Best posts made by jrbidlot

See the last part of my answer in
https://community.windytv.com/topic/3603/different-definitions-for-the-wave-periods-are-shown

Hi, the issue with the wave period being different between ECMWF and NOAA was relayed to me on Friday (3/3/2017).

I am Jean Bidlot, the main person in charge of wave modelling at ECMWF

I went on the windty website and got the following screen shots
for ECMWF:
for WaveWatch3:

I could not figure out how to plot the wave period as a map but the white text in the middle of the south Atlantic indicates
that ECMWF has a wave period of 9 s and WW3 21 s at the selected point !!!

why is it so different?
It's simple: they are showing two different wave periods.
for ECMWF, they appear to be using the mean wave period of the total sea (more likely MWP in the request below)
but for WW3, they are clearly not! A mean wave period of 23 s is very very unlikely!

We do produce however the peak period and if you compare

for our mean period
with

for our peak period

you clearly see the difference over the middle of the south Atlantic.

So why such a difference,
mean periods are derived from weight integrals of the wave spectrum (the distribution of wave energy in frequency and direction)
whereas the peak period is the reciprocal of the frequency that correspond to the peak of the spectrum
Over the south atlantic what you see is that the long waves Tim was talking about in the weather discussion, that has affected the Azores and Morocco, have continued their propagation southwards
At that location, they are the dominant wave system, but obviously they are other wave systems and so in the mean the wave period if lower but the peak period is high.
More information on the different wave parameters derived at ECMWF can be found in chapter 10 of
http://www.ecmwf.int/sites/default/files/elibrary/2016/17120-part-vii-ecmwf-wave-model.pdf

In order to be compared the same definition for the period should be use for both systems.

for surfers, I can see that the peak wave period is more related to the waves that would interesting, but in opinion it is not the ideal quantity to use.
On Monday, I will access the archived data for the 18-19 January to show why.