open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi Developers,

Just wondering whether it'd be possible to implement a Charnock wind drag formulation for T2D and T3D whereby the user could define a constant Charnock parameter in the CAS, or alternatively provide spatially and temporarily varying maps inside the Atmospheric Binary File under a new variable name.

I'm not sure whether TOMAWAC provides Charnock as an output in the case of coupling.

Many Thanks,
Toby

Have implemented some modifications myself and will report back upon testing.

Regards,

Hello Toby,

FYI, a former intern from colleagues of mine implemented Charnock formulation for a study in TELEMAC-2D. You can read what she did here:
henry.baw.de/items/d4f6dbfc-0b98-4c1b-9076-d62d5fdb09f6

It was just a try and not included in the official release but it may help you.

Chi-Tuan

Hi Chi-Tuan,

Thanks very much. Actually, that paper and another co-authored by Alexander were the inspiration for investigating this.

I've currently implemented the drag formulation and am looking at the sensitivity to wind setup using:

- ERA5 Charnock parameter fields
- Constant Charnock parameters; and
- The default formulation (Flather I believe).

Regards,
Toby

Hi Toby,

Indeed we have implemented a Charnock wind drag, and use it quite regularly. According to my colleague, it work somewhat better during storms than the Flather relation implemented in TELEMAC.
We tried spatially varying charnock coefficients, but did not see much better results, by the way, so for the moment, my advice would be to just implement a constant value in the .cas file.

Maybe we should indeed consider integrated Charnock in the official version if Chi-Tuan agrees.

By the way, TOMAWAC at the moment does not yet have Charnock as an output value, but I don't think it would be very complicated to do so.

Kind regards,

Alexander

Hello Alexander and Toby,

Do not hesitate to suggest Merge Requests, it could then be looked.

Chi-Tuan

OK. Need to do some code cleaning first.

I keep you updated.

That'd be great.

I have the modifications available allowing Charnock to be defined the CAS file (VALUE OF THE CHARNOCK PARAMETER) or alternatively via atmospheric binary file. However, there is no implementation for choosing Charnock over Flather. Another dictionary variable would be necessary to select the equation.

On another note, the implementations of Charnock I've seen reported previously in TELEMAC use a simplified formula to avoid implicit calculation of the wind drag. The paper below provides an alternative formulation which is what I've implemented and avoids this issue. The resulting drag is indeed different to considering Ustar = U10/25 so perhaps it needs some discussion.

www.ecmwf.int/sites/default/files/elibra...utral-wind-speed.pdf

Alexander - I'll try to reach out to you to discuss further.

Cheers,
Toby

Hi Toby,
We used Newton's method to solve the implicit relations. It would be interesting to compare.
Let's continue through private private email and try to have a branch in the first part of next year.

Kind regards,

Alexander