Hello Karl,
There have been extensive debates in the observational oceanographic community on the concept of 'sea surface temperature' - much of it on this list led by Craig Donlan - that resulted in the addition of sea_surface_foundation_temperature, sea_surface_skin_temperature and sea_surface_subskin_temperature in 2008, giving four SST Standard Names, not three. If you dig back into the archives you'll find the case was very strongly argued and for that reason alone I would counsel against deprecating sea_surface_skin_temperature.
I totally agree that sea_surface_temperature is an imprecise term which is why Craig's community argued so strongly for its augmentation, but I'm pretty sure that the scope of their considerations was open water and not water under sea ice. Whilst I agree that surface_temperature and sea_surface_skin_temperature should theoretically be numerically equal to my (observationally-biased) viewpoint the former is a measurement in the atmosphere (based on the CF definition of surface) whilst the latter is a measurement in the water column. Again, I would counsel against deprecating sea_surface_skin_temperature.
As regards land skin temperature I really don't have any experience in that domain, but my feelings are that labelling a measurement of terra firma as the temperature of the lower boundary of the atmosphere is pushing implied semantics too far. Consequently, I would support the new Standard Name.
Cheers, Roy.
Please note that I now work part-time from Tuesday to Thursday. E-mail response on other days is possible but not guaranteed!
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Karl Taylor
Sent: 19 June 2013 21:44
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] new standard name: land_surface_skin_temperature
Dear all,
I think for any conceivable purposes land_surface_skin_temperature is identical to surface_temperature. If not, the definition should explain how they differ. Is there really an important difference between the temperature representative of the top few micrometers and the temperature of the top few molecules of the surface? I know in models there isn't. Also, over land presumably some sensors are inferring this temperature from radiation emitted by some combination of both vegetation and bare ground. "upper boundary of the land" sounds like bare ground to me, so maybe this should be clarified.
Rather than define land_surface_skin_temperature to maintain consistency with sea_surface_skin_temperature, I would prefer to deprecate sea_surface_skin_temperature in favor of surface_temperature. As Jonathon G. recalled, there was some rather extensive discussion about sea_surface_skin_temperature when it was proposed, which should be reviewed before deciding to deprecate. [I can't remember it, and I don't think I participated in that earlier discussion, but I can't see a need for skin_temperature.]
Reading over the 3 standard names relevant to SST (surface_temperature, sea_surface_temperature, sea_surface_skin_temperature) here are some comments:
For *models*: In ice-free regions all three quantities are identical. In sea-ice-covered (or partially sea-ice-covered) regions only sea_surface_skin_temperature and sea_surface_temperature are identical (although one might argue that sea_surface_skin_temperature should not be defined under sea ice because longwave radiative fluxes are normally neglected between the sea ice and ocean below.) Globally all three are the same if cell_method="area: mean where ice_free_sea".
For *observations*: There is a difference between sea_surface_skin_temperature and sea_surface_temperature. In addition sea_surface_temperature is imprecise because it depends on the measuring device (e.g., ship intake temp., bucket temp. ). In ice-free regions sea_surface_skin_temperature should be nearly identical to surface_temperature (How big can the difference be between a weighted average of temperature over the top 10-20 micrometers of the ocean and the temperature of the top few molecules of the ocean??). One might argue that sea_surface_skin_temperature should not be defined under sea ice because the radiometers used to measure it would not normally penetrate the sea-ice. If so, then surface_temperature with a cell_method="area: mean where ice_free_sea" would be the same as "sea_surface_skin_temperature" and they both would undefined under sea ice. They would, of course, differ from sea_surface_temperature.
My conclusion is that sea_surface_skin_temperature is not needed and should be deprecated and for similar reasons land_surface_skin_temperature should not be added. We need to retain both sea_surface_temperature and surface_temperature.
best regards,
Karl
On 6/19/13 8:32 AM, Jonathan Wrotny wrote:
Dear Jonathan G and John G,
Thanks to both of you for your replies to the questions on land surface skin temperature. I do agree with Jonathan G's points and see how the land surface temperature is likely very similar in value to the surface (interface) temperature, but my opinion is that for consistency of names within the CF and to alleviate general confusion (e.g. the inevitable questions of why there is not a land surface skin temperature when there is one for the sea surface), its seems useful to add a standard name for land_surface_skin_temperature. I think having the analogue to the sea surface quantity is complementary. I realized, however, that my initial definition of land surface skin temperature was a little too specific in attempting to quantify the depth of the skin layer. My general impression is that the depth of the land skin layer is more variable and not as well quantified as the depth of the sea surface skin. Thus, I generalized the definition to emphasize that the skin temperature is a radiometric quantity and
removed the reference to the skin depth. Here is my current proposal:
Standard Name: land_surface_skin_temperature
Definition: The land surface skin temperature is the aggregate temperature of the "skin" of the land surface, where the "skin" is the upper boundary of the land which emits infrared radiation directly to space through the atmosphere.
Canonical Units: K
Any comments are appreciated.
Sincerely,
Jonathan Wrotny
On 6/17/2013 10:17 AM, John Graybeal wrote:
As I understand it, the reason for the introduction of the skin temperature (and other sea surface temperatures) is that originally sea_surface_temperature encompassed everything (in its 100 years of use) from a bucket somewhere in the first 10 meters, to a satellite measuring the first few millimeters. More precise names were needed.
I expect a similar situation applies on land. Even if past practice may only penetrate the service 10 centimeters, that's noticeably different than a satellite measuring a few millimeters. If that's at all true, having the refined term makes sense to me.
