# calculate height of first model layer

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#### Lefteris

##### New member
Hello everyone,

I would like to ask you how you can determine the height of the first model layer? Is it HGT parameter, z (geopotential /g) or is there another parameter which determines the first model layer?

Lefteris

Lefteris,
In wrfout, you will find PH and PHB, (PH + PHB) /9.8 gives you the height of each model level.

Hello, Ming
I have a question regarding this one.
As you have suggested that (PH + PHB)/9.8 gives the height of each level, can we estimate the height of the first layer grids by using the result of (PH + PHB)/9.8 for the first layer and subtract by HGT (terrain height level)?

I am not quite sure about how to relate the eta levels with the height levels.
I suppose that the first layer grids should be very close to the ground surface, and the height of the first layer cells should be smaller than 2m.
Because the WRF models can output the temperature at 2m height, so there should be at least one grid cell within that height (2m). is it correct?

Thank you!

Chang

Chang,
The height of model levels above ground can be obtained by [(PH+PHB)/9.8 - terrain height].

Detailed description of vertical coordinate used in WRF can be find here: https://www2.mmm.ucar.edu/wrf/users/docs/technote/v4_technote.pdf

(Full) Eta level described in the above document starts from the surface and extends to the model top. Mass quantitates are calculated in" half" eta levels, which are the middle between two adjacent "full" eta levels.

T2, Q2, and 10m winds are diagnosed based on similarity theory. They are not variables located at any model levels.

Thank you, Ming. This solves my question.

I am wondering is there any way to obtain the wind speed at 2m height?
For this purpose, should I learn some similarity theory to infer the value at 2m by using the wind speed at 10m, I guess it should be related to a log-law or power-law profile derived by the local atmospheric roughness value?
Or should I interpolate/extrapolate the value at 10m by using the simulated values at different levels?

Thanks!

Chang

Change,
I don't think you can extrapolate wind from 10m to 2m. There is no physical laws you can follow to do so.
As for the similarity theory, probably you can refer to the code that diagnoses 10m wind and try to apply it to obtain wind at 2m.

Ming Chen said:
Chang,
The height of model levels above ground can be obtained by [(PH+PHB)/9.8 - terrain height].

Detailed description of vertical coordinate used in WRF can be find here: https://www2.mmm.ucar.edu/wrf/users/docs/technote/v4_technote.pdf

(Full) Eta level described in the above document starts from the surface and extends to the model top. Mass quantitates are calculated in" half" eta levels, which are the middle between two adjacent "full" eta levels.

T2, Q2, and 10m winds are diagnosed based on similarity theory. They are not variables located at any model levels.

Hi Ming,
From your replys, I think you understand WRF well. I have some doubts about " the height of model level" in WRF.
My questions are:
1] Does the model height locate in the middle of two eta_levels in WRF? For example, is the first model height in the middle of the eta_level 1 and the eta_level 2?
2] Is the ground surface defined as eta_level = 1.0 in WRF?

Hi Yonghang,