<p>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.</p>

<p>For sigma coordinates, you still define the eta-levels. (Note that it sigma coordinates is just a normalisation of pressure)
<span class="math-container">$$
\sigma = \frac{p-p_t}{p_s-p_t}
$$</span>
where <span class="math-container">$p$</span> is specified through
<span class="math-container">$$
p=A+Bp_s.
$$</span>
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.</p>

CC BY-SA 4.0
2019-11-20T14:55:47.597Z
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.
BarocliniCplusplus
CC BY-SA 4.0
2019-11-20T21:44:37.700Z
@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?
Love_qq_xq
CC BY-SA 4.0
2019-11-21T07:14:15.640Z
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](https://www.ncl.ucar.edu/Document/Functions/WRF_arw/wrf_user_interp_level.shtml) 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:](https://unidata.github.io/MetPy/latest/examples/sigma_to_pressure_interpolation.html)
Whir
CC BY-SA 4.0
2019-11-20T09:29:48.573Z
2019-11-20T09:29:48.573Z
Whir
<p>Could you tell me what the  WRF's vertical coordinate system is? Sigma or eta?</p>

<p>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?</p>

CC BY-SA 4.0
2019-11-20T21:46:15.623Z
@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?
Love_qq_xq
<meteorology><climate-models><wrf>
Does WRF simulation use sigma or eta vertical coordinate system?
Love_qq_xq
# Does WRF simulation use sigma or eta vertical coordinate system?

(Asked by: username_0 on 2019-11-20)

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?

**Tags:** meteorology, climate-models, wrf

> **Comments:**
> **username_0**: @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?  

---

## 1 answers

### Answer by username_1 on 2019-11-20. Score: 5

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.

> **Comments:**
> **username_2**: 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.  
> **username_0**: @username_1 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?  
> **username_1**: 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](https://www.ncl.ucar.edu/Document/Functions/WRF_arw/wrf_user_interp_level.shtml) 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:](https://unidata.github.io/MetPy/latest/examples/sigma_to_pressure_interpolation.html)  

---

