高空槽对9711号台风变性加强影响的数值研究

NUMERICAL STUDY ON IMPACTS OF UPPER-LEVEL WESTERLY TROUGH ON THE EXTRATROPICAL TRANSITION PROCESS OF TYPHOON WINNIE(1997)

9711号台风Winnie是一个在中国大陆长久维持(2—3 d)并产生强降水的热带气旋(TC),在其深入内陆过程中变性加强为一个温带气旋。用MM5V3对不同强度高空槽影响下Winnie的变性加强过程进行了数值研究。结果表明:(1)Winnie变性加强过程表现为强锋面侵入台风内部、冷空气包裹台风中心、一个温带气旋在近地层锋面上强烈发展的过程;(2)Winnie在陆上的变性加强与西风带高空槽的强度密切相关。TC与不同强度高空槽相互作用过程中,较深槽携带较强冷平流、正涡度平流以及较强的槽前高空辐散,从而有利于TC的维持和变性发展。数值试验中,高空槽越强,Winnie变性加强越明显,温带气旋的发展越快;(3)模拟结果的位涡分析表明,Winnie的温带变性发展与对流层高层正位涡下传、低层锋区和TC低压环流三者之间的相互作用有关。

Extratropical transition （ET） is a special process for a tropical cyclone to obtain the baroclinic energy. Previous studies demonstrated that the ET process of tropical cyclones is closely related to baroclinic waves including upper trough, front zone etc. The baroclinic potential energy can be converted into TC kinetic energy through the interaction between TC and baroclinic systems. This is the important way through which a TC depression intensifies again during its ET process. Typhoon Winnie made landfall in Zhejiang province of China on 18 August 1997. It sustained over land and produced heavy rainfall in the next 3 days because of its transformation and re-intensification process. In this study, we modified an upper-level westerly trough with a different intensities using the Legendre scale filtering method, then used the mesoscale model MM5v3 to examine the effects of different modified troughs on the ET process of Winnie. The results indicate that the upper-level trough played an important role in the ET and re-intensification process of Winnie, in which a cold front intruded the TC circulation, cold air cyclonically wrapped up the TC center and an extratropical cyclone developed distinctly on the front zone in lower levels. The ET process is sensitive to the intensity of the upper trough, which affected the ET process in terms of temperature advection, vorticity advection and the upper level divergence ahead of the trough. The stronger the upper trough is, the faster the TC re-intensifies. Besides, the PV analysis of simulation results show that the ET and re-intensification of Winnle was related to the interactions of the TC circulation with the lower layer front, and the downward transportation of PV from the upper troposphere cold air on the ET process need farther study in details. However, the impacts of the degree and range of