Hi,
I'm also coming to grips with CF-compliance for wave data and model
products, so think I should comment.
The various quantities Heinz and Beate describe are all derived from the
directional spectrum. In the most general case (and explicitly so inside
a spectral wave model) this is a function S(t,x,y,f,theta) of five
dimensions:
t = time,
x,y = spatial coordinates (e.g. longitude & latitude)
f = frequency
theta = direction
S has the standard name
"sea_surface_wave_directional_variance_spectral_density"
Integrating over direction:
S1 = integral[S d(theta)], with the standard name
"sea_surface_wave_variance_spectral_density"
The parameters of interest can be derived from moments of the spectra,
i.e. if
M(n) = integral[S1*f^n df]
then the various definitions of mean period are
Tm-1 = M(-1)/M(0)
Tm1 = M(0)/M(1)
Tm2 = sqrt(M(0)/M(2))
Also:
Hm0 = 4*sqrt(M(0)) = "sea_surface_wave_significant_height"
while "mean direction" and "directional spread" come from various
directional moments of S, e.g.
integral[[S cos(theta) df] d(theta)]
Now the problem is how to describe these variables where the frequency
and directional dimensions are not necessarily explicitly present: i.e.
our data are Hm0(t,x,y), etc. These may come from a model which has
derived them from spectra, or from satellite data where the spectrum
hasn't been used explicitly at all.
If we accept that these quantities are well defined in terms of these
accepted definitions, we could just give them standard names as Heinz
and Beate propose (except I'd suggest
>> for the periods from the moment of order 2:
>> sea_surface_wave_tm2_period;s
>> sea_surface_wind_tm2_peak_period;s
>> sea_surface_swell_tm2_peak_period;s
should read:
sea_surface_wave_tm2_period;s
sea_surface_wind_wave_tm2_period;s
sea_surface_swell_wave_tm2_period;s
for consistency).
But in some cases it could be helpful to spell out how these are
derived. For example, ideally the frequency range is 0 to infinity, but
in practice a limited frequency range might have been used, especially
if we are distinguishing swell and wind waves. Then we could give
explicit frequency bounds, and use a cell method description, e.g. for Tm1:
standard_name = "sea_surface_wave_tm1_period"
cell_method = "sea_surface_wave_variance_spectral_density: normalised
inverse of first frequency moment" or similar.
Unfortunately it's rather wordy, and that's one of the simpler ones:
directional spread would be quite messy to define in words!
Regards,
Richard
On 12/11/2006 2:59 a.m., Jonathan Gregory wrote:
> Dear Heinz and Beate
>
>> we propose new standard_names for serveral variables concerning wave
>> periods deduced from the one dimensional frequency wave spectrum:
>
> These concepts look to me like different statistical methods for characterising
> the spectrum, and as such, can they be expressed by cell_methods? For instance,
> if we define a standard name of
> probability_density_function_of_sea_surface_wind_wave_period
> we can express the "peak" period (I presume this is the mode of the pdf - is
> that right?) with a standard name of sea_surface_wind_wave_period and a
> cell_methods of
> "probability_density_function_of_sea_surface_wind_wave_period: maximum"
>
> I imagine that something like that could done for the others, but I am not
> clear what the "mean" is over, or what "tm" means. "spread" also sounds like
> a statistic of some kind, that would need definition, or is it perhaps a
> standard deviation?
>
> Cheers
>
> Jonathan
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>
--
=========================================================
Richard Gorman
National Institute of Water and Atmospheric Research
PO Box 11-115, Hamilton, 3251, New Zealand
Tel: +64 7 856 1736 Mob: 021 074 7490 Fax: +64 7 856 0151
Email: r.gorman at niwa.co.nz Web: http://www.niwa.co.nz
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Received on Wed Nov 15 2006 - 23:24:33 GMT
