The Effect Of Warm Water On The Rapid Intensification Of Typhoon Hato(2017)

Typhoon Hato went through a rapid intensification(RI)process in the nearshore region.According to observation data,before the RI process,Hato firstly moved over a large area of warm water,then two warm core rings successively.SST satellite data shows the SST cooling(SSTC)induced by Hato is about 0.73℃ which is much weaker than general cases.The research is carried out to find out whether those warm water features contributed to the RI process and the underlying mechanism,if they did.Two air-sea coupled simulation on Hato were conducted,and it is found that around 20% of the RI is contributed by the warm water.According to the SST budget analysis,the SSTC is taken up mostly by turbulent mixing while advection played a much minor part.On one hand,the warm water impaired the turbulent mixing between different layers of sea water;on the other hand,the cold advection did not bring about much SSTC either.Therefore,the negative feedback the sea applies on Hato is highly weakened,and that’s how the warm water helped the RI.Further analysis is taken on the atmospheric process of the RI.It turns out that the heat flux on the air-sea interface is also enhanced by 20% due to the warm water,while around 87.5% of it is latent heat,so the warm core of Hato took shape and maintained under the effect of the diabatic heating.Besides,the dynamic structure of Hato is significantly strengthened,especially the vertical motion.The intense ascending motion triggers convective bursts which helps ascending motion in turn by giving out latent heat.This positive feedback is an important mechanism for the intensification of Hato.Upward mass flux brought by ascending motion strengthens the inertial stability which increases the heating efficiency of latent heat.Therefore,the warm core is effectively maintained and the RI process is promoted.Although there might be other factors that also take part in the RI process,this study draws more attention to the air-sea interaction in the tropical cyclone forecast and sheds light on the impact of warm water on the intensity and structure of a tropical cyclone.