Effects Of The Air-Sea Interaction On The Secondary Eyewall Formation And Evolution Of Typhoon Sinlaku(2008)

The concentric eyewalls(CEs)are very common features in intense tropical cyclone(TCs).The secondary eyewall formation(SEF)and the eyewall replacement cycle(ERC)will lead to sudden changes in the intensity and structrues of TCs,which increase the difficulties in forecasting the intensity and precipitations of TCs.The air-sea interaction is an important factor which will influence the TCs' development.The cold wake and strong waves caused by TCs directly affect the energy exchanges betweem the ocean and the air.To explore the air-sea interaction on the CEs structure,an ERC of Typhoon Sinlaku(2008)was reproduced by using the high-resolution coupled ocean-atmosphere-wave-sediment transport(COAWST)model.The numerical simulations were evaluated with a wide range of observational datasets.Moreover,the importance of the ocean to ERC is discussed by comparing four sensitive experiments,i.e.,two uncoupled experiments with different time-invariant sea surface temperatures(A_FNL_SST and A_HYCOM_SST),an ocean-atmosphere coupled experiment(OA),and an ocean-atmosphere-wave fully coupled experiment(OAW).The causes of SEF and characteristics of ERC under different sea conditions have been compared and anylayzed according to the simulations.On this basis,the paper is mainly focused on the response of the sea surface temperature(SST),waves and their feedbacks to TCs.At the last,conceptual models of the SEF and ERC were built when the air-sea interaction was considered.The results of sensitivity experiments indicate that the structure and strength of the CEs are very sensitive to the variation of the underlying ocean.The SEF and ERC of Sinlaku are successfully simulated by all of the four experiments.Among them,the OAW experiment has the best performance that the TC track and intensity are most closed to the TC best track datasets and the aircraft observations.The OAW experiment also successfully reproduces the ERC of Sinlaku which lasted for 31 hours.Unlike the classical conceptual model of CEs,the outer eyewall simulated by OAW experiment failed to contract inward after the SEF;instead,it always remains within the radius of 90-110 km.The CEs simulated by the OA experiment which only considered the cooling of ocean was weak,so that the inner eyewall faded quickly,and the ERC just lasted for 20 hours.If the oceanic effects are not taken into consideration,the CEs simulated by the single atmospheric model are too strong,with a shorter ERC demonstrating more deviation from the reality.According to the analysis of the TC axisymmetric dynamics methods,the development of the SEF and ERC can be summarized as follows.Before the SEF,there is a growth and development of the outer rainbands above the SEF region,and the transverse circulation was triggered by the latent heat released which would invigorates the enhancement of the radial inflow.On one hand,the enhanced inflow increases the radial gradient of equivalent potential temperature,causing the loacal warming up of the SEF region,exacerbating the thermal instability and lowing the level of free convections,which is conductive to the convections downward from the upper air to the boundary layer.On the other hand,the enhanced inflow can enhance the outward expansion of the tangential wind.Finally,the outer eye wall forms.In this study,we find that the supergradient wind above the boundary layer is the dynamic responses to the SEF.The time of the sudden change of the supergradient wind has nothing to do with when the second maximum value of tangential wind forms,but with when the convections extending to the ground.After the SEF,the outer eyewall can obtain more energies from the ocean due to its intense winds.At the same time,the supergradient wind is instrumental in the strong convergence and upward motion within the boundary layer,releasing more diabatic heating to the atmosphere,which can promote the development of the secondary eyewall.With the development of TC,the outer eyewall contracts inward and the downdraft area(moat area)between the CEs become narrow.The vertical development of the enhanced outer eyewall not only cuts off the high entropy supply of the inner eyewall,but also inhibits the ventilation effet of the upper layer.Therefore,the inner eyewall disappears rapidly which means the ERC ends.When the ocean response is considered,the ocean affects the characteristics and the maintenance of the ERC significantly,although the physical mechanism of SEF does not change.The strong wind stress and the ocean turbulence kinetic energy caused by TCs mainly focus below the eyewalls.Affected by the Coriolis force and TC movement,the velocity of surface ocean current behind Sinlaku is stronger than the other quadrants.Under the combined action of the horizontal advection and the vertical advection,the ocean cooling in the moat area is significant,which directly influences the formation position and intensity of the outer eyewall.After the SEF,two SST cooling centers caused by the ocean entrainment effect and a large area of the Ekman pumping velocity appear,which reinforce the extent and scope of the could water upwelling.In the late stage of the simulation,a strong cold wake exists below the whole TC circulation.The cold wake can reduce the transport of the sensible heat flux and latent heat flux from the ocean to the air,and decrease the total energy of the TC,therefore the outer rainband in the coupling experiment is weak and has a limited inhibition effect on the inner eyewall.It also can reinforces the stability of the local boundary layer and inhibits the occurrence and the development of convections.Therefore,the outer eyewall generates at the edge of the strong SST cooling area.In addition,the asymmetric distribution of the TC convections become more obvious due the cold wake.The convections are mainly concentrated in the front side of the TC track.The weak convections behind the TC help the radial inflow enhance,which provide more energy for the inner eyewall.Therefore,compared with the atmospheric experiment,the ERC in the OAW coupling experiment lasts longer.When the ocean waves are considered,the time of ERC duration is extended by approximately double length.The analysis indicates that the energy supplied by the ocean to the inner eyewall is the reason why the ERC endures for a relatively lengthy time in the OAW experiment.After SEF,under the nonlinear interaction of the ocean waves,the maximum value of the significant wave height is mainly located in the moat area between the two CEs.Owing to the presence of high waves and low wave steepness,even in the eye of TC and the moat region where the wind is weak,the sea surface heat exchange coefficient can well be maintained at a higher value,which is conductive to the heat enthalpy fluxes,and to slow down the dissipation of the inner eyewall.When the cold wake is strong enough,the energy gained by the inner eyewall from the ocean can not continued to support the development of the weak convections,the inner eyewall disappears.