Hi Jonathan,
Thank you for your thoughtful reply which summarises very well our
discussions thus far. From your reply I deduce that you are accepting
the following 5 proposed names:
sea_surface_wave_root_mean_square_amplitude
sea_surface_wave_zeroth_spectral_moment
sea_surface_wave_mean_crest_period
sea_surface_wave_significant_wave_period
sea_surface_wave_period_at_second_peak_of_the_spectrum
This if fine except that I think
'sea_surface_wave_period_at_second_peak_of_the_spectrum'
should be re-named
'sea_surface_wave_period_at_second_largest_peak_of_the_energy_spectrum'
as the word 'largest' will make it less ambiguous.
For the other 4 proposed parameters, you propose to construct meaning using a
combination
of a standard name (concept) 'sea_surface_wave_height' and an appropriate cell
method
as shown in the following list:
CF std. name; cell_method; constructed meaning (name)
sea_surface_wave_height; time: root_mean_square;
sea_surface_wave_height_root_mean_square
sea_surface_wave_height; time: mean_of_upper_decile;
sea_surface_wave_height_mean_of_upper_decile
sea_surface_wave_height; time: mean; sea_surface_wave_height_mean
sea_surface_wave_height; time: maximum; sea_surface_wave_height_maximum
The above will require 2 new cell methods: root_mean_square and
mean_of_upper_decile.
In can see the advantages of using the cell methods as you explained however
how well that will fit into existing vocabularies and the idea of having a
'common concepts' is
up to others on the list to discuss.
As you noted for our project (IMOS+AODN) we can continue using long_name(s) for
the above
9 parameters and I propose to do that for now because I can see it will take
some
time before the CF metadata community reaches agreement. However I think we have
made a good start as 5 out of 9 proposed isn't too bad!
In answer to your questions:
1) Root mean square amplitude (Yrms)
This is derived from the wave energy spectrum E(f) analysis not from the wave
height recording.
Yrms is the square root of the zeroth spectral moment (M0) i.e Yrms=sqrt(M0)
where M0 is the
area under the wave energy spectrum curve.
The 'sea_surface_wave_root_mean_square_amplitude' is calculated from the wave
energy
spectrum not to be confused with the 'sea_surface_wave_height_root_mean_square'
which
is calculated from zero crossing analysis of the wave height recording.
2) Zeroth spectral Moment (M0)
Yes M0 is related (mathematically) to the existing 2 CF standard names i.e
sea_surface_wave_mean_period_from_variance_spectral_density_first|second_frequency_moment.
Here is the maths:
M0=integral( E(f) df ) - area under the spectrum curve
M1=integral( E(f)*f df )
M2=integral( E(f)*f^2 df )
and
sea_surface_wave_mean_period_from_variance_spectral_density_first_frequency_moment
[T(m1)]
T(m1) = M0/M1
sea_surface_wave_mean_period_from_variance_spectral_density_second_frequency_moment
[T(m2)]
T(m2) = sqrt(M0/M2)
Andrew Walsh
Data Facililator - Australian Ocean Data Network
----- Original Message -----
From: "Jonathan Gregory" <[email protected]>
To: "andrew walsh" <[email protected]>; <[email protected]>
Cc: "Lowry, Roy K" <[email protected]>; <[email protected]>; "Mark Kulmar"
<[email protected]>; <[email protected]>
Sent: Tuesday, October 12, 2010 05:09
Subject: [CF-metadata] Seeking new CF standard names (9) for sea surface wave
parameters
Dear Andrew et al.
This is to summarise briefly the discussion we have had in emails not on
the list. You proposed the following standard_names:
sea_surface_wave_mean_wave_height
sea_surface_wave_root_mean_square_wave_height
sea_surface_wave_mean_wave_height_of_highest_one_tenth
sea_surface_wave_maximum_wave_height
sea_surface_wave_root_mean_square_amplitude
sea_surface_wave_zeroth_spectral_moment
sea_surface_wave_mean_crest_period
sea_surface_wave_significant_wave_period
sea_surface_wave_period_at_second_peak_of_the_spectrum
where the heights are in m, the moment is in m2 and the periods are in s.
I agree with the proposals for new wave periods. I note that we have another
such existing name viz sea_surface_wave_zero_upcrossing_period.
You note that sea_surface_wave_significant_height is an existing standard_name
and your proposals have this style. Roy explained that these measures of wave
height are evaluated from very high-frequency sampling. I think they can be
described with cell_methods of the time axis. The advantages of using
cell_methods are that (a) it is consistent with CF treatment of other
quantities (b) it clarifies which dimension "mean" etc applies to - time in
this case, not space (c) it reduces the number of standard names required.
I therefore propose we introduce a standard_name of sea_surface_wave_height,
and new cell_methods of root_mean_square and mean_of_upper_decile (highest one
tenth); we already have cell_methods of mean and maximum, of course. I have
not understood quite what root_mean_square_amplitude means, and how it relates
to the wave height distribution sampled at high frequency. I note that the
significant wave height could be described by a cell_method of
mean_of_upper_tercile, but I would not propose that because it's a widely
used term and we have a standard_name for the corresponding period.
You commented that this means splitting up the information which describes a
quantity into two attributes. That's quite true, but it's exactly what we have
done in other such cases. The aim of the proposed common_concepts convention,
on which the discussion has not been concluded, is to provide an additional
attribute to "label" such groups of metadata. But for your own use, or for a
particular project, you could of course define your own additional convention,
which standardises the long_name, for example, to serve this purpose.
Regarding the moment, I wonder whether this is related to the moments
referred to by the existing standard_names sea_surface_wave_mean_period_from_
variance_spectral_density_first|second_frequency_moment.
Best wishes
Jonathan
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