Dear Jonathan G.
[The following discussion has little relevance to CF standard names (so
most of you shouldn't bother reading this), but scientifically I think
it's worth discussing further, so hopefully Jonathan will at least
continue.]
I was startled when you wrote earlier that in models, surface
temperature "does not necessarily have any matter associated with it",
and now you've repeated that assertion. In models I'm familiar with the
temperature is calculated such that the net exchange of energy with the
atmosphere matches the net flux into/out of the surface soil layer. As
you know, some of the fluxes are independent of the surface temperature
(e.g., surface solar fluxes and the downwelling longwave radiation), but
others depend on this temperature (e.g., the upwelling longwave
radiation and sensible heat fluxes). Surely the temperature used to
calculate the upwelling longwave radiation is based on the model's true
"skin" temperature, which, of course, is generally not the same as the
average temperature in the uppermost layer of the soil or of the
ocean. That is why the current sea_surface_temperature definition says
"Sea surface temperature is usually abbreviated as "SST". It is the
temperature of sea water near the surface (including the part under
sea-ice, if any), and not the skin temperature, whose standard name is
surface_temperature."
When this was written, sea_surface_skin_temperature had not yet been
defined, and it was clear that surface_temperature was meant to
represent the skin temperature, which might differ from a bucket
temperature or the temperature of a model's upper-most layer.
Similarly, note that sea_ice_surface_temperature is defined as
"The surface temperature is the (skin) temperature at the interface, not
the bulk temperature of the medium above or below....."
You'll note that surface_temperature is again said to be the same as
skin temperature. The fact that a model only uses skin temperature in
the surface flux calculations, not in the heat content calculations
doesn't mean that the skin temperature isn't associated with matter. It
is associated with the matter right at the model's surface, not the bulk
matter of the model's surface *layer*.
Finally, doesn't basic physics say that temperature is a measure of the
average kinetic energy of molecules, which implies it can't be defined
unless matter is involved. I would contend this is true both in the
real world and in models. [In models, of course, the molecules involved
need not necessarily be realistically represented, but surely there must
be some assumed form of these molecules in order for a temperature to
exist.]
cheers,
Karl
On 6/20/13 9:24 AM, Jonathan Gregory wrote:
Dear Karl
As I wrote in a previous posting, I think surface_temperature is either a some-
what vague concept, to be used when it is not critical to say exactly what is
meant (that's fine - standard names have always supported a range of precision
in concepts), or it's an idealisation which really refers to an energy balance
at the interface. The latter concept is applicable in models, and then does
not necessarily have any matter associated with it.
Best wishes
Jonathan
----- Forwarded message from Karl Taylor <[email protected]> -----
Date: Thu, 20 Jun 2013 09:15:36 -0700
From: Karl Taylor <[email protected]>
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.5; rv:16.0)
Gecko/20121026 Thunderbird/16.0.2
To: [email protected]
Subject: Re: [CF-metadata] new standard name: land_surface_skin_temperature
Dear all,
O.K. I withdraw my suggestion to deprecate sea_surface_skin_temperature.
I do think the definitions should say how skin temperature differs
from surface temperature. Maybe someone can explain that in a few
words.
As I understand it, temperature is only defined when molecules are
involved. So surface_temperature I think should be defined as the
temperature of the surface molecules on the ocean or land/vegetated
surface. I don't think there are any useful observational
measurements of this temperature either in the ocean or land.
Models do calculate these a surface temperature, and as I understand
it models use this as their surface radiating temperature so in that
sense the temperature is identical to skin_temperature, I would
think.
It sounds to me like in land observations, at least, the
skin_temperature is not precisely defined because the effective
radiating layer depends presumably on what wavelengths are being
sensed. To precisely say what the temperature represents one would
have to show what fraction of the radiation originated from
different depths. saying 10-20 microns of course gives an idea
about this, but it isn't precise.
Also, the definition of land_surface_skin_temperature should clearly
indicate (when it represents an area mean) whether it is meant to be
the area mean of the soil or of the "solid or liquid surface" as
seen from above which might include vegetation, puddles, etc. [as
an aside, I wonder if the thickness of the layer producing the
radiation varies much from one material to the next.]
It does seem a shame to me that users looking for
surface_temperature information will now have to search both for
surface_temperature and surface_skin_temperature, but I'll accede to
the clear majority that thinks both are necessary.
best regards,
Karl
On 6/20/13 4:56 AM, Jonathan Gregory wrote:
Dear Karl
Like Roy, I don't think we should deprecate sea_surface_skin_temperature.
Although I cannot remember the arguments - which must be apparent in the
mailing list archive - I do recall that it was a careful and long discussion
with Craig which led to the introduction of the various SST names.
Therefore adding land_surface_skin_temperature seems fine to me if there is
a need to be precise about this as an observable quantity, which relates
to a particular layer, even though it's very thin. The definition should note
that if this precise meaning is not intended, the name surface_temperature
could be used, which strictly refers to the temperature at the interface.
Best wishes
Jonathan
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