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?
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$\begingroup$ @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? $\endgroup$– Love_qq_xqNov 20, 2019 at 21:46
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WRF is using the sigma (terrain-following) vertical coordinate. However, as @gansub has already referred, in WRF V3.9 you can now select a hybrid sigma-pressure vertical coordinate. The advantage of this is that the coordinate is terrain-following near the surface, but it 'converts' to pressure levels at higher levels, which improves the accuracy of the calculation of the pressure gradient. Over very steep terrain the sigma coordinate extends this steepness throughout the atmosphere which is not very ideal.
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.
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2$\begingroup$ It's also worth knowing that even though it is sigma coordinates, the namelist variable to specify specific values of sigma is called eta_levels. So even though it may be referred to as eta, if is actually sigma. Confusing, I know. $\endgroup$ Nov 20, 2019 at 14:55
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$\begingroup$ @Whir Thanks for your detailed explanation. The version of WRF I used is WRF 3.6.1. It 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? $\endgroup$ Nov 20, 2019 at 21:44
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$\begingroup$ As far as I recall, "wrf_user_interp_level" is an NCL function? Personally I have not used NCL for 6-7 years, but by looking at their documentation I think it is correct. Normally I do it myself, but it is then, of course, important that you know the values of A and B. You can also use "MetPy" which have a nice example of doing this with WRF output: $\endgroup$– WhirNov 21, 2019 at 7:14