[Wrf-users] CFL criterion - general question

Fri Nov 2 03:00:33 MDT 2012

Hi Jan,

Thanks! The CFL was happening over the ocean (Malacca Strait) which is quite strange. But is true that maybe the problem was coming from the Time step as I was setting the parent_time_ratio = 1, 2, 3 but the parent_grid ratio = 1, 3, 3 so the CFL errors were coming from domain 2!

Just strange that it run till the end and just a couple of this CFL errors were printed....

Thanks a lot and all the best for you too!

Elena

From: Jan Harlass [mailto:jharlass at geomar.de] 
Sent: Friday, 2 November, 2012 4:41 PM
To: Elena Maria Pison San Pedro
Cc: wrf-users at ucar.edu
Subject: Re: [Wrf-users] CFL criterion - general question

Hi Elena, 

another thing you could try is to add:
smooth_cg_topo = .true.
under &domains in the namelist when you run real. This smoothes the model topography to match the low resolution topography that comes with the driving data, if CFL happens along boundary zones. If CFL occurs near complex terrain, you may try to set epssm=0.2 ( up to 0.5) to see if that helps.
[from http://www.mmm.ucar.edu/wrf/users/FAQ.html]

Also remember to scale your time step with the maximum map factor of your domain. This value is written out in the log files of real.exe. The time step is then: dt (sec) = 6*dx (km) / (maximum map factor)

All the best!

    Jan

Am 01.11.2012 06:51, schrieb Elena Maria Pison San Pedro:

	Hello,

	I've remarked that sometimes when I run a simulation (WRF3.4) I got messages saying " x points exceeded cfl=2 at time..." but the simulation run to completion.

	How can this affect the results? What can I do to solve this instabilities, run a higher time step?

	Thanks a lot in advance!

	Best regards,

	Elena

Elena Pison San Pedro

M.Sc. in Atmospheric Sciences

DHI-NTU Research Centre and Education Hub,

DHI Water & Environment (S) Pte Ltd

1 Clean Tech Loop

#03-05 CleanTech One

Singapore 637141

Tel:  

+65-6777-6330

emp at dhi.com.sg

www.dhigroup.com <http://www.dhigroup.com/> 

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Dipl.-Met. Jan Harlaß
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Helmholtz-Zentrum für Ozeanforschung Kiel (GEOMAR)
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