[Wrf-users] CFL criterion - general question

[Kevin Matthew Nuss]

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> 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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