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!Environment Canada - Atmospheric Science and Technology License/Disclaimer, 
!                     version 3; Last Modified: May 7, 2008.
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***s/p  gzcalcul
*

      subroutine gzcalcul(gzm,tvirt,mrgasd,s,ni,nk)
*
#include "impnone.cdk"
      integer ni,nk
      real mrgasd
      real gzm(ni,nk),tvirt(ni,nk),s(ni,nk)
*
*author
*          A-M.Leduc (June 2002)
*
*
*object
*         To calculate the geopotential from virtual
*         temperatures and sigma levels
*
*arguments
*
*          - output -
* gzm        result
*
*          - input -
* tvirt    virtual temperature
* s        sigma levels
* mrgasd   gas constant (negative value)
* ni       horizontal dimension
* nk       vertical dimension
*
*notes
*         gzm (*,nk) is initialized to zero in phyexe1
*         just before the call to this routine
**
*
      integer j,k
      real tavg, savg


*           calculation of geopotential
*
        do k=nk,2,-1
         do j=1,ni

           tavg=(tvirt(j,k)+tvirt(j,k-1))*0.5
           savg=(s(j,k)+s(j,k-1))*0.5
           gzm(j,k-1)=gzm(j,k) + mrgasd *tavg *(s(j,k)-s(j,k-1))/savg
         
         end do
       end do

      return
      end