a short band has been generated. The simulation shown in Figure \[fig:forcing\_amplitudes\](b) was considered. The sharp deactivation is at $t=550$, at which the short band generated by the force is visualised in Figure \[fig:forcing\_amplitudes\](e). The smooth deactivation is at $t_0=600$ and the force starts to decrease approximately at $t=550$. Panel (d) shows the kinetic energy of the flow and that the band continue to grow in both deactivation cases. Therefore, these tests suggest that the deactivation of the force does not introduce instabilities and velocity fluctuations, and that the specific deactivation process does not significantly affect the band generation, as long as the band has sufficiently developed under the forcing. However, it can be noticed that the gradual deactivation slightly delays the growth of the band in terms of kinetic energy compared to the sharp deactivation case, see panel (d). A possible reason is that, the force does not contribute to the instability anymore as the strength gradually reduces, whereas it might slightly affect the formation of a natural local instability, or a natural head, that is needed for the turbulent band to grow. Therefore, we recommend a sharp deactivation for simplicity.]{}]{}

To sum up, it is shown in this 