How to draw geopotential height at 850 hPa for WRF-Chem ouput via NCL?

[YKim]

Hello,

I'd like to draw geopotential height at 850 hPa for WRF-Chem output via NCL.

I have variables as below.
PH(Time, bottom_top_stag, south_north, west_east): perturbation geopotential [m2 s-2]
PHB(Time, bottom_top_stag, south_north, west_east): base-state geopotential [m2 s-2]
P(Time, bottom_top, south_north, west_east): perturbation pressure [Pa]
PB(Time, bottom_top, south_north, west_east): BASE STATE PRESSURE [Pa]

I think I can calculate geopotential height as follows.
step1.
GPH = (PH + PHB)/(g), where g = 9.80665 m s-2 (gravitational acceleration)
step2.
interpolate GPH to 850 hPa by using "P+PB" and ncl function "wrf_interp_3d_z"

However, it does not work because bottom-top level between geopotential and pressure is different(bottom_top_stag and bottom_top).

In this case, how can I get geopotential height at 850 hPa via NCL?

Thanks,
YKim.