------------------------------
> Dr Philip Cameron-Smith, pjc at llnl.gov, Lawrence Livermore National Lab.
> -----------------------------------------------------------------------
>
>
>
>> -----Original Message-----
>> From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf
>> Of John Graybeal
>> Sent: Friday, October 04, 2013 6:04 PM
>> To: CF Metadata List
>> Subject: Re: [CF-metadata] Surface temperatures
>>
>> Philip, to be clear, are you saying one should, or should not, compare values
>> from a model to observational values? We don't distinguish between them in
>> CF names that I know of, so I'm assuming it's OK.... (And to Jonathon
>> Wrotny's point: Considering a fundamental concept like "temperature of an
>> observable feature" somehow different just by virtue of being in a model, is
>> just way too big a Pandora's box to open up. In my humble opinion.)
>>
>> For me, 'equivalent names' means equivalent _names_. If land_surface_skin
>> temperature is equivalent to surface_temperature:cell_methods="area:
>> mean where land", then sea_surface_skin_temperature must be equivalent
>> to surface_temperature:cell_methods="area: mean where sea". And I bet I
>> could find quite a few other 'equivalences' by using a cell method like "area:
>> mean where land/sea".
>>
>> Not only are such equivalences quite uncommon so far, to me it is not all that
>> equivalent. (Perhaps that is my ocean background, where 'surface' is still a bit
>> indeterminate -- rightly or wrongly!)
>>
>> John
>>
>> P.S. IIRC, sea_surface_temperature used to be the only sea surface
>> temperature; other definitions were added because our view of the top of
>> the ocean -- through satellites or models or thermometers -- was much more
>> refined. So we needed more refined terms to make things comparable again.
>>
>>
>> On Oct 4, 2013, at 17:28, "Cameron-smith, Philip" <cameronsmith1 at llnl.gov>
>> wrote:
>>
>>> Hi Jonathan (Wrotny),
>>>
>>> The general practice of CF is that quantities that are 'equivalent', ie close
>> enough that it is meaningful to take the difference between them, should
>> have the same std_name (ie, they are both trying to calculate or measure
>> the same physical quantity).
>>> IMHO, this provides huge value to users, since it tells them when they can,
>> or shouldn't, compare two quantities (eg, compare the surface temperatures
>> from a model dataset with satellite observations of surface temperatures). If
>> 'equivalence' is treated too strictly, then no variable can ever be compared to
>> another.
>>> Unfortunately, there is a grey zone between quantities are equivalent and
>> quantities that are not, and then long discussions usually occur.
>>> From the description of the quantity you describe, it seems to me that
>> land_surface_skin_temperature and
>> surface_temperature:cell_methods="area: mean where land", should be
>> deemed to be 'equivalent'.
>>> If you agree, then one advantage for you is that you don't have to do any
>> more work on this email list ;-).
>>> Best wishes,
>>>
>>> Philip
>>>
>>>
>>>
>>> ----------------------------------------------------------------------
>>> - Dr Philip Cameron-Smith, pjc at llnl.gov, Lawrence Livermore National
>>> Lab.
>>> ----------------------------------------------------------------------
>>> -
>>>
>>>
>>>> -----Original Message-----
>>>> From: Jonathan Wrotny [mailto:jwrotny at aer.com]
>>>> Sent: Thursday, October 03, 2013 12:40 PM
>>>> To: Cameron-smith, Philip
>>>> Cc: Jonathan Gregory; cf-metadata at cgd.ucar.edu
>>>> Subject: Re: [CF-metadata] Surface temperatures
>>>>
>>>> Dear Philip,
>>>>
>>>> My take is that the land_surface_skin_temperature and the
>>>> surface_temperature are likely very close in value, since the
>>>> surface_temperature is an infinitesimally thin layer at the bottom
>>>> level of the atmosphere which interfaces with the land skin (soil)
>>>> below - hence, the definition stating that they can be taken to be
>> equivalent.
