Intensity Change Of Noru (2017) During Binary Tropical Cyclones Interaction In Monsoon Trough

Compared with the track changes,the intensity changes during the binary interaction caught less attentions.A few of studies examine the TC intensity changes during the binary TC interaction.It is suggested that the merger of binary TCs may cause a dramatic TC intensity change.In this study,the intensity change of tropical cyclone(TC)Noru(1705)is investigated by using numerical simulations.It is found that the TC Noru experienced a weakening stage during the binary interaction with TC Kulap(1706).In the presence of Kulap(1706),the model captures well the observed intensity change.On the contrary,once Kulap is removed,it fails to reproduce the weakening stage of Noru(1705).Possible mechanisms are proposed as follows: During the mutual interaction,the competition of moisture supplies will lead to a dry air layer wrapped around Noru(1705).Meanwhile,due to the circulation of nearby Kulap(1706),the vertical shear is relatively larger in the adjacent of Noru(1705).As proposed by previous studies,the negative impacts by dry air layer will be enhanced through two possible pathways.The first is that the dry air directly impacts the TC inner core convection under vertical wind shears.The second is through the downward entropy flux into boundary layer,by which reducing the boundary layer entropy and thus convection.Dynamically,along with the approach of Kulap,it will induce barotropic instability at the outer region,by which greatly enhances the inner-core asymmetry.The asymmetric component will grow at the expense of the mean flow,and thus TC Noru(1705)intensity weakens.Since TC Noru(1705)and Kulap(1706)are embedded into the monsoon trough with a higher RH.Hence,the critical radius is likely larger and the partial merger process occurs eventually.Furthermore,there are a series of idealized simulations carried out to explore the influence of monsoon trough on the merger of binary TCs.The results show that in the presence of the monsoon trough,it is indeed conducive to the merger of binary TCs.And within a certain range,the stronger the monsoon trough,the faster the merger of binary TCs.Relative humidity as an important parameter can influence the speed of the merger of binary TCs.Analysis on the idealized simulations that changed environmental relative humidity indicates that the speed of merger of binary TCs is not promoted if reduced half of relative humidity.Though analysing of idealized simulations it is found that the both the monsoon trough and the environmental relative humidity have an important effect on the merger of the two vortices.