[Wrf-users] Some Notes About CFLS

Tue Oct 3 01:44:12 MDT 2017

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

-----Message d'origine-----
De : Wrf-users [mailto:wrf-users-bounces at ucar.edu] De la part de wrf-users-request at ucar.edu
Envoyé : lundi 2 octobre 2017 17:10
À : wrf-users at ucar.edu
Objet : Wrf-users Digest, Vol 158, Issue 1

Send Wrf-users mailing list submissions to
	wrf-users at ucar.edu

To subscribe or unsubscribe via the World Wide Web, visit
	http://mailman.ucar.edu/mailman/listinfo/wrf-users
or, via email, send a message with subject or body 'help' to
	wrf-users-request at ucar.edu

You can reach the person managing the list at
	wrf-users-owner at ucar.edu

When replying, please edit your Subject line so it is more specific than "Re: Contents of Wrf-users digest..."

Today's Topics:

   1. Some Notes About CFLS (Barry Lynn)

----------------------------------------------------------------------

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.ucar.edu/pipermail/wrf-users/attachments/20171001/cc30c564/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: namelist.input
Type: application/octet-stream
Size: 6290 bytes
Desc: not available
URL: <http://mailman.ucar.edu/pipermail/wrf-users/attachments/20171001/cc30c564/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: plot_grids.ncl
Type: application/octet-stream
Size: 5359 bytes
Desc: not available
URL: <http://mailman.ucar.edu/pipermail/wrf-users/attachments/20171001/cc30c564/attachment-0001.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: namelist.wps
Type: application/octet-stream
Size: 1672 bytes
Desc: not available
URL: <http://mailman.ucar.edu/pipermail/wrf-users/attachments/20171001/cc30c564/attachment-0002.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: wps_new_dom.pdf
Type: application/pdf
Size: 247937 bytes
Desc: not available
URL: <http://mailman.ucar.edu/pipermail/wrf-users/attachments/20171001/cc30c564/attachment.pdf>

------------------------------

Subject: Digest Footer

_______________________________________________
Wrf-users mailing list
Wrf-users at ucar.edu
http://mailman.ucar.edu/mailman/listinfo/wrf-users

------------------------------

End of Wrf-users Digest, Vol 158, Issue 1
*****************************************