>>>> The land_surface_skin_temperature proposal is motivated by a new
>>>> observational data product which is the radiating temperature of a
>>>> very thin, top layer of the land surface. This quantity does not
>>>> currently exist in the CF standard name set, but has an analogue in
>> sea_surface_skin_temperature.
>>>> The surface_temperature name was added to CF because it is a standard
>>>> model variable, I believe. Someone please correct me if I am wrong,
>>>> but the radiating temperature of the Earth in models is often simply
>>>> referrred to as the "surface temperature," so I wanted to draw a
>>>> connection between the model quantity and the observable
>>>> land_surface_skin_temperature in the definition such that they are
>>>> effectively the same thing. This seems to be one of those situations
>>>> where there are two quantities, one created for an observed quantity
>>>> and the other for a model quantity, but the two quantities likely
>>>> have very similar values. I guess the question is whether or not this is
>> permissible within CF.
>>>> Sincerely,
>>>>
>>>> Jonathan Wrotny
>>>>
>>>> On 10/3/2013 1:30 PM, Cameron-smith, Philip wrote:
>>>>> Hi Jonathan (Wrotny), Jonathan (Gregory), et al.,
>>>>>
>>>>> I am a little surprised.
>>>>>
>>>>> It is explicitly stated in the proposed description that
>>>> land_surface_skin_temperature "can be taken to be equivalent to"
>>>> surface_temperature over land areas.
>>>>> In the description for surface_temperature, it indicates that it can
>>>>> apply to
>>>> just land using cell_methods. Indeed, in the CF convention, example 7.6
>>>> explicitly states this:
>>>>> Example 7.6. Mean surface temperature over land and sensible heat
>>>>> flux
>>>> averaged separately over land and sea.
>>>>> float surface_temperature(lat,lon);
>>>>> surface_temperature:cell_methods="area: mean where land";
>>>>>
>>>>> I also note that surface_temperature is already an alias for
>>>>> surface_temperature_where_land (which I think is deprecated)
>>>>>
>>>>> Why is a new std_name needed? What am I missing?
>>>>>
>>>>> It is true that there is a variable called
>>>>> sea_surface_skin_temperature, but
>>>> it appears that this was introduced for different reasons. Specifically, it
>> looks
>>>> like sea_surface_temperature was created to refer to the water _near_
>> the
>>>> surface to distinguish it from the 'skin'. sea_surface_skin_temperature
>> then
>>>> differs from surface_temperature because it refers to the interface
>>>> under sea-ice rather than above sea-ice.
>>>>> Best wishes, as always :-),
>>>>>
>>>>> Philip
>>>>>
>>>>> --------------------------------------------------------------------
>>>>> --
>>>>> - Dr Philip Cameron-Smith, pjc at llnl.gov, Lawrence Livermore National
>>>>> Lab.
>>>>> --------------------------------------------------------------------
>>>>> --
>>>>> -
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On
>>>> Behalf
>>>>>> Of Jonathan Gregory
>>>>>> Sent: Thursday, October 03, 2013 9:35 AM
>>>>>> To: cf-metadata at cgd.ucar.edu
>>>>>> Subject: Re: [CF-metadata] Surface temperatures
>>>>>>
>>>>>> Dear Jonathan
>>>>>>
>>>>>> The new proposal looks fine to me. Thanks. I see that you don't
>>>>>> have to define the thickness of the layer; instead, you are
>>>>>> defining it implicitly through the method of diagnosis. Others may
>>>>>> have views, of
>>>> course.
