# Does WRF simulation use sigma or eta vertical coordinate system?

Could you tell me what the WRF's vertical coordinate system is? Sigma or eta?

According to the User WRF Guide document, uses a terrain-following hydrostatic pressure coordinate while the WRF namelist has eta level. Are both of them same?

• @gansub Thanks for your comments. The version of WRF I used is WRF 3.6.1. I have not read through the new document of WRF3.9. I think the coordinate system in WRF 3.6.1 should be the sigma vertical coordinate. Could you be willing to tell me about how to interpolate variables of WRF to pressure level? I tried to do such conversion by the function "wrf_user_interp_level". Is it a right way to do this? Nov 20, 2019 at 21:46

For sigma coordinates, you still define the eta-levels. (Note that it sigma coordinates is just a normalisation of pressure) $$\sigma = \frac{p-p_t}{p_s-p_t}$$ where $$p$$ is specified through $$p=A+Bp_s.$$ It has been a few years since I played around with WRF, so I don't know how this change affects computational efficiency. If you get any results on this that will be interesting to hear.