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Correct value of timestep in WRF (CFL error related)

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Dear WRF-experts,

I have some questions related to the CFL error in WRF:

[1] It is recommended that the timestep be equal to 6*dx, where dx is the horizontal resolution in km. I wonder where the "6" comes from.
Do you have documentation explaining why this value is recommended?

[2] I encountered a CFL error. Many online posts suggested reducing the time step. Indeed, when I reduced the timestep, WRF worked perfectly fine.
However, I wonder as to what extent one should reduce the time step?
Right now I am guessing the values of the time step. But I want to know if there is a more objective method on getting the correct timestep in a WRF run.


(1) Please see the document here, Section 3.3 gives detailed description how to set the time step.

(2) Usually the time step is set to 6dx. In some cases for example over complex terrain or running with complex microphysics, time step can be reduced to 4dx. We won't recommend time step smaller than 3dx/