Growth rate of axismmyetric modes measured in the laminar-wind simulations for different k_x (red diamond makers). The blue-dashed curve shows the theoretical growth rate predicted from the model extension in Appendix B (assuming a pure laminar stress). The green plain curve shows the same model but taking into account the u_zB_x term in the induction equation (this term is simply estimated by calculating the ratio u_zB_x/u_xB_z associated with the unstable mode in simulations.