Dear Jonathan,
Thank you for drawing my attention to what is a glaring error in the existing wave height Standard Name definitions. I've no idea how I failed to spot it before - trying to fit CF into spare moments leading to too much scan reading or perhaps they were set up before I became involved in CF. The problem is that 'wave height' is the distance from trough to crest and not the distance of the crest from a plane of reference!
Whilst existing cell methods COULD be used be used in some of these cases, I really would prefer if we can follow the precedent set by the half dozen existing Standard Names such as ' sea_surface_wave_mean_period_from_variance_spectral_density_first_frequency_moment' and avoid using cell methods for wave statistics, especially where this would take very well-known terms in the oceanographic community such as 'significant height', 'mean height' and 'maximum height' out of the Standard Name. Leaving them in will both make CF wave data more discoverable, semantically more transparent and also prevent ourselves getting tied up in knots with some of the more complex wave statistics not yet under consideration.
I have yet to read Elodie's proposed definitions in detail, but certainly the observation period should not be fixed at 30 minutes. We hold wave statistics with observation periods varying from 10 minutes to 3 hours.
I propose to proceed as follows:
1) I will propose new definitions for the three existing wave height Standard Names:
2) I will go through Elodie's e-mail of yesterday and check her definitions.
3) I will comment on Elodie's proposals for energy, direction, steepness and period.
4) I will go through the remaining existing wave Standard Names.
This may take me a few weeks as I'm away on holiday for a week from Friday. For a start, here are my proposals for revised wave height Standard Names.
sea_surface_swell_wave_significant_height
Wave height is defined as the distance from a wave trough to the following wave crest. Significant wave height is a statistic computed from wave measurements collected during an observation period that approximates to the wave height that would be recorded visually by a human observer during that observation period. Swell waves are the low frequency portion of a bimodal wave frequency spectrum.
sea_surface_wave_significant_height
Wave height is defined as the distance from a wave trough to the following wave crest. Significant wave height is a statistic computed from wave measurements collected during an observation period that approximates to the wave height that would be recorded visually by a human observer during that observation period.
sea_surface_wind_wave_significant_height
Wave height is defined as the distance from a wave trough to the following wave crest. Significant wave height is a statistic computed from wave measurements collected during an observation period that approximates to the wave height that would be recorded visually by a human observer during that observation period. Wind waves are the high frequency portion of a bimodal wave frequency spectrum.
Cheers, Roy.
-----Original Message-----
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jonathan Gregory
Sent: 03 May 2016 15:49
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Waves
Dear Elodie
Thanks for your postings. I hope you don't mind my making some more comments.
> - *sea_surface_wave_significant_height*
> The already existing definition is "Height is the vertical distance
> above the surface." We believe that this definition defines the
> "sea_surface_wave_height" part, but not the "significant" part. Could
> it be possible to revise it to make it more precise? We think that a
> more precise definition could be :"Height is the vertical distance
> above the surface. The significant height is defined as the average
> height of the highest third of observed sea surface waves over a 30 minutes period."
"height" by itself in CF names means above the land surface or the sea surface. I don't think that's what you mean in "wave height", really. What is the level with respect to which the wave height is measured?
I agree that "significant wave height" can't be described by existing cell_methods. We could introduce a new cell_method for this purpose i.e. the mean of the largest third. This would be an alternative to naming it as a different quantity from "wave height". However, mean wave height and maximum wave height could be described by existing cell_methods.
I wonder why the definitions all refer to a 30-minute period. If the data variable has a time dimension, it could indicate any time interval to which the statistical operations apply.
> - *sea_surface_wave_height_of_highest_tenth*
> Height is the vertical distance above the surface. The height of the
> highest tenth is defined as the average height of the highest tenth of
> observed sea surface waves over a 30 minutes period.
This is a similar sort of definition to "significant height", but with a different percentile (1/10 instead of 1/3). Again, we could define a cell_method for this. If it's named specifically, why is it not mean_wave_height_of_highest_tenth?
> - *sea_surface_wave_height_of_the_highest_crest*
> The crest is the vertical distance above the mean sea surface to the
> maximum in a wave. Height of the highest crest is the maximum value of
> wave crests observed over a 30 minutes period.
>
> - *sea_surface_wave_deepest_trough*
> Trough is the vertical distance below the mean sea surface to the
> minimum in a wave. The deepest trough is the maximum value of wave
> troughs observed over a 30 minutes period.
These two names look like they should be opposites, but they are not named in a corresponding way.
What's the difference between maximum wave height and height of the highest crest?
Best wishes
Jonathan
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Received on Wed May 04 2016 - 04:54:15 BST
