[Wrf-users] Some Notes About CFLS

[Barry Lynn]

Hello:

I had some problems with CFLS when setting up my 12/4 km domain.

I had particular points which (at almost every time step) had CFL "errors."

There are few things to do about this.

1) Change epssm to be closer to 0.5, rather than 0.1 (or 0.2).

2) Set smooth_option = 2 (this smoothes near the boundaries of the inner
domain.)

3) Set smooth_cg_top = .true.  (smoothes topography near boundary of outer
domain.)

These steps together can reduce the number of CFLs.

4) damp_opt = 3 (reduces upper level instability; CFLs occurring at upper
levels).

However, one other, but very important note, is to be sure to place the
boundaries of the inner domain away from steep terrain (see attached pdf. I
had CFLS near the right boundary until I moved the right upper boundary
into the water.)

5) Lastly, if you get CFLs during severe or strong storms, you may have to
reduce the timestep from 6 to 5 or even 4 times the grid size (at least
until you've passed the period of greater model instability).

Barry Lynn



-- 
Barry H. Lynn, Ph.D
Senior Lecturer,
The Institute of the Earth Science,
The Hebrew University of Jerusalem,
Givat Ram, Jerusalem 91904, Israel
Tel: 972 547 231 170
Fax: (972)-25662581

C.E.O, Weather It Is, LTD
Weather and Climate Focus
http://weather-it-is.com
Jerusalem, Israel
Local: 02 930 9525
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