CDO

I found that geopotential heights are a bit lower (about 3-5%) than the observations after the vertical interpolation from hybrid levels to pressure levels using the command 'ml2pl'.
No error messages happen during the interpolation process. The spatial patterns after interpolation are reasonable, except the values are a bit lower.
This problem only occurs in geopotential height, not the other 3D variables.

I noticed the sentence in the manual: 'It is assumed that the geopotential heights are located on the hybrid layer interfaces.'
However, the geopotential heights I need to interpolate are located on the hybrid layer mid-points. I didn’t know if the problem came from this.

How can I fix this problem? Thank you!

float Z3(time, lev, lat, lon) ;
                Z3:mdims = 1 ;
                Z3:units = "m" ;
                Z3:long_name = "Geopotential Height (above sea level)" ;
                Z3:cell_methods = "time: mean" ;
        double lev(lev) ;
                lev:long_name = "hybrid level at midpoints (1000*(A+B))" ;
                lev:units = "hPa" ;
                lev:positive = "down" ;
                lev:standard_name = "atmosphere_hybrid_sigma_pressure_coordinate" ;
                lev:formula_terms = "a: hyam b: hybm p0: P0 ps: PS" ;
        double hyam(lev) ;
                hyam:long_name = "hybrid A coefficient at layer midpoints" ;
        double hybm(lev) ;
                hybm:long_name = "hybrid B coefficient at layer midpoints" ;
        double P0 ;
                P0:long_name = "reference pressure" ;
                P0:units = "Pa" ;
        double ilev(ilev) ;
                ilev:long_name = "hybrid level at interfaces (1000*(A+B))" ;
                ilev:units = "hPa" ;
                ilev:positive = "down" ;
                ilev:standard_name = "atmosphere_hybrid_sigma_pressure_coordinate" ;
                ilev:formula_terms = "a: hyai b: hybi p0: P0 ps: PS" ;
        double hyai(ilev) ;
                hyai:long_name = "hybrid A coefficient at layer interfaces" ;
        double hybi(ilev) ;
                hybi:long_name = "hybrid B coefficient at layer interfaces" ;