A Study On The Numerical Simulation Of The Formation Mechanism Of Secondary Eyewall And Moat In Sinlaku(0813)

Eyewall replacement cycle(ERC)is common in the development of strong tropical cyclone(TC),which has important influences on TC inten sity and structure change.This will bring great difficulties to TC intensity and weather forecast,so it becomes a hot issue in recent years.In this paper,ERC and secondary eyewall formation(SEF)of typhoon Sinlaku(0813)are successfully simulated by using high-resolution Weather Research and Forecasting(WRF)model.Six experiments are conducted with different cumulus schemes.The results show that the radii of the inner eyewall in different experiments are basically the same,but the radial locations of the secondary eyewall vary.In general,the wider the moat,the larger the location of the outer eyewall and the longer the ERC time.Correspondingly,TC outer rainbands are also significantly different.It is found that the precipitation-driven downdraughts in the outer region of TC will act to import low equivalent potential temperature air from middle troposphere into the boundary layer,reducing the equivalent potential temperature in the boundary layer.This will increase the air-sea temperature difference and the subsequent transport of surface heat flux from the underlying ocean.Then the upward motion will be promoted,resulting in the generation of more vortical hot towers(VHTs)in the outer region of TC.At the beginning of ERC,these VHTs locate and develop solely.At this time,the strong small-scale vorticity dipoles appear.They are highly asymmetric with weaker anti-cyclonic vorticity and stronger cyclonic vorticity.As the time evolves,VHTs start to merge with each other.Meanwhile,the cyclonic vorticity moves inward,while the anti-cyclonic vorticity moves outward.There is a strong rotation difference on the outside of the cyclonic vorticity,which will stretch the vorticity into a filamentous structure,and thus the axisymmetric vorticity ring forms at the location of the outer eyewall.The more the VHTs,the more enhanced the vorticity ring,followed by an enhanced secondary maximum of tangential wind and outer eyewall.With an active secondary eyewall,the updrafts are greater,leading to a stronger upper-level compensating subsidence within the moat,which will suppress the lower-level convection.Therefore,the moat is dominated by the downward motion,causing a wider moat width and a larger radial location of SEF.By calculating the concentration index(CI)defined in this paper,it is found that under the condition of similar outer precipitation amount,when the rainbands are more active and compact,the CI will be smaller,leading to an increased transfer of the surface heat flux induced by a greater air-sea temperature difference.The TC outer rainbands distributions and their influences on the air-sea complex interactions result in significant differences between some important parameters during ERC period such as the radial location of secondary eyewall,the width of moat and the ERC duration time.In order to further study the physical process in SEF,the vertical velocity and vorticity budget of TC secondary eyewall are diagnostically analyzed.The results show that(1)the advection process is always the fundamental reason of vertical motion generation throughout the whole ERC,where the temperature advection plays the most important role and the vorticity advection contributes relatively less,which is about one-tenth of the temperature advection.While the contribution of adiabatic heating is always the smallest.But there is a positive feedback between the adiabatic heating and temperature advection,so it is also important to SEF.(2)At the beginning of ERC,the total vorticity budget is negative at the lower level and positive at the upper level,which represents the development of TC outer eyewall.At this time,the horizontal advection,convergence and divergence of the vorticity play a major role directly.While the vert ical advection of vorticity contributes to the whole budget indirectly and the the tilting of vorticity due to the velocity gradients is basically positive throughout the whole layer.With the time evolving,the convergence and divergence of the vorticity start to make negative contributes and other terms remain unchanged,which will convert the whole vorticity budget from positive to zero or even negative,representing the end of ERC.In addition,the range of the rapid filamentation zone is not consistent with moat.The filamentation time of the region near the inner eyewall in moat is basically less than 30 min,but there are still strong convections in this area.While the region near the outer eyewall has larger filamentation time and weaker upward motion.It is found that the formation and maintenance of moat are influenced by three factors,which are compensating subsidence induced by the inner and outer eyewall,the rapid filamentation and the vortex Rossby waves(VRWs)propagation.The compensating subsidence can suppress the upward motion and rapid filamentation can stretch the convection,both of which are not conducive to the development of the lower-level convective activity.On the contrary,VRWs can bring the convection to moat by propagating out ward from the primary and secondary eyewall.The interactions between the three mechanisms have great impact on moat.