John
On Jun 17, 2013, at 05:41, Jonathan Gregory <j.m.gregory at reading.ac.uk><mailto:j.m.gregory at reading.ac.uk> wrote:
Dear Jonathan and Jim
In my previous email, I was trying to explain why an interface temperature
is a physical meaningful concept, which Jonathan asked about. This is actually
an applicable concept in models, as I said, and it is the idea which I (at
least) had in mind when the name was put in the standard name table. Like CF
in general, the standard name table was originally created for the purpose of
model metadata, and was later to extended to observations. This quantity is an
idealisation, not an observable quantity.
The heat capacity of a layer 12 micrometers thick is so small that I suppose
there is practically no difference between the skin temperature and the
interface temperature, on the timescales you're interested in. Is that correct,
do you think? If so, it seems to me that it would be fine to use the existing
name of surface_temperature for this quantity. You propose the new name on the
analogy of the sea_surface_skin_temperature. The same argument would apply to
that as well. I can't remember the reason why it was thought necessary to make
a distinction between surface_temperature and sea_surface_skin_temperature,
though I do recall quite a lot of discussion about it.
It is also fine to introduce land_surface_skin_temperature as well, I would
say. The data-writer has a choice. They could use surface_temperature if that
is accurate enough, but if they wish to be more precise about what material
layer it applies to, the skin temperature names could be used.
Best wishes
Jonathan
13/06/17 13:20:03 house
On Fri, Jun 14, 2013 at 02:45:28PM -0400, Jonathan Wrotny wrote:
Date: Fri, 14 Jun 2013 14:45:28 -0400
From: Jonathan Wrotny <jwrotny at aer.com><mailto:jwrotny at aer.com>
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130509
Thunderbird/17.0.6
To: Jim Biard <jim.biard at noaa.gov><mailto:jim.biard at noaa.gov>
CC: "cf-metadata at cgd.ucar.edu List"<mailto:cf-metadata at cgd.ucar.eduList> <cf-metadata at cgd.ucar.edu><mailto:cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] new standard name: land_surface_skin_temperature
Dear Jim,
Thanks for your comments. They all make sense to me and I'm on
board with your suggested definition. I'll just wait for others to
comment, if needed, then we can converge on a final definition.
Sincerely,
Jonathan
On 6/14/2013 2:11 PM, Jim Biard wrote:
Jonathan,
I still don't believe that the surface temperature concept that
Jonathan Gregory has ever been what people were intending when
they make the surface_temperature standard name, but I'll abide by
whatever folks decide.
On a different front, I don't think the definition of the standard
name should include statements about technology used (measured by
an infrared radiometer?). The definition should speak only to the
measured quantity, without reference to the way in which you
happen to be measuring it. Likewise, there is no need for the
statement regarding variability of the quantity. Also, the
surface in this name is not the lower boundary of the atmosphere.
It is the upper boundary of the land. An non-volatile object in a
hard vacuum has a surface skin temperature.
Given all that, I'd suggest this for your definition:
Standard Name: land_surface_skin_temperature
Definition: The land surface skin temperature is the aggregate
temperature of the "skin" of the land surface, which extends
vertically approximately 12 micrometers below the land surface.
If people really think it needs to be spelled out even further,
add the sentence "The land surface is the upper boundary of the
land."
Grace and peace,
Jim
Jim Biard
Research Scholar
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On Jun 14, 2013, at 1:54 PM, Jonathan Wrotny <jwrotny at aer.com<mailto:jwrotny at aer.com>
<mailto:jwrotny at aer.com><mailto:jwrotny at aer.com>> wrote:
Dear Jonathan Gregory,
Thanks for your reply...this certainly helps to clear things up
for me. I now better understand the meaning of the
"surface_temperature" standard name with the temperature defined
by heat fluxes at an interface, and not based on an actual
medium.
This also makes it obvious to me that my proposed standard name
"land_surface_skin_temperature" does not currently exist within
CF and could serve as an analogue to
"sea_surface_skin_temperature." To summarize, here is my
current proposal:
Standard Name: land_surface_skin_temperature
Definition:The surface called "surface" means the lower boundary
of the atmosphere. The land surface skin temperature is the
temperature measured by an infrared radiometer, but measurements
from microwave radiometers operating at GHz wavelengths also
exist. It represents the aggregate temperature of the skin
surface where ?skin? means the surface medium viewed by a sensor
to a vertical depth of approximately 12 micrometers.
Measurements of this quantity are subject to a large potential
diurnal cycle which is primarily due to the balance between
heating during the day by solar radiation and continual cooling
from terrestrial (long-wave) radiation emitted by the skin
surface.
Canonical Units:K
Sincerely,
Jonathan Wrotny
On 6/14/2013 1:22 PM, Jonathan Gregory wrote:
Dear Jonathan
I defer to Roy about the various sea water temperature names.
It is physically meaningful to have a temperature which doesn't relate to any
material layer. If there is no matter associated with it, it must have zero
heat capacity, so the temperature is determined by requiring an exact balance
of heat fluxes. For this to be possible, the heat fluxes concerned must depend
on the temperature, which of course they generally do. Obviously this is an
idealisation, but a surface interface temperature of this kind really can
exist in a model, although it's not an observational quantity. A model can
obtain such a temperature by solving simultaneously for the heat fluxes that
are balanced at the interface.
Best wishes
Jonathan G
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