>>>>>> Cheers
>>>>>>
>>>>>> Jonathan
>>>>>>
>>>>>> ----- Forwarded message from Jonathan Wrotny <jwrotny at aer.com>
>>>>>> -----
>>>>>>
>>>>>>> Date: Wed, 2 Oct 2013 11:26:27 -0400
>>>>>>> From: Jonathan Wrotny <jwrotny at aer.com>
>>>>>>> User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130801
>>>>>>> Thunderbird/17.0.8
>>>>>>> To: Jonathan Gregory <j.m.gregory at reading.ac.uk>, "cf-
>>>>>> metadata at cgd.ucar.edu"
>>>>>>> <cf-metadata at cgd.ucar.edu>
>>>>>>> Subject: Re: [CF-metadata] Surface temperatures
>>>>>>>
>>>>>>> Dear Jonathan Gregory,
>>>>>>>
>>>>>>> I am getting back to this reply after a long time - sorry, I was
>>>>>>> pulled in a few different directions lately. Hopefully, it is
>>>>>>> possible to bring back to life a submission that I had made for
>>>>>>> the land_surface_skin_temperature.
>>>>>>>
>>>>>>> Revisiting my previous proposal and a few e-mails by Karl Taylor
>>>>>>> and Evan Manning, I have made some modifications to the definition
>>>>>>> of this standard name so that I can incorporate some suggestions
>>>>>>> by Karl and Evan. Here is my current proposal:
>>>>>>>
>>>>>>> Standard Name:land_surface_skin_temperature
>>>>>>>
>>>>>>> Definition:The land surface skin temperature is the temperature of
>>>>>>> a land point or the land portion of a region as inferred from
>>>>>>> infrared radiation emitted directly towards space through the
>>>>>>> atmosphere. Not all of the emitted surface radiation originates at
>>>>>>> the soil.Some may come from various terrestrial features (e.g.,
>>>>>>> vegetation, rivers, lakes, ice, snow cover, man-made
>>>>>>> objects).Thus, the land surface skin temperature is the aggregate
>>>>>>> temperature of an effective layer which includes the soil and
>>>>>>> terrestrial features at the surface (if they occur).In models, the
>>>>>>> radiating temperature of the surface is usually the
>>>>>>> "surface_temperature", which then can be taken to be equivalent to
>>>>>>> land_surface_skin_temperature or sea_surface_skin temperature,
>> depending on the underlying medium.
>>>>>>> Canonical Units:K
>>>>>>>
>>>>>>> Thanks for still considering this proposal. Sincerely,
>>>>>>>
>>>>>>> Jonathan Wrotny
>>>>>>>
>>>>>>> On 8/1/2013 12:56 PM, Jonathan Gregory wrote:
>>>>>>>> Dear all
>>>>>>>>
>>>>>>>> I agree with Karl than in CF standard names "land" means
>>>>>>>> "non-sea", whereas sea-ice is part of sea. Hence I would support
>>>>>>>> adding land_surface_skin_ temperature, for use by applications
>>>>>>>> which classify
>>>>>> locations as land or sea.
>>>>>>>> However I also agree with Evan that one can approach this more
>>>>>>>> generally, and therefore I would also support the addition of
>>>>>>>> surface_skin_temperature, with which an area-type could be
>>>>>>>> specified, if anyone wants to follow that approach (we only add
>>>>>>>> names when they
>>>>>> are needed).
>>>>>>>> The quotations that Evan made show that we need to change the
>>>>>>>> definitions where they mention "skin". This is because in these
>>>>>>>> new names "skin" is being given a more precise and practical
>>>>>>>> meaning, motivated by observational methods, whereas the
>>>> surface_temperature
>>>>>>>> names were introduced for models, in which the skin can be a
>>>>>>>> notional
>>>>>> and infinitesimally thin layer.
>>>>>>>> Best wishes
>>>>>>>>
>>>>>>>> Jonathan
>>>>>>>> _______________________________________________
>>>>>>>> CF-metadata mailing list
>>>>>>>> CF-metadata at cgd.ucar.edu
>>>>>>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>>>>> ----- End forwarded message -----
>>>>>> _______________________________________________
>>>>>> CF-metadata mailing list
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>>>>> _______________________________________________
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>> ------------------------------------
>> John Graybeal
>> Senior Data Manager, Metadata and Semantics
>>
>> T +1 (408) 675-5545
>> F +1 (408) 616-1626
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>>
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Received on Wed Oct 09 2013 - 05:40:34 BST
