[Wrf-users] Some Notes About CFLS
Barry Lynn
barry.h.lynn at gmail.com
Tue Oct 3 02:59:16 MDT 2017
Hi Thierry:
Thanks for making me aware of this.
It looks like it could be very helpful.
At the moment, with the setup I have, I haven't had a CFL complaint yet,
but the run has a long way to go.
I wonder how sensitive results are to time step. I mention this because if
you are trying to compare forecasts at different resolutions and the time
steps are different (meaning as a multiple of grid spacing), then this
might affect the validity of the comparison.
Barry
On Tue, Oct 3, 2017 at 10:44 AM, HEDDE Thierry 137147 <thierry.hedde at cea.fr>
wrote:
> Hi Barry and thanks for the CFLs notes
>
> I would add to point 5 (reduce time step)
> 6) Use the adaptative_time_step option which is working well with WRF 3.9.1
> Here is the classical tuning, I'm using for it (-1 means : automatic) for
> a 2 domains run (9km, 3km) fed with gfs 0.25°
>
> &domains
> use_adaptive_time_step = .true.,
> step_to_output_time = .true.,
> target_cfl = 1.2, 1.2
> target_hcfl = .84, .84,
> max_step_increase_pct = 5, 51,
> starting_time_step = -1, -1,
> max_time_step = -1, -1,
> min_time_step = -1, -1,
> adaptation_domain = 1,
>
> I'm testing it for my operational daily prognostics over France and as far
> as now, I've noticed no crash, it's quicker and the prognostic quality
> looks the same (qualitatively)
> The timestep is mainly at 8*DX during the run but sometimes adapts itself
> down to 4*DX
> Cheers
> Thierry
>
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> 1. Some Notes About CFLS (Barry Lynn)
>
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 1 Oct 2017 19:36:17 +0300
> From: Barry Lynn <barry.h.lynn at gmail.com>
> To: wrf-users at ucar.edu
> Cc: wrfhelp <wrfhelp at ucar.edu>
> Subject: [Wrf-users] Some Notes About CFLS
> Message-ID:
> <CAN7Bv264m961RoHS2DXr42pJkF8yiHEutjBTXQ3Ok1by990FfA at mail.
> gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hello:
>
> I had some problems with CFLS when setting up my 12/4 km domain.
>
> I had particular points which (at almost every time step) had CFL "errors."
>
> There are few things to do about this.
>
> 1) Change epssm to be closer to 0.5, rather than 0.1 (or 0.2).
>
> 2) Set smooth_option = 2 (this smoothes near the boundaries of the inner
> domain.)
>
> 3) Set smooth_cg_top = .true. (smoothes topography near boundary of outer
> domain.)
>
> These steps together can reduce the number of CFLs.
>
> 4) damp_opt = 3 (reduces upper level instability; CFLs occurring at upper
> levels).
>
> However, one other, but very important note, is to be sure to place the
> boundaries of the inner domain away from steep terrain (see attached pdf. I
> had CFLS near the right boundary until I moved the right upper boundary
> into the water.)
>
> 5) Lastly, if you get CFLs during severe or strong storms, you may have to
> reduce the timestep from 6 to 5 or even 4 times the grid size (at least
> until you've passed the period of greater model instability).
>
> Barry Lynn
>
>
>
> --
> Barry H. Lynn, Ph.D
> Senior Lecturer,
> The Institute of the Earth Science,
> The Hebrew University of Jerusalem,
> Givat Ram, Jerusalem 91904, Israel
> Tel: 972 547 231 170
> Fax: (972)-25662581
>
> C.E.O, Weather It Is, LTD
> Weather and Climate Focus
> http://weather-it-is.com
> Jerusalem, Israel
> Local: 02 930 9525
> Cell: 054 7 231 170
> Int-IS: x972 2 930 9525
> US 914 432 3108
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--
Barry H. Lynn, Ph.D
Senior Lecturer,
The Institute of the Earth Science,
The Hebrew University of Jerusalem,
Givat Ram, Jerusalem 91904, Israel
Tel: 972 547 231 170
Fax: (972)-25662581
C.E.O, Weather It Is, LTD
Weather and Climate Focus
http://weather-it-is.com
Jerusalem, Israel
Local: 02 930 9525
Cell: 054 7 231 170
Int-IS: x972 2 930 9525
US 914 432 3108
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