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Hi

What are the units of the BOUNDARY DIRECTIONAL SPREAD 1 keyword? Is it the one-sided width of the spectrum in degrees?

Cheers

Fay

Hello,

the BOUDARY DIRECTIONAL SPREAD 1 parameter is NOT the one-sided width of the spectrum in degrees.
It corresponds to the width parameter s, which is related to the one-sided directional width theta expressed in radians, via the following expression:

theta = sqrt( 2/(s+1) )

Kind regards,

Giovanni

Thanks

I'm sorry. I'm still confused.

I use data from ANEMOC (wave hindcast with TOMAWAC). I have "un étalement directionnel moyen" given in degrees. I'd say it corresponds to SPD, the mean directionnal spreading which, according to APPENDIX 7 is equal to :
sqrt(2(1-sqrt(a^2+b^2)))*180/pi

So what you said is that the mean directionnal spreading is different from the Boundary directional spread (SPRE1L) with
SPD(in radian)=sqrt(2/(SPRE1L+1)). Is that correct ?
Is SPRE1L is an angle ?

Thank you very much for your help.
Pauline

I answer to myself if others wonder the same.

The Boundary directional spread (or Initial) correspond to "s" page 63 of the manual. The relationship between "s" and the mean directional spreading SPD, expressed in degrees, depends on the Angular fonction distribution used and is not easy to find.
Graphically, I reproduced the angular function distribution function of the theta for different value of s. The SPD value seems to correspond to 95% of the distribution or something like this but I'm still confused here. It can be coincidence...

Nevertheless, in my case, I have chosen Angular Fonction Distribution 1 (in cos^2s) and I tried different values of "s" until I found the good SPD. It corresponds to s=1.6 to SPD=21°. It is time-consuming (effective but not efficient !!) but at least, it works...

Hi,

if you choose the angular function distribution 3 (Mitsuyasu type), the relation between the directional width SPD (mean directional spreading in TOMAWAC) in degrees, and the width parameter s (boundary or initial directional spread in TOMAWAC) is:
SPD=(180/pi)*sqrt(2/(s+1))

Kind regards,

Giovanni