Hi Nan,
I think the statement "It is the temperature of sea water near the
surface (including the part under sea-ice, if any)" means that *under
sea ice* it is only "near the surface", not at the surface (which is the
interface between the surface and the atmosphere). Elsewhere, I think
"sea_surface_temperature is really *at* the surface, not *near* the
surface. Does anyone disagree?
For surface_air_pressure, on the other hand, the location is everywhere
at the interface between the atmosphere and underlying surface (be it
land, ice, sea ice, or water).
And surface_temperature is exactly analogous to surface_air_pressure and
is the same as sea_surface_temperature over the oceans, except where
there is sea ice.
As Jonathan says, then, when "surface" is specified, there is no need to
indicate a vertical coordinate value (which normally will vary from one
place to another). That's the simplicity of precisely defining the
"surface".
For surface air temperature and surface winds, the standard names
air_temperature and eastward_wind and northward_wind are used, and
normally a singleton dimension is defined giving the height coordinate
(standard_name=height) at which the measurement is made (typically 2m
for temperature and 10m for winds). I'm not sure how one should
indicate the heights for a set of stations, if the stations make
measurements at different heights. Perhaps someone else can provide
guidance about this.
regards,
Karl
On 3/15/12 12:17 PM, Nan Galbraith wrote:
Thanks Roy, Karl, and Jonathan -
I guess I was hoping for confirmation that one or both terms
(air_pressure_at_sea_level and surface_air_pressure) would be
analogous to sea_surface_temperature, which is defined as
being *near the surface*. In the case of SST, a depth coordinate
is allowed, as far as I can tell from the standard name table.
It looks like surface_temperature would be used without a
depth coordinate, based on the definition below, and I'm
not exactly sure about sea_surface_skin_temperature, which
includes a depth (10 - 20 micrometers) in its definition.
For in situ data, it's preferable to record the measured barometric
pressure and provide a sensor height, rather than correct to sea
level, because the correction algorithm is inexact and because
sensor height can be fully described using attributes - important
on a surface buoy, where the waterline may not be perfectly known.
We could use the standard name air_pressure, but some systems, like
SeaDataNet, distinguish between pressures used as Z coordinate variables
(for balloon or sonde measurements) and pressures that are 'science
observables'. This is a valid distinction, and I'd like to maintain it -
although
that may not be part of the CF approach.
*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. For the temperature of sea water at a
particular depth or layer, a data variable of sea_water_temperature
with a vertical coordinate axis should be used.
*surface_temperature* The surface called "surface" means the lower
boundary of the atmosphere. The surface temperature is the
temperature at the interface, not the bulk temperature of the medium
above or below.
Thanks for your input on this -
Nan
On 3/15/12 2:27 PM, Jonathan Gregory wrote:
Dear Nan
If the quantity has "surface" in its standard name, it should not have a
z-coordinate, because "surface" quantities are defined as being values on
"the surface" (i.e. bottom of the atmosphere) z(x,y). Given x and y, z
is known. The same applies to quantities with standard names that contain
other named surfaces, such as toa (rather a vague surface!) or sea_floor.
Best wishes
Jonathan
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