Hi Alison:
(Ken and JF, the thread can be picked up here:
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2007/date.html#1609)
I missed your mail and the subsequent threads for some reason - the
most up to date entries I have are from the recent message sent by Nan
(thanks Nan!). So I'll try and cover the details as best I can for
all concerned.
All of the names that are being proposed have been the subject of
great debate within the GHRSST-PP community and are the result of long
discussions at several international meetings. The names are used for
every data set produced by GHRSST-PP every day 24/7 and as the
GHRSST-PP uses netCDF (and tries to be CF-1.0 Compliant) it was
appropriate to try and obtain ratification for the long and standard
names proposed. It will be very, very difficult to change many of the
names for GHRSST-PP now but of course, this can be done with effort.
For the benefit of everyone concerned, here are the full definitions
(reported in Donlon et al., J Climate 2002 and updated and reported in
BAMS this August). The following is taken from
https://www.ghrsst-pp.org/SST-Definitions.html
SST is a difficult parameter to define exactly because the upper ocean
(~10 m) has a complex and variable vertical temperature structure that
is related to ocean turbulence and the air-sea fluxes of heat,
moisture and momentum. A framework is required to understand the
information content and relationships between measurements of SST made
by different satellite and in situ instruments, especially if these
are to be merged together. The definitions of SST developed by the
GHRSST-PP SST Science Team (agreed at the 2nd and 3rd GHRSST-PP
workshops) achieve the closest possible coincidence between what is
defined and what can be measured operationally, bearing in mind
current scientific knowledge and understanding of how the near surface
thermal structure of the ocean behaves in nature.
The interface temperature (SSTint)
At the exact air-sea interface a hypothetical temperature called the
interface temperature (SSTint) is defined although this is of no
practical use because it cannot be measured using current technology.
(NOTE- this is probably not required but was included for
completeness)
[Proposed CF-names: SSTint, interface_SST, interface_sea_surface_temperature]
The skin sea surface temperature (SSTskin)
The skin temperature (SSTskin) is defined as the temperature measured
by an infrared radiometer typically operating at wavelengths 3.7-12 ?m
(chosen for consistency with the majority of infrared satellite
measurements) that represents the temperature within the conductive
diffusion-dominated sub-layer at a depth of ~10-20 ?m. SSTskin
measurements are subject to a large potential diurnal cycle including
cool skin layer effects (especially at night under clear skies and low
wind speed conditions) and warm layer effects in the daytime. (NOTE:
Discussion on frequency is of secondary importance here - the main
issue is that the SSTskin is that retrieved using an infrared
radiometer)
[Proposed CF-names: SSTskin, skin_layer_SST, skin_layer_sea_surface_temperature]
The sub-skin sea surface temperature (SSTsub-skin)
The subskin temperature (SSTsubskin) represents the temperature at the
base of the conductive laminar sub-layer of the ocean surface. For
practical purposes, SSTsubskin can be well approximated to the
measurement of surface temperature by a microwave radiometer operating
in the 6-11 GHz frequency range, but the relationship is neither
direct nor invariant to changing physical conditions or to the
specific geometry of the microwave measurements. (NOTE: Discussion on
frequency is of secondary importance here - the main issue is that the
SSTsub-skin is that retrieved using a microwave radiometer)
[Proposed CF-names: SSTsubskin, Sub-skin_SST, sub-skin_sea_surface_temperature]
The surface temperature at depth (SSTz or SSTdepth)
All measurements of water temperature beneath the SSTsubskin are
referred to as depth temperatures (SSTdepth) measured using a wide
variety of platforms and sensors such as drifting buoys, vertical
profiling floats, or deep thermistor chains at depths ranging from
10-2 - 103m. These temperature observations are distinct from those
obtained using remote sensing techniques (SSTskin and SSTsubskin) and
must be qualified by a measurement depth in meters (e.g., or SST(z)
e.g. SST5m).
(NOTE: I'm happy with Alison and Nan suggestions regarding the use of
sea_water_temperature followed by a depth attribute. Without the
depth attribute the data are only of marginal use in data assimilation
systems and for blending. Many of the problems we have in the
satellite SST community today stem from the fact that depth was not
included as an essential component of the measurement. Now that some
satellite radiometers are providing more accurate and consistent
measurements of SST than can be obtained in situ (O'Carroll et al
JTECH, 2007) depth is essential!!) So to answer Alisons question
"We already have the standard name sea_water_temperature which is
defined as follows: "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." Would this meet your
needs?" The answer is YES it does and we will work on this within
GHRSST-PP.
The foundation temperature (SSTfnd) (Please look at figure provided at
https://www.ghrsst-pp.org/SST-Definitions.html to make sense of this)
The foundation SST, SSTfnd, is defined as the temperature of the
water column free of diurnal temperature variability (daytime warming
or nocturnal cooling) and is considered equivalent to the SSTsubskin
in the absence of any diurnal signal. It is named to indicate that it
is the foundation temperature from which the growth of the diurnal
thermocline develops each day (noting that on some occasions with a
deep mixed layer there is no clear SSTfnd profile in the surface
layer). Only in situ contact thermometry is able to measure SSTfnd and
analysis procedures must be used to estimate the SSTfnd from
radiometric satellite measurements of SSTskin and SSTsubskin. SSTfnd
provides a connection with the historical concept of a "bulk" SST
considered representative of the oceanic mixed layer temperature and
represented by any SSTdepth measurement within the upper ocean over a
depth range of 1-20+m. SSTfnd provides a more precise, well-defined
quantity than previous loosely defined "bulk" SST and consequently, a
better representation of the mixed layer temperature. In general,
SSTfnd will be similar to a night time minimum or pre-dawn value at
depths of ~1-5 m, but some differences could exist. Note that SSTfnd
does not imply a constant depth mixed layer, but rather a surface
layer of variable depth depending on the balance between
stratification and turbulent energy and is expected to change slowly
over the course of a day.
[Proposed CF-names: SSTfnd, Foundation_Temperature,
foundation_sea_surface_temperature]
(NOTE: The main purpose of SSTfnd is to remove from the vocabulary of
scientists the totally confusing and lazy term bulk SST which is
meaningless in the context of modern measurements, for data
assimilation and simply perpetuates confusion and erroneous
assumptions (Bit harsh, I know, but essentially true!)). To answer
Alison's question:
"Does this mean that the foundation temperature is measured/modelled
at the base of the thermocline so that the values are intrinsically
free of diurnal variation [YES], or is the diurnal variation
statistically removed from the data [it could be - especially when
blending data for statistical analyses]?" The answer is yes in both
cases. I hope that a new 'surface' and associated name can be
introduced to represent this quantity which is the quantity that most
global SST analysis systems produce today. There is a nice figure
showing what SSTfnd is at
https://www.ghrsst-pp.org/SST-Definitions.html
I should note that I had a small WMO/IOC JCOMM task team set up as
part of the SOT to look at this a year or so ago which concluded at
this years SOT. The definitions presented above above were adopted by
the WMO CBS. I Hope this makes things clearer and more straight
forward and I sense that the main concern is with the in situ folk
regarding the details and not the principles behind SSTz.
Take care and with best regards
Craig
--
Dr Craig Donlon
Director of the International GODAE SST Pilot Project Office
Met Office Hadley Centre,
Fitzroy Road, Exeter, EX1 3PB United Kingdom
Tel: +44 (0)1392 886622 Mob:07920 235750
Fax:+44 (0)1392 885681
Skype ID:crazit
SkypeIn: +44 0141 416 0882
E-mail: craig.donlon at gmail.com
Received on Wed Jul 25 2007 - 07:56:50 BST
