Hi All,
'Anomaly' is one of those words, like 'coupled', that is widely used but means
totally different things to different people, depending on their field and
their current interest. Hence, I would prefer we avoided it for CF. However,
there are already four std_names that use it, and in all four cases 'anomaly'
is defined to mean 'difference from climatology'. Hence, our options are 1)
stick with 'anomaly' meaning difference from climatology, 2) change the
existing std_names to get rid of 'anomaly', or 3) use 'anomaly' to refer to
multiple different types of anomaly.
I really dislike (3) because it will mess up the CF vocabulary that has been
developing, since it would mean different things in different std_names. The
only way I can see to go with option (3) is that 'anomaly' would require
definition of both the reference and altered state in the std_name, which I
think would open up a can of worms.
This means we have to choose between (1) and (2). If we could go back in a
time-machine, then I would recommend option (2). But since that isn't
possible I think the best option is (1).
Yours truly,
Philip
-----------------------------------------------------------------------
Dr Philip Cameron-Smith, [email protected], Lawrence Livermore National Lab.
-----------------------------------------------------------------------
From: [email protected]
[mailto:[email protected]] On Behalf Of Bert Jagers
Sent: Thursday, January 26, 2012 10:07 AM
To: CF metadata list
Subject: Re: [CF-metadata] Sea surface height
Hi all,
The term "sea level anomaly" is indeed used in the context given by Stéphane; I
know my colleagues are using it in exactly the same way. So, I don't agree with
Jonathan that the term "anomaly" wouldn't be correct, however, he's probably
equally right when indicating that it's in other contexts used to refer to the
difference from a multi-year climatology. Since in climate research the sun is
the most important celestial body besides the earth, I can imagine that it's
sufficient to compare a value with the average value for the same date/time in
multiple years. However, in tidal analysis the sun is not the only
gravitational force with significant influence, the attraction of the moon is
also very important and the moon's orbit is not perfectly periodic over the
period of a year. In this context an sea level anomaly is the deviation of the
water level from the level that could have been foreseen by means of a proper
tidal analysis (either based on long-term local measurement series or a good
hydrodynamic model). Tidal analysis may show periodic behavior with frequencies
up to 18.6 years which in the absence of storms enables you to predict rare but
deterministic flooding events of low cities like Jakarta many years ahead.
Storms cause local and remote "random" deviations from this tidal signal and
that's what Stéphane is referring to.
The two variables suggested by Phil could indeed be used to store both the
reference tidal signal (water_surface_reference_datum_altitude) and the
deviation thereof (water_surface_height_above_reference_datum). However:
a) this will make the water_surface_reference_datum_altitude time dependent
which is unusual for reference levels, and
b) nobody will recognize the physical interpretation of the two variables
without further metadata, and
c) would one be allowed to store only the "sea level anomaly" variable
(water_surface_reference_datum_altitude) without the corresponding
water_surface_height_above_reference_datum variable?
Best regards,
Bert
----------------------
Bentley, Philip wrote:
Hi Stephane,
Would one or both of the following two standard names cover your use-case? The
first is in effect your residual, I think.
--------------
standard name: water_surface_height_above_reference_datum
canon units: m
description: 'Water surface height above reference datum' means the height of
the upper surface of a body of liquid water, such as sea, lake or river, above
an arbitrary reference datum. The altitude of the datum should be provided in a
variable with standard name water_surface_reference_datum_altitude. The surface
called "surface" means the lower boundary of the atmosphere.
--------------
standard name: water_surface_reference_datum_altitude
canon units: m
description: Altitude is the (geometric) height above the geoid, which is the
reference geopotential surface. The geoid is similar to mean sea level. 'Water
surface reference datum altitude' means the altitude of the arbitrary datum
referred to by a quantity with standard name
'water_surface_height_above_reference_datum'. The surface called "surface"
means the lower boundary of the atmosphere.
--------------
Regards,
Phil
________________________________
From: [email protected]<mailto:[email protected]>
[mailto:[email protected]] On Behalf Of Stephane TAROT
Sent: 26 January 2012 13:58
To: [email protected]<mailto:[email protected]>
Subject: [CF-metadata] Sea surface height
Hi,
In the MyOcean european project, we are using
sea_surface_height_above_sea_level for the insitu data mesured by tide gauges.
By substracting these measures and the predicted tide sea level, we calculate
the residual due to atmospheric (pressure and wind) conditions.
Is there an existing standard name for the residual ?
Should we consider that the predicted tide sea level is a sort of climatology
and therefore use "sea_surface_height_above_sea_level_anomaly" for the residual
?
Thanks for your help
Stéphane Tarot (Ifremer)
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