Forecast question

zeta's picture
zeta started the topic in Thursday, 5 Mar 2015 at 1:07pm

Hi Guys,

I have a question about the wave heights displayed in the forecast. If it says 5m swell at 13s, are we talking significant wave height? If so, is it calculated the same way as in WaveWatchIII with H_s = 4 sqrt(E)?
I am trying to gauge if a certain swell is behaves as predicted compared with realtime buoy measurements.

Also, if it says lets say 4ft, does that mean 4ft on the sets or on average?

Thanks alot,
Cheers

udo's picture
udo's picture
udo Thursday, 5 Mar 2015 at 6:20pm

.

wellymon's picture
wellymon's picture
wellymon Thursday, 5 Mar 2015 at 7:02pm

Means nothing zeta , zilch absolutely zilch, always changes every day, day to day.
The only real wave heights you're gonna get are the one you experience in the water surfing;)

caml's picture
caml's picture
caml Friday, 6 Mar 2015 at 12:35am

If forcast says 5m @ 13 & your nearest swell buoy ( real time ) is the same then alls perfect ? Right

thermalben's picture
thermalben's picture
thermalben Friday, 6 Mar 2015 at 5:59am

Not necessarily Caml. Depends on what data you're looking at (both forecast and observation). Which location are you referring to? The Margs forecast and the Naturaliste buoy?

caml's picture
caml's picture
caml Friday, 6 Mar 2015 at 9:24am

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caml's picture
caml's picture
caml Friday, 6 Mar 2015 at 9:22am

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zeta's picture
zeta's picture
zeta Friday, 6 Mar 2015 at 10:10am

I am looking at http://www.bom.gov.au/products/IDT65014.shtml

I would like to understand the forecast better. I mean its a good tool, it follows a certain set of equations and understanding what is actually shown is well worth it.

Sheepdog's picture
Sheepdog's picture
Sheepdog Friday, 6 Mar 2015 at 11:58am

Gday zeta...... What exactly don't you get?

zeta's picture
zeta's picture
zeta Friday, 6 Mar 2015 at 1:02pm

As stated in my first post. The wave heights predicted by the forecast, are they effective wave heights? Or what wave heights are we talking?

thermalben's picture
thermalben's picture
thermalben Friday, 6 Mar 2015 at 1:23pm

Zeta, the model forecasts are for deepwater swell trains.

However the live buoy observations - whilst also for deepwater swells - are for Significant wave heights and Maximum wave heights (which we don't plot). Instead, we plot individual swell trains (which is more accurate for surf forecasting).

Nevertheless, there is a reaosnably good correlation between the Cape Sorell buoy and our Cape Sorell forecast: https://www.swellnet.com/reports/australia/tasmania/cape-sorell/forecast

As you can see from the forecast below, the primary swell (which is the only swell train modeled to be in the water) is expected to be somewhere between 7.0m (6am) and 5.5m (6pm). 

This roughly equates with today's Significant wave height observations from Cape Sorell (blue line).

 

The red line is maximum wave height, which we don't calculate in our forecasts because it's not as useful for surf forecasting.

Where things come unstuck (re: buoy data vs model forecasts) is in the period measurements.

Most buoy software programs don't calculate individual swell periods by default.. they just display Average periods (also known as Zero Crossing), or Significant periods (the average period of the highest 1/3 of all waves), plus Maximum Period. These are different to the swell periods of individual swell trains.

Sometimes they'll match up (ie leading edges with Tmax, and strong solitary swell sources) but as soon as there's a number of swells in the water - for example, when there's a lot of windswell - the Significant period values can become 'contaminated'.

Not sure if that explains things a little better for you?

Sheepdog's picture
Sheepdog's picture
Sheepdog Friday, 6 Mar 2015 at 1:26pm

Cheers, Ben.... There you go, zeta...... I don't use wavewatch3.... I just go by the charts like the one you supplied in your 10.10am post.... I'm just an old school basic sort of forecaster....
You do know how to interpret those charts? as in Hmax etc? That's about all I could help you with....
Oh and using ascat and calculating swell size and arrival.....

wellymon's picture
wellymon's picture
wellymon Friday, 6 Mar 2015 at 2:50pm

Settle

zeta's picture
zeta's picture
zeta Friday, 6 Mar 2015 at 3:56pm

Thanks ben that is very helpful.

Sheepdog wrote:

You do know how to interpret those charts? as in Hmax etc? That's about all I could help you with....
Oh and using ascat and calculating swell size and arrival.....

I am familiar with it from a research point of view as I am working a bit with ocean storms.
That's why I was interested how the wave height is defined in this swell forecast.
I am living only 2 years next to the ocean now so I do not have enough experience to gauge swells like you old dogs.

I have not heard of ascat but I will have a look at that.

Thanks again for all the help.
Cheers