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<div class="moz-cite-prefix">Hi All,<br>
<br>
The maximum value of the CFL number is set by the
Courant-Friedrichs-Lewy condition (see WRF Description, Section
3.3 Stability Constraints). It is a necessary condition for the
convergence of the finite difference integration schemes.<br>
<br>
Generally CFL = u_i*dt/dx_i, where u_i is the maximum velocity in
the ith direction, dt is the timestep and dx_i is the grid
spacing in the ith direction. For a simple 1D advection equation
with an explicit solver the requirement is CFL < 1 everywhere
in the domain. The limiting value is different for other equations
and higher order finite difference schemes.<br>
<br>
If you get an error message saying that the CFL was violated you
should consider the solution associated with that simulation to be
spurious. To resolve this problem you can either reduce dt (which
increases the computation time) or you can increase dx (which
reduces resolution) and rerun the simulation. Keep in mind that
u_i is the maximum velocity in the ith direction so a large
velocity event at even one grid point can cause you simulation to
"blow up" in just a couple of time steps. Thus it is important to
consider the CFL in three dimensions and then allow some small
factor of safety in the design of dt or dx.<br>
<br>
Regards,<br>
<br>
Anders<br>
<br>
<br>
<div class="moz-signature">-- <br>
Anders Nottrott<br>
<i>PhD Student, Mechanical and Environmental Engineering</i><br>
University of California, San Diego<br>
Department of Mechanical and Aerospace Engineering<br>
Website: <a href="solar.ucsd.edu/nottrott">solar.ucsd.edu/nottrott</a></div>
<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:part4.05020502.04030104@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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Pison San Pedro</span></p>
<p class="MsoNormal">
<span
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