al to 2*dt = time(i+1) - time(i-1)

         Tri_surf -- type(surf_diff_type)
             information passed from  information 
               surface modules and gcm_vert_diff_down 


         dt_t, dt_q -- real, dimension(:,:,:)

             The tendencies modified by the effects of diffusion
             Because the algorithm is implicit, the final tendency
                depends on which other processes have contributed to 
                the tendency before, and which after, this call

            