boudary conditions for real case LES || WRF-LES specialists help please.

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Peter_Wilson
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Joined: Sat Mar 20, 2021 3:52 pm

boudary conditions for real case LES || WRF-LES specialists help please.

Post by Peter_Wilson » Sat Apr 03, 2021 7:43 pm

Hi everyone, I want to run WRF LES(Large eddy simulation) in a real case with real lateral boundary conditions.
Which weather model is best for simulating with a 100–150 m grid spacing?
I tried GFS(28km) and ECMWF(9km) but they do not resolve turbulent scales at this grid spacing, obviously they're too coarse.
The best I've found is HRRR with 3km grid, is it worth trying? Or is there something better for real case LES?
Last edited by Peter_Wilson on Wed Apr 07, 2021 1:29 pm, edited 1 time in total.

kwerner
Posts: 2251
Joined: Wed Feb 14, 2018 9:21 pm

Re: lateral boudary conditions for real case LES

Post by kwerner » Mon Apr 05, 2021 9:10 pm

Hi,
When you're running this with coarser input (e.g., GFS/ECMWF) are you using nesting so that you have coarser outer domain(s) around your 100-150m domain of interest?
NCAR/MMM

Peter_Wilson
Posts: 7
Joined: Sat Mar 20, 2021 3:52 pm

Re: lateral boudary conditions for real case LES

Post by Peter_Wilson » Mon Apr 05, 2021 11:45 pm

Yep, i did a one-way nested run with d01 3km grid and d02 166m (1:18 grid ratio) using HRRR boundary conditions for a coarsest domain. And this is complete nonsense, the model does not resolve any large eddies, there is no eddy resolving deep convection. Also the physical structure of cloud,rain,snow,ice fields remains the same as if it was a 3km grid run. Increasing the horizontal resolution only made them smoother, otherwise everything is the same.
This cannot be compared with idealized WRF-LES, that is where everything is present and all large eddies (depending on the resolution) are resolved. And here in real case is complete nonsense, although everything is correct in the input file.

kwerner
Posts: 2251
Joined: Wed Feb 14, 2018 9:21 pm

Re: lateral boudary conditions for real case LES

Post by kwerner » Tue Apr 06, 2021 11:04 pm

The problem could very well be the 1:18 grid ratio. We recommend using a ratio of either 3:1 or 5:1 (typically odd ratios are best, but not mandatory), but no higher than 5:1. It may be necessary to add several nests in between to get down to the resolution you're interested in. For that to work, though, you may have to use ndown since the number of processors you'll be able to use for high-res domains (and likely larger domain sizes) will be limited by the number you can use for the coarser (smaller number of grid points) domains. Here are a few informational pages that may be useful for you:

Best practices for namelist.wps settings and namelist.input settings

Presentation about WRF Nesting

Explanation about allowable/reasonable number of processors, based on the size of your domain(s).
NCAR/MMM

Peter_Wilson
Posts: 7
Joined: Sat Mar 20, 2021 3:52 pm

Re: lateral boudary conditions for real case LES

Post by Peter_Wilson » Wed Apr 07, 2021 1:28 pm

Not sure if the problem is with the nesting itself. Nesting cannot improve the resolution of the initial boundary conditions of the coarsest region.
I think that the situation can be changed either by forcing the structures of large eddies, or by increasing the resolution of the boundary conditions themselves.
But I don’t know how to do a forcing of large eddy structures. And I have not seen boundary conditions with a resolution of 166 meters at all, most likely they do not exist.

In general, I want someone who is really engaged in large eddy simulations with real boundary conditions to help me.

kwerner
Posts: 2251
Joined: Wed Feb 14, 2018 9:21 pm

Re: boudary conditions for real case LES || WRF-LES specialists help please.

Post by kwerner » Fri Apr 09, 2021 11:08 pm

Hopefully someone on here will see this and be able to help you out. In the meantime, in case you haven't seen this, here is a physics presentation that has some information on real LES cases.
NCAR/MMM

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