Re: [getm-users: 1566] GOTM: Charnock constant

[Hans Burchard]

Bert,

this value is used by Burchard (2001), and I took it from Craig & Banner
(1994), not being aware of Wu (1982) (references below).

In my paper, I discuss however on page 3140 that the resulting z0 seems to
be quite low compared to estimates in the literature.

So, if nobody disagrees, we will change the default value to 18500.

Thanks a lot for pointing this out.

Regards, Hans.

References:

Burchard, H., 2001, Simulating the wave-enhanced layer under breaking
surface waves with two-equation turbulence models, J. Phys. Oceanogr., 31,
3133-3145.

Craig, P. D., and M. L. Banner, 1994: Modelling wave-enhanced turbulence
in the ocean surface layer. J. Phys. Oceanogr., 24, 2546�2559.

Wu, J., 1982: Wind-Stress Coefficients Over Sea Surface From Breeze to
Hurricane, J. Geophys. Res., 87(C12), 9704�9706,
doi:10.1029/JC087iC12p09704.

> Can anyone tell me why the default Charnock constant in GOTM 1400
> instead of, say, 0.0185*rho**2, which would be about 18500?
>
> 0.0185 is the value suggested by Jin Wu in a 1982 JGR paper. What am I
> missing?
>
> -Bert Rubash
>

--
Hans Burchard
Leibniz Institute for Baltic Sea Research Warnemuende
Dept. for Physical Oceanography and Instrumentation
Seestrasse 15
D-18119 Rostock-Warnemuende
Germany

Tel. : +49 - 381 - 5197 -140
Fax : +49 - 381 - 5197 -114
web : http://www.io-warnemuende.de/hans-burchard-en.html
E-Mail: hans.b...@io-warnemuende.de
skype : hans.burchard

[adolf...@jrc.ec.europa.eu]

Dear Bert and Hans,

This is actually a quite good point, because indeed the value of 1400 did not produce 

a good simulation of the near surface layer, only when using 14000 satisfactory results were

achieved, see the reference below, where this is discussed.

Therefore indeed the value should be changed to 18500, regards Adolf

"Measurement and simulation of viscous dissipation in the wave affected surface layer."

Adolf Stips, Hans Burchard, Karsten Bolding, Hartmut Prandke, Andre ́ Simone, Alfred Wu ̈ est

Deep Sea Research Part II: Topical Studies in Oceanography, 2005 vol. 52 (9-10) pp. 1133-1155,

doi:10.1016/j.dsr2.2005.01.012

[Adolf STIPS]

Hi Hans and Bert,

Adolf,

I do not unterstand from where the factor rho**2 should come.

I would stay therefore with the oceanographic literature and charnock_val=1400 

( This Charnock - function is its own research field..)

The Charnock formulas comes from the atmosphere (and only from there)

Charnock 1955:
charnok = g * z0 / U*^2
U*^2 = Cd * U10^2

In the atmosphere following values are used:
charnok = 0.012     (Charnock 1955)
charnok = 0.0144   (Garatt 1977)
charnok = 0.0185   (Wu 1980)

Fairall ea 1996:
charnok = g * (z0 - 0.11 ny/U*) / U*^2

Foreman & Emeis 2010 plot charnok as a decreasing function of wind speed
(from 0.08 - 0.0012)

In GOTM this concept is applied to the ocean part of the boundary

It must be:
 stress(atmos) == stress(ozean) 

->

rho_w *U_w*^2  = rho_a * U_a*^2

rho_w /charnok_w  * zo_w = rho_a / charnok_a *zo_a 

charnok_w = rho_w/rho_a * zo_w/zo_a  * charnok_a

# there is NO factor rho^2

only with assuming the roughness length:
zo_a ~ 0.0002
zo_w ~ 0.2  

(but that is certainly a to extreme choice) 

charnok_w   ~  1000/1.25 * 1000 * chanok_a

A possible range of charnok_w  is therefor from ca 1000 to ca 20.000.

Bourassa 2000 measured charnock in water and got ca. 850.
(ca 10^5 * charnock_atmosphere according to him)

Waves will complicate the story much more and likely lead to bigger charnok values.

conclusion:

1) A factor of rho^2 does not come from the physics according to my understanding

2) A basic choice of charnok=1400 is okay, as it is also well referenced  

3) If somebody does simulations with wave influence he has to dig the literature 

and might need to adjust the charnok_val (higher values).

So charnok_val in GOTM will not be changed (in agreement with Hans last proposal)

Ciao Adolf