periods in OEWC / sectors vs. wind types

[Sinisa Knezevic]

Dear,

I have a few questions regarding the calculation of OEWC:

1 What is the principle in determining the number and length of periods?

2 How is the all-sector value determined? Why is it not the same as the maximum of sector 50yr extreme winds? In the help file the all-sector value is even larger then all sector values - that seems strange to me.

3 Would it be more accurate to calculate expected 50yr extreme wind based on wind types, rather then based on direction sectors? I assume that the division in sectors is the approximation of wind type analysis.

Thank you,

Sinisa

[Duncan]

Generally, each period should be a year. We don't recommend anything else, as far as I know.

The all-sector U50max is calculated from the sectorwise values, and should usually be greater than any of the sectors (not the maximum value from all sectors). I see that the help file does not explain why this should be. I will try to find a reference if you want.

I don't quite understand your third point. What do you mean here by wind 'type'?

Duncan

[Sinisa Knezevic]

Hi,

Thank you very much for the answer. If you manage to find any reference for the calculation of all-sector U50, that would be great.

As for the third question, the wind type refers to types of winds we have in Croatia, for example Bora wind (strongest winds are almost always Bora) blows usually in 0° to 45° and depending on the location the range of direction may vary a bit. The other most frequent wind (Jugo) is in 140°- 190° range (cca) and so on. So the question was in the sense if it is "more correct" to divide data bins by wind types rather then strict sectors, because as I understand, the Gumbel method refers to "extreme events" and types of extreme wind events.

Anyway, I've tested the Climate Analyst in a sense that I've changed the number of sectors, but I've got pretty much the same all-sector result.

Thank you,
Siniša

[Morten]

Hi Siniša,

You ask whether it is most correct to separate the extreme wind analysis of a mixed extreme wind climate. Normally this is unnecessary but there are exceptions, see e.g. the paper by L. Gomes, B.J. Vickery (1978) “Extreme wind speeds in mixed wind climates” in Journal of Wind Engineering and Industrial Aerodynamics, Volume 2, Issue 4, January 1978, Pages 331-344. These authors investigated extreme winds in cities on the East coast of Australia. The extreme-wind climate to the North was dominated by tropical cyclones and the extreme winds to the South was dominated by mid-latitude storms. Tropical storms are extremely violent but very rare, which means that their Gumbel fit tend to be steeper than for mid-latitude storms. The key question is which Gumbel distribution dominates at the fifty years recurrence time? Gomes and Vickery were able to separate the two phenomena by looking at synoptic pressure data maps. After individual Gumbel analysis of both phenomena, it seemed that half way up the coast, at Brisbane, both kinds of storms influenced the fifty-year wind estimate.

Put in another way: If you are measuring at a coastal topical site for say four years you might never observe a tropical cyclone/typhoon/hurricane, because the extreme wind field is too small to frequently pass the site. But it is very dangerous to ignore the phenomena for the U50 estimate.

I am not too familiar with your Bora and Jugo winds. What you might try is to separate the phenomena and do independent Gumbel analyses to test whether the Gumbel slope parameters are significantly different. I am not sure how you can use Climate Analyst for this. Just dividing into sectors is probably not enough. You need to evaluate the meteorological situations. I mean the Bora is a kind of Katabitic wind (right?) so you should test whether there actually is a baroclinic contribution to each suspected Bora wind.

Cheers,
Morten