long wait betwen sets
By the forecast period of the swell. Longer period means it's traveled a longer distance which means the individual waves have had a chance to align themselves into swell trains, or as we know it sets.
Not quite that simply Don.
Sure, longer period swells are generally made further away, and hence have time to draw out in period, but we can also get strong period swells that are very consistent because they are made but a very powerful system close to our coast.
The "Expect long waits between sets" call is usually made when swells are made a good 1,500 - 2,000km's away or so. And the further past this point the less consistent the swell will be.
Yep, good point Craig.
Thanks, that's useful to know about the distance to generation.
I'm familiar with the transfer of energy to longer periods over distance, but what you're describing seems to be a modulated signal. Where the frequency of the modulation decreases over distance. The other interesting thing it is very peaky- long flat periods with a short time of large waves. This is very different to say two string vibrating at similar frequencies where the beat smoothly fades in and out. Does this tend to be associated with a narrow spectral peak? And is it more noticeable when the swell first arrives or as its dying away?
Haven't really looked at at in that sort of detail before Mods.
What I do know and probably something you've observed first hand yourself is that with a new building swell, the sets are more consistent than when the swell is dropping or fading away.
This is a result of the spectral energy being positioned more towards the start of the swell event rather than the tail, but this all depends on how the system evolved and moved as it produced the swell.
Say for example we have a strong cold front moving through the Southern Ocean up towards Victoria generating gale-force winds. If this system travelled at a similar speed to the swell it's creating and then just before nearing the Victorian coast it vanished (hypothetically). We would then expect to see a large proportion of this swell contained in the higher periods, with a strong swell front, and a rapid increase in size across the coast ahead of a similarly rapid decrease in size after the peak of the swell.
If however the system was moving slightly slower than the swell it's creating we would see a slower increase in size across the coast (to a smaller peak size), and also a slower decline. This is due to the system acting on a longer stretch of open ocean. Also the periods with this smaller swell would be lower due to the strong winds not being maintained over the same stretch of ocean as in the first case. I.e the energy would be distributed out over a wider range than the example above.
Don't know if this helps answer any of your questions at all?
Kind of, but I still need to think about it a bit more. Thanks for taking the time to consider it and respond.
Cheers
Hi,
Just wondering how you get to the conclusion that there will be a 'long wait between sets' in the forecast.