Abstract:  Based on the reanalysis data from the National Centers for Environmental Prediction and cloud images from THORPEX-Pacific Asian Regional Campaign, the formation of super Typhoon Sinlaku(2008)over the
Northwest Pacific is analyzed in detail in this work.The result reveals that Sinlaku is originated from a large easterly wave system cons}stmg of several smaller easterly wave perturbations. As the easterly wave system propagated
westwards and then interacted with the monsoon trough, the propagating speed of the easterly wave disturhances decreased and critical layers of the waves appear, which are the "sweet spots" for tropical cyclone formation, In the
areathe near the critical layer, two easterly wave disturbances merged and finally developed into Typhoon Sinlaku under effect of the distinct convergence of tonal wind. However, although the easterly wave disturbance preceding the
 recursor of Sinlaku was situated in the critical layer in most of the time, it failed to develop into a tropical cyclone due to the lack of considerable convergence.Therefore, though the area near the critical layer is a sweet spot for
tropical cyclone formation, it is not sufficient enough for tropical cyclogenesis unless it is located in the lager-scale tonal wind convergence zone, which implies that the scale contraction of the disturbances and wave energy
accumulation mechanisms play a more important role in the formation of tropical cyclone.