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<div class="moz-cite-prefix">As a follow on to my earlier message
and in response to the comment:<br>
<br>
"If CFLs always appear at the same coordinates such as a mountain
peak..."<br>
<br>
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.<br>
<br>
Regards,<br>
<br>
Anders<br>
<br>
On 11/1/2012 11:37 AM, Kevin Matthew Nuss wrote:<br>
</div>
<blockquote
cite="mid:CALZketE39xdFT2dXXLWHDHY0-0i-6P=70stYtzV3dovkFf5beQ@mail.gmail.com"
type="cite">Hi Elena,
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>If CFLs always appear at the same coordinates such as a
mountain peak, EPSSM can be adjusted or the grid changed.</div>
<div><br>
</div>
<div>If you need more information and detail about what I do to
avoid CFL errors, I have some on my website: <a
moz-do-not-send="true"
href="http://www.nusculus.com/wrf-errors">http://www.nusculus.com/wrf-errors</a></div>
<div><br>
</div>
<div>I don't claim to be an expert; I have simply suffered through
a lot of bad WRF runs.</div>
<div><br>
</div>
<div>But most people just keep reducing the time step until the
CFL errors go away.</div>
<div>
<br>
</div>
<div>Kevin<br>
<br>
<div class="gmail_quote">On Wed, Oct 31, 2012 at 11:51 PM, Elena
Maria Pison San Pedro <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:emp@dhi.com.sg"
target="_blank">emp@dhi.com.sg</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div link="blue" vlink="purple" lang="EN-SG">
<div>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""> </span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif"">Hello,</span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""> </span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif"">I’ve
remarked that sometimes when I run a simulation
(WRF3.4) I got messages saying “ x points exceeded
cfl=2 at time…” <u>but the simulation run to
completion</u>.</span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""> </span></p>
<p class="MsoNormal"><span><img
src="cid:part3.06020707.02060600@ucsd.edu"
width="1037" height="576"></span><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""> </span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif"">How
can this affect the results? What can I do to solve
this instabilities, run a higher time step?</span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""> </span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif"">Thanks
a lot in advance!</span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""> </span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif"">Best
regards,</span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif"">Elena</span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif""> </span></p>
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<p class="MsoNormal"><span
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Pison San Pedro</span></p>
<p class="MsoNormal">
<span
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