 diffusivity for heat, water vapor, tracers (m*2/s)

             shape must conform to those of u, v, t, q
             diff_m(:,:,k) and dsiff_t(:,:,k) are the diffusivities
                 evaluated at the interface between the k-1 and k levels
             the values diff_m(:,:,1) and diff_m(:,:,1) are not used

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

         p_full 