[Wrf-users] CFL criterion - general question

[Anders Nottrott]

As a follow on to my earlier message and in response to the comment:

"If CFLs always appear at the same coordinates such as a mountain peak..."

This is likely occurring due to terrain following coordinates in WRF. 
Near steep terrain (as in mountainous regions) grid cells become 
stretched or compressed due to deformation of the terrain following 
grid. This can cause local values of dx_i to be smaller than in the rest 
of the domain, resulting in larger than expected local values of the CFL.

Regards,

Anders

On 11/1/2012 11:37 AM, Kevin Matthew Nuss wrote:
> Hi Elena,
>
> I have had the same thing happen. I don't know exactly how that 
> affects the results, so when it has happened to me, I did not trust 
> the results.
>
> Yes, reducing the time step is the standard way to avoid CFL errors. 
> There are some alternatives like using restart files: when CFLs 
> appear, stop, go back a little way in time, use small time steps for a 
> while, stop and continue forward using the larger timesteps. That is 
> mostly done for very long runs like climate downscaling when it is not 
> practical to use small timesteps for an entire run.
>
> If CFLs always appear at the same coordinates such as a mountain peak, 
> EPSSM can be adjusted or the grid changed.
>
> If you need more information and detail about what I do to avoid CFL 
> errors, I have some on my website: http://www.nusculus.com/wrf-errors
>
> I don't claim to be an expert; I have simply suffered through a lot of 
> bad WRF runs.
>
> But most people just keep reducing the time step until the CFL errors 
> go away.
>
> Kevin
>
> On Wed, Oct 31, 2012 at 11:51 PM, Elena Maria Pison San Pedro 
> <emp at dhi.com.sg <mailto:emp at dhi.com.sg>> wrote:
>
>     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
>
>
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>     emp at dhi.com.sg <mailto:emp at dhi.com.sg>
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