waves, swell, swell2 & wind waves. Oh my good!

[idefix37]

@jose_fley
As explained in my post you have read :

The seastate that you can observe in a given place, is a mix of different type of waves:
The windsea (also called wind waves) is produced by the local wind.
The main swell is produced by a strong wind in a remote place. The swell travels great distances to where you observe it.
Secondary swells are produced also by winds in other remote places.
All of them have different heights, come from different directions and show different periods (i.e. the time between 2 wave crests)
Swell 1 is the main swell,
Swell 2 and 3 are secondary swells

Waves is the sea state (also called total sea) that you can observe in a given place = Wind Waves + Swell1 + Swell2 + Swell3
Wind waves is produced by the local wind
Swell 1 is the main swell produced by remote wind
Swell 2 is a smaller swell produced by an other remote wind
Swell 3 mostly is very small and neglected.

What is the interest of knowing these different types of waves?
For Surfers the best conditions are a strong Swell 1 with low Wind Waves and low Swell 2/3. With strong Wind Waves and strong Swell, waves are very disorganised especially if their directions are different ... « like in a washing machine »
For Sailors, Wind Waves at 90º of the Swell direction give a «cross sea » tougher than a sea with all kind of waves in the same direction
...etc...

Hope it’s clear for you now.

[jose_fley]

Is exactly that. for sailor. I have now a little boat 3.30mts.

I can not understand well which one I should observe, which one is more important or that affects me directly 200, 300 or 500 meters far from the coast

Yesterday I was observing the sea on the coast. On the map of Windy it showed that there was waves 2mts, swell (swell 1) of 2.1 meters. the waves was about 2 meters when broke on solid ground about , but 200 meters inside the sea, only 0,6/0,8mts.

The wind was exactly as showed in the Windy map. 6km / h.

[idefix37]

@jose_fley
I see.... Both Weather Models ECMWF and GFS, and their respective Waves Models, used by Windy are global models which cover the planet. Their resolution is good but quite coarse for local forecasts in some areas (mainly because of the coarse bathymetric model embedded). I don’t know where is your navigation zone but at 500m the swell is much weaker if you are in shallow waters.

As example today in France ECMWF gives this waves forecast in south of Britanny. The swell is well established and the Total Sea (Waves in Windy) is 2.8m near the coastline.

If we consider at same time the forecast of a French Waves Model with a higher resolution and probably a better mapping of the seabed, we have just more than 1m (1mt) at the same place.

For you it’s probably better to consider « Wind Waves ».
« Waves » and « Swell » are overestimated if you sail at short distance of the coast in shallow waters.

[jose_fley]

Thanks for the explanation.
just one question more:

how many is for you "short distance of the coast."10 meters? 20 meters?,50, 100??

[idefix37]

@jose_fley
I can’t say at which distance you get smaller waves. It depends on the seabed profile.

You see that high waves (green) become smaller (blue) from 20, 10m, and much more from 5m (light blue). On top of the map, the 20m sounding line are at 1.96NM (nautical miles) or 3.6km.
On the right part of the map you see that these sounding lines are very close to the seashore and high waves too (green).

Then, you must consider that the sea is tougher as the soundings decrease. Distance between wave crests is reduced, waves become sharper and start to break. So sailing close to the coastline may be dangerous.

http://weather.mailasail.com/Franks-Weather/How-Waves-And-Swell-Form

In which location do you navigate?

[jose_fley]

In the north of Tenerife (Canary Island). Exactly in Garachico city.

That picture is from Navionics, right?.
Thanks

[idefix37]

@jose_fley
Yes it’s a picture from Navionics map. Do you use them?

Along which part of the sea shore do you sail? We could have a look to the sea bottom.

[jose_fley]

Navionics and Windy are my favorite apps.
This week the sea weather is very very bad here
My sail zone is around the green fish that you can see in the map,
the exact coordinates of the little port from where i get out are; 28.378502, -16.813212

[Gkikas LGPZ]

@jose_fley
@idefix37
I want to point out:
a) the resolution of the wave model is 0.1 degree=6 NM = 11 km,
so we must not expect different wave height at 200, 300 or 500 m far from the coast.
Also https://community.windy.com/topic/4541/is-model-resolution-important
b) Jose's boat is only 3.3 m long.

[jose_fley]

Thanks for your reply

Best regards

[矢澤旭泰]

I would like to add a question: How is the direction of "waves" (as called "all wave types combined (swell and wind waves)" in the map forecast or waves_direction-surface in the point forecast API) technically calculated? I assume it somehow combines swell and wind waves, but how are the different directions of swell and wind waves averages to get the "total wave" direction? Thanks a lot for any explanation.

[idefix37]

@矢澤旭泰
Wave model is based on spectrum analysis of waves and direction of total sea is deducted from it. See page 10.

https://confluence.ecmwf.int/download/attachments/59774192/wave_parameters.pdf

[moritz.huetten]

@idefix37 Thanks a lot for the response. How is it done for GFSwave (point forecast API) and ICON?

[idefix37]

@moritz-huetten
These wave models are also based on spectral analysis.
GFS wave.
https://gfs.sourceforge.net/wiki/index.php/GfsWave
ICON
The WAM model developed by DWD was the original model used and improved by ECMWF.
So it should be very similar:
https://www.dwd.de/DE/leistungen/opendata/help/modelle/legend_ICON_wave_EN_pdf.pdf?__blob=publicationFile&v=3

[moritz.huetten]

@idefix37 Thanks a lot for your response. By the way, how do you extrapolate the wave forecasts beyond the original forecast domain (e.g. GFS Wave), to obtain values close to the coast or even on land?

[idefix37]

@moritz-huetten
Do you refer to a precise location? A screenshot would be useful.