Significant Wave Height (Seas)

My understanding is that in swell forecast we get the significant wave height. That is, the average of the highest one-third (33%) of waves (measured from trough to crest) that occur in a given period.
For example, if our buoy measurements are 3,3,3,4,4,4,5,6,7, then the top one third is 5,6,7, and the average of that is 6.

Now, I wonder, can I get the full data? And if so, would it be useful?
For example, if we get a measurement of 6,6,6,6,6,6,6,6,6, then the most significant wave height is also 6. But this sequence is totally different from the one above.

Would love to hear what number folks are looking at 

Not as detailed as what you are looking for, but this buoy in Santa Cruz gives average and peak:

http://cdip.ucsd.edu/themes/cdip/?d2=p70:s:254

I'm not sure if this is true of other buoys...

Thanks for the reference. So it looks like the deviation is not significant and the most significant wave is just a little more than others - maybe by a foot or two

I emailed with NOAA on this topic a while back, I'll post the exchange and links below.  TLDR is that they don't summarize the data on sub dominant swells for you, but the raw data is available.

Surfline and other commercial sites do the work of doing a deeper analysis and publishing the full spectrum of swells, but you have to pay for that.

.    To Whom It May Concern -

I'm writing to ask if there is a way to get raw data on sub dominant swells form the buoy web pages.  For example, I live on the Monterey Bay, and when there is a long period but low height SW swell in the water (i.e. 2 feet at 17 seconds), along with a short period but taller swell from the NW (i.e. 6 feet at 9 seconds), the buoy data that I can ind on the buoy web pages does not show the sub dominant SW swell at all.  Is there a place I can look to get info on these sub dominant wells?  This has been bugging me for years, so I would greatly appreciate any help you can give.  Thanks very much.

PS, a few years back I had the opportunity to work as a technician for MBARI on the Kaiimmoana tending the TAO array, so I have a bit of insight into the incredible effort that goes into maintaining the fantastic system of buoys you guys operate.  Thanks, and keep up the good work.

Sincerely,

Jon


Jon,

Good to hear from you, and glad you are using our data.

Unfortunately we do not calculate sub dominate swells on our web page, but we do have the spectral data located here:

http://www.ndbc.noaa.gov/data/realtime2/46042.data_spec

This is the energy measured by the buoy into frequency bands. From that you could write a script to calculate and/or plot the frequency and compute height. Information on this can be found at:

http://www.ndbc.noaa.gov/wavecalc.shtml

What you would need to do is determine the wind swell separation frequency, and then use the energies in the swell portion you are interested in. For example, in the image below the dominate swell is at 7 seconds (.14hz)  with a sub dominate swell at 16 seconds (.063hz). The separation frequency is .1hz. So I would sum up the energies from .05-.1hz as sub-dominate and .1-.2hz for dominate and calculate wave height from the formula on the web page. Plus, I know my frequencies so I can look up the direction here:

http://www.ndbc.noaa.gov/data/realtime2/46042.swdir

and find the sub dominate direction is 196 degrees and dominate swell is 304 degrees.


I hope this helps you,

Walt McCall
Physical Scientist
     

Thanks Jon.
Very interesting.
I wish they went the extra mile and provided the calculations on the data.