How to resolve cfl errors?

[Felipe_SM]

Hello,

I have been attempting to run WRF in central Chile, a region characterized by complex topography due to the Andes mountain range. I used three domains of 36, 12, and 4 km with time steps of 6dx, 4dx, and 3dx (216, 144, and 108 seconds, respectively). I am using the ERA5 dataset with 50 vertical layers. In all cases, upon reviewing the rsl.error and rsl.out files, I encounter messages like the following:

“rsl.error.0007

2000-06-13_21:23:24 MAX AT i,j,k: 97 62 17 vert_cfl,w,d(eta)= 2.01002645 -2.92913198 4.20067906E-02”
“rsl.error.0007

2000-06-13_21:25:12 1 points exceeded cfl=2 in domain d01 at time 2000-06-13_21:25:12 hours”

I have attached the following summary table:

Where dt is the time step, #CFL2(error) is the number of times when the CFL message appeared, CFL Maximo (error) is the max value of CFL in all rsl.error* files and #CFL>2: dom i, is the amount of time the CFL message appeared in domain i.
It is worth noting that all my simulations successfully complete the WRF execution, and precipitation results look reasonable, I only find these messages when analyzing the WRF outputs. In my search for solutions, I found various recommendations on the forum, such as:

• w_damping = 1: This option activates vertical velocity damping to reduce numerical noise in the vertical wind component.
• diff_6th_opt = 2: This setting applies sixth-order numerical diffusion to improve model stability and accuracy.
• damp_opt = 2: This parameter activates Rayleigh damping to help control gravity wave reflections at the model top boundary.
• epssm = 0.9: This value controls the sound waves that propagate vertically. If the model domain is over a complex terrain area with large topography gradients, it is recommended that this value be increased.

I tried using these previous options separately and in combination, but the results were similar in all cases. When I reduced the time step, I observed significantly fewer CFL warnings, and their values decreased considerably.

How can I solve this problem? What would you recommend to avoid reducing the time step and prevent this instability issue?

 

[Ming Chen]

When CFL violation occurs, we usually try to overcome the problem by reducing time step, turning on w_damping and increasing the value of epssm, which are exactly what you have done. These approaches can effectively suppress CFL violation, if not completely eliminate it.
I would say that if your case can run successfully, you don't need to worry for the CFL errors. The CFL violation messages in RSL files only tell that the model may be temporarily unstable in its numerical integration. However, the model can manage to overcome the instability and continue to run successfully. In this case, we can get reasonable simulations.

 

[Arty]

• epssm = 0.9: This value controls the sound waves that propagate vertically. If the model domain is over a complex terrain area with large topography gradients, it is recommended that this value be increased.

Hello,

Note that epssm "should not be set higher than 0.5" as told by Kelly Werner HERE.

About decreasing time-step, I've also been advised to be careful : currently running sensitivity tests, it might affect the way I compare physics options on my outputs. This is worth noting.