See the section at the end of Chapter 4 of the WRF Users Guide, titled "Setting Model Vertical Levels."
A good procedure to follow may be:
1. Determine the model top pressure.
2. Determine how thick your lowest layer is (50 m by default)
3. Determine maximum thickness (1 km by default).
Then based on your needs, the stretching rates can be chosen using the table in chapter 4 of UG (link above) as a guideline.
1. Choose the stretching rate(s) - larger if you want fewer levels but it’s not recommended that the rate be > 1.3.
2) Find the minimum number of levels that works for that stretching factor by iterating running real and changing e_vert. The minimum is the one that gives the smoothest variation in DNW, which can also be viewed in wrfinput as part of the process
It is worth mentioning that when one chooses this method, the real program prints out the thickness of the layer, and it can be very good information for making adjustments for the stretching parameters.
- Smoothly varying vertical levels can be very important for model stability. Tests have shown that two different sets of vertical levels gives different numbers of CFL prints.
- For cases with dz > dx (as can occur for dx < 1 km at the top), tests have also shown that continuous stretching works through to the model top better than having many equal dz levels.