HowTo: Calculating the true pressure from the hybrid sigma-pressure levels of LMDZ

HowTo: Calculating the true pressure from the hybrid sigma-pressure levels of LMDZ

The 4D variables (x,y,z,t) of LMDZ, such as ta, ua, va, etc are output on hybrid sigma-pressure levels.

The 4D variable (x,y,z,t) "pres" contains the pressure values of each mesh for each vertical level and time step. When "pres" is not available, we can use the "geop" variable (which corresponds to the geopotential of each mesh for each vertical level and time step) and the surface pressure to calculate the pressure on the vertical.

When the "pres" or "geop" variables are not available, the true pressure levels of the "true_presnivs" model can be reconstructed, using the ap and bp coefficients and the surface pressure, called "preff" here.

In the situation where the variables ap and bp are given at the interfaces of the model (ap and bp are of dimension klevp1=80), we calculate "true_presnivs" by:

  DO l = 1, klev
     true_presnivs(l) = 0.5 *( ap(l)+bp(l)*preff + ap(l+1)+bp(l+1)*preff )
  ENDDO

with klev=79.

In the case where ap and bp are given at the middles of the layers (with ap and bp of dimension klev), then :

  DO l = 1, klev
     true_presnivs(l) = ap(l)+bp(l)*preff
  ENDDO

The surface pressure "preff" is also a 4D variable and is a priori included in the 4D files (like ta*.nc, ua*.nc, va*.nc, etc).