- real, dimension(:,:,:) 
             kinematic diffusivity (m*2/s)
             shape must conform to that of xi
             diff(:,:,k) is the diffusivity evaluated at the interface
                 between the k-1 and k levels
             the value diff(:,:,1) is not used

         p_half -- real, dimension(:,:,:)
             pressure at interface between model levels
             size(p_half,3) must = 1 + size(xi,3)
             units = Pascals

         z_full -- real, dimension(:,:,:)
            