A Study On The Mechanism Of The Influence Of The Northwest Pacific Monsoon Vortex And High-level Cold Vortex On The Intensity And Track Of Tropical Cyclones

Skillful prediction of typhoon intensity and sudden track change remains a challenge for numerical models in western North Pacific.Recently,it has been founded that some tropical cyclones(TC)may experience track and intensity changes through interactions with low-level monsoon gyre(MG)and upper-tropospheric cold low(UTCL).Understanding these complex interactions are conducive to strengthen the monitoring and forecasting of typhoons and reduce disaster losses.The main conclusions are as follows:(1)The vertical structure and moisture distribution of the MG could induce the different track of TC.According to ideal numerical simulation,it is found that when the TC interacts with a MG with a relative deep initial vertical structure,it may experience a sudden northward turn and have a larger intensity and outer size.TCs always locate to the east side of the MGs at the initial time,a larger outer size of the TC also favors a faster northwestward propagation under ? effect propagation from its initial position,also the large relative vorticity gradient by MG also makes the TC move towards MG center through the vorticity segregation process.Once the coalescence of their centers,the total vorticity tendency for the TC wavenumber-one component nearly vanishes as the vorticity advection by the MG cancels the vorticity advection by the TC.Duo to the total horizontal advection is weak and points to the north,the TC experiences a sharp northward turn.In addition,the superposition of the two systems enhances the Rossby wave energy dispersion and the enhanced southwesterly wind on the east side of TC acts as a steering flow for the sharp northward turn.Moreover,the spatial moisture distribution in the MG could also affect the nearby TC track.When the MG is accompanied by higher(lower)relative humidity in the eastern(western)semicircle,the TC could experience a sharp northward turning.(2)Regardless of the initial difference of thermal conditions,MG may slow down the TC development through three potential dynamical mechanisms.Firstly,the superposition of TC and MG circulations produce a larger outer size for the TC.A TC with a larger size develops slowly and has a reduced capacity to organize convection through vorticity segregation process.Deep convection is concentrated mainly outside the radius of maximum wind(RMW),resulting in slower intensification rates.Secondly,when the TC is embedded on the eastern flank of the MG,its outer region becomes barotropic unstable.Through the interaction between the inner and outer zones,the barotropic instability in the outer area could increase the asymmetric disturbances in the inner area.The asymmetric perturbations exhibit an up-shear titling,which extracts kinetic energy from the primary vortex and thus hinders TC intensification.Thirdly,the vertical baroclinic nature of the MG imposes an ambient vertical wind shear(VWS)on the TC,leading to a vertical tilting and increased asymmetric perturbation,thus a larger ventilation index hinders TC intensification.The MG related relative humidity could also impact TC intensification.(3)In addition,upper-level systems also could affect the movement and development of tropical cyclone.Typhoon Jongdari(2018)had an unusual looping path and intensity change(first intensifies and then weakens)with a nearby UTCL,which posed a forecasting challenge.CTL run could simulate this track and intensity change.On this basis,an additional experiment(RCL run)is designed to compare with CTL run.UTCL is removed in the initial field of RCL run,the storm tends to move westward and strengthens steadily.The diagnosis of potential vorticity tendency suggests that horizontal advection term and diabatic heating term under Fujiwhara interaction are the main contributors for Jongdari's motion in CTL run.Furthermore,the intensity change of Jongdari could be discussed in two stages.The UTCL contributes to the intensification of Jongdari at the early stage by enhancing the eddy flux convergence of angular momentum and reducing inertial stability,and it contributes to the storm weakening via enhanced vertical wind shear at the later stage when moving closer to Jongdari.Different sea surface temperatures and other environmental conditions along the different storm tracks also contribute to the intensity differences between the CTL and RCL run.