Namelist in LES simulations of flow over topography

Anything related to the idealized simulations in WRF.

Namelist in LES simulations of flow over topography

Postby Clément » Wed Dec 13, 2017 10:02 am

Hello everyone,
I am quite new in the WRF business, and I am working on flow over topography.
I would like to run an idealized LES simulation, but I have some questions about the namelist file.
In fact, I found several different version to set up the namelist file.
On this web-page :
http://www2.mmm.ucar.edu/wrf/users/docs/user_guide_V3/users_guide_chap5.htm
in the "Planetary Boundary layer (bl_pbl_physics)" part, they suggest for a large-eddy-simulation (LES) boundary layer :
Code: Select all
bl_pbl_physic = 0,
isfflx = 1,
sf_sfclay_physics and sf_surface_physics "are selected" ( = 1 ??)
diff_opt = 2,
km_opt = 2 or 3,
isfflx = 0 or 2


what is a little bit different from the namelist in the test case, em_les, which is

Code: Select all
bl_pbl_physic = 0,
isfflx = 1,
sf_sfclay_physics =1
sf_surface_physics = 0
diff_opt = 2,
km_opt = 2
isfflx =  2


So, is there some physics differences between these two configurations, which one could be considered as a "real" LES simulations ?
I hope my questions are clear enough, don't hesitate to ask me more about the problem.

Thank you very much,

Clement
Clément
 
Posts: 4
Joined: Wed Dec 13, 2017 9:27 am

Re: Namelist in LES simulations of flow over topography

Postby moulin » Tue Jan 02, 2018 3:06 pm

Hi,

To my understanding, the second setup is for a idealized test case of free convective boundary layer driven by a constant surface heat flux. That's why the
Code: Select all
isfflx = 2
(I check the namelist.input in em_les, it is not equal to 1 as you wrote in your post), which use drag from sf_sfclay_physics and heat flux from tke_heat_flux with
Code: Select all
bl_pbl_physics=0.
So there is no need for extra land-surface model for providing surface flux as it has been specified explicitly. That's why sf_surface_physics is set to 0 there

But I think if you intend to run a "real" LES, with specific land-use information and topography, you should use the first setup to allow the corresponding land-surface forcing kick in.

Hope it could help you.

Mou Lin
Wind Engineering and Renewable Energy Laboratory
École polytechnique fédérale de Lausanne
moulin
 
Posts: 3
Joined: Wed Mar 01, 2017 8:44 am

Re: Namelist in LES simulations of flow over topography

Postby Clément » Wed Jan 31, 2018 8:31 am

Hello,
Thank you for your answer.
Sorry, I wrote 2 times isfflx in the second set up but I meant "isfflx = 2" this time.
In fact I tried the two possibilities, providing or not fluxes with sf_sfclay_physics and heat flux from tke_heat_flux, but it was not so concluding.

Thank you,

Clément
Clément
 
Posts: 4
Joined: Wed Dec 13, 2017 9:27 am


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