Hello Elodie,
This is a large proposal that I think raises a number of issues to be discussed. Trying to tackle everything at once could easily tie everybody in knots resulting in a stalled proposal. So, I will try and focus on one area at a time so that we actually get somewhere!
Let's start with a brief introduction and raising one of the topics. I'm a technical consultant at BODC working on oceanographic data management for 35 years. My knowledge of wave data was given to me by Laurie Draper when I privileged to know him as a work colleague in the 1980s.
My first comment concerns your proposals for significant wave height. My understanding from Laurie is that this was the best approximation of wave height as estimated by a visual observer that could be obtained form analysis of wave measurements. Using his classical methodology, which I know as Tucker-Draper analysis, but I think is what you call zero-upcrossing analysis, this was the height of the highest one-third of the waves. Later this was approximated as Hrms*4 to simplify digital data processing (some call this characteristic wave height) and later still it was determined as you describe in your definition for sea_surface_wave_spectral_significant_height from spectral wave data.
This raises a point as far as CF is concerned. First, is that Laurie's 'Significant Wave Height' is what CF would call a geophysical variable or O&M would call on observed phenomenon. My understanding of CF is that the Standard Name refers to the geophysical variable, no matter how it is derived. I would therefore propose that your 1.a, 1.b and 1.c should use the existing Standard Name (maybe we could improve the definition) sea_surface_wave_significant_height. You could the use the long_name field to differentiate Hs on the basis of derivation methodology.
Jonathan, do you agree I've correctly interpreted the CF principles here?
Cheers, Roy.
Please note that I partially retired on 01/11/2015. I am now only working 7.5 hours a week and can only guarantee e-mail response on Wednesdays, my day in the office. All vocabulary queries should be sent to enquiries at bodc.ac.uk. Please also use this e-mail if your requirement is urgent.
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Elodie Fernandez <elodie.fernandez at mercator-ocean.fr>
Sent: 28 April 2016 16:18
To: cf-metadata at cgd.ucar.edu
Cc: mar at puertos.es
Subject: [CF-metadata] Waves
Hi all,
So here are our proposals for wave variables that will be available through the European Copernicus Marine service. It's split in two "categories": names for the whole spectrum and names for the partitions. I added in copy for this topic Marta de Alfonso Alonso-Munoyerro who is an expert in waves from Puertos del Estado.
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Whole partition
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1. Height
Significant height can be measured using different methods, so we think that the already existing definition should be more precise and that new names should be added.
1.a "sea_surface_significant_height" (modification of definition)
unit m
Height is the vertical distance above the surface. Significant wave height can be estimated by zero upcrossing analysis and by spectral analysis. The generic significant wave height is used when the estimator is unknown.
1.b "sea_surface_wave_spectral_significant_height"
unit m
Height is the vertical distance above the surface. It can be defined from spectral analysis. The wave directional spectrum can be written as a five dimensional function S(t,x,y,f,theta) where t is time, x and y are horizontal coordinates (such as longitude and latitude), f is frequency and theta is direction. S has the standard name sea_surface_wave_directional_variance_spectral_density. S can be integrated over direction to give S1= integral(S dtheta) and this quantity has the standard name sea_surface_wave_variance_spectral_density. Frequency moments, M(n) of S1 can then be calculated as follows: M(n) = integral(S1 f^n df), where f^n is f to the power of n. Spectral significant wave height is defined as 4* sqrt (M(0)) = 4 * sqrt ( integral(S1 df) )
1.c "sea_surface_wave_zero_upcrossing_significant_height"
unit m
Height is the vertical distance above the surface. The significant wave height is defined from zero upcrossing analysis as the average height of the highest one third waves.
1.d. "sea_surface_wave_zero_upcrossing_average_height_of_highest_tenth"
unit m
Height is the vertical distance above the surface. The average height of highest tenth waves is defined from zero upcrossing analysis as the average height of the highest one tenth waves.
1.e "sea_surface_wave_zero_upcrossing_average_height"
unit m
Height is the vertical distance above the surface. The average height is defined from zero upcrossing analysis as the average of wave heights.
1.f "sea_surface_wave_maximum_height"
unit m
Height is the vertical distance above the surface. Estimated maximum wave height is not measured but estimated from others parameters like significant wave height.
1.h "sea_surface_wave_zero_upcrossing_maximum_height"
unit m
Height is the vertical distance above the surface. Maximum zero crossing wave height is the measured maximum height of the waves separated by zero upcrossing analysis.
