===============================================

 subroutine gcm_vert_diff_down (is, js, delt,                              &
                          u, v, t, q, tr,                                  &
                          diff_m, diff_t, p_half, p_full, z_full,          &
                          tau_u, tau_v, dtau_du, dtau_dv,                  &
                          flux_tr,                                         &
                          dt_u, dt_v, dt_t, dt_q, dt_tr,                   &
 