1.i "sea_surface_wave_crest_through_maximum_height"
unit m
Height is the vertical distance above the surface. Maximum crest trough wave height is the measured maximum height of the waves separated by crests method.
1.j "sea_surface_wave_deepest_through"
unit m
Trough is the vertical distance below 0 to the minimum in a wave. Depth of the deepest trough is the maximum value of wave troughs.
1.k "sea_surface_wave_height_of_the_highest_crest"
unit m
Crest is the vertical distance above 0 to the maximum in a wave. Height of the highest crest is the maximum value of wave crests.
2. Energy
2.a "sea_wave_spectrum_peak_energy"
unit mms (meter*meter*second)
The wave directional spectrum can be written as a five dimensional function S(t,x,y,f,theta) where t is time, x and y are horizontal coordinates (such as longitude and latitude), f is frequency and theta is direction. S has the standard name sea_surface_wave_directional_variance_spectral_density. S can be integrated over direction to give S1= integral(S dtheta) and this quantity has the standard name sea_surface_wave_variance_spectral_density. Wave spectrum peak energy is the maximum value of the variance spectral density (max(S1)).
3. Period
3.a "sea_surface_wave_mean_period"
unit s
A period is an interval of time, or the time-period of an oscillation. Mean or averaged wave period is the average value of the wave periods and can be estimated by zero upcrossing analysis and by spectral analysis. The generic average wave period is used when the estimator is unknown.
3.b "sea_surface_wave_zero_upcrossing_significant_wave_period"
unit s
A period is an interval of time, or the time-period of an oscillation. The significant wave period is defined from zero upcrossing analysis as the average period of the highest one third waves.
3.c "sea_surface_wave_zero_upcrossing_average_one_tenth_wave_period"
unit s
A period is an interval of time, or the time-period of an oscillation. The average period highest one tenth waves is defined from zero upcrossing analysis as the average period of the highest one tenth waves.
3.d "sea_surface_wave_maximum_period"
unit s
A period is an interval of time, or the time-period of an oscillation. The maximum wave period is defined from zero upcrossing analysis as the maximum period of the waves.
3.e "sea_surface_wave_period_of_highest_wave"
unit s
A period is an interval of time, or the time-period of an oscillation. The period of the highest wave is defined from zero upcrossing analysis as the period of the highest wave.
4. Direction
4.a "sea_surface_wave_from_mean_direction"
unit degree
from_direction is used in the construction X_from_direction and indicates the direction from which the velocity vector of X is coming. The mean wave direction is the average direction from which waves are coming.
4.b "sea_surface_wave_from_direction_at_spectral_peak"
unit degree
from_direction" is used in the construction X_from_direction and indicates the direction from which the velocity vector of X is coming. The spectral peak is the most energetic wave in the total wave spectrum. The wave direction at spectral peak is the direction from which waves are coming at the spectral peak.
5. Steepness
5.a "sea_surface_wave_maximum_steepness"
unit dimensionless
The wave steepness is defined as the ratio of the wave height divided by the wavelength. The maximum wave steepness is the maximum value.
-------------------------------------
Partitions
-------------------------------------
We also wish to add names for the three "main" variables defining waves for three partitions: the wind wave, the primary swell wave and the secondary swell wave. These three variables are
- the spectral significant height (1.b of our proposal sea_surface_wave_spectral_significant_height)
- the direction (4.b of our proposal sea_surface_wave_from_direction_at_spectral_peak)
- the period (already existing name sea_surface_wave_mean_period_from_variance_spectral_density_first_frequency_moment)
So we propose for all three to replace in the name "sea_surface_wave" with "sea_surface_wind_wave", "sea_surface_primary_swell_wave" and "sea_surface_secondary_swell_wave". And we believe the definitions should be the definitions already existing or proposed on this email, with the addition at the end of the definition of the partition itself: "The directional wave spectrum can be separated into several partitions: wind wave contribution (WW), primary swell (SW1) contribution (the most energetic swell) and secondary swell contribution (SW2).", with a bit more detail for the wind wave: "Wind waves are waves on the ocean surface generated by the local wind."
For example:
sea_surface_secondary_swell_wave_from_direction
unit degree
The directional wave spectrum can be separated into several partitions: wind wave contribution (WW), primary swell (SW1) contribution (the most energetic swell) and secondary swell contribution (SW2). The mean wave direction is the average direction from which waves are coming for the secondary swell wave partition.
Regards,
Elodie Fernandez
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