登陆热带气旋维持的次天气尺度环流特征

THE CHARACTERISTICS OF SUB-SYNOPTIC SCALE CIRCULATION OF TROPICAL CYCLONES SUSTAINING OVER LAND

971 1 (Winnie)和 94 0 6 (Tim)是 2个在中国登陆后长久维持 (2～ 3d)并产生严重降水的热带气旋。在深入内陆过程中 ,Winnie变性并再次加强为一个温带气旋 ,Tim则衰减消亡。采用移动坐标以及云顶亮温 (TBB)和常规资料 ,对它们在陆上维持期间的次天气尺度环流特征及其与次天气尺度系统之间的相互作用进行对比分析和诊断。结果表明 :(1 )Winnie和Tim登陆过程中具有螺旋波状的次天气尺度云系 ,这种螺旋波结构有利于热带气旋的维持。陆上维持后期 ,Winnie的螺旋云系继续维持并有所加强 ,Tim的螺旋云系趋于零散、消亡。Winnie的螺旋波特征比Tim明显 ,因而维持更长时间 ;(2 )热带气旋次天气尺度环流也具有环绕热带气旋中心的明显波动特征 ,此波动结构能否维持与热带气旋活动的天气尺度背景密切相关。在与西风槽相互作用过程中 ,Winnie的次天气尺度波动结构维持 ,而Tim的次天气尺度波动结构遭到破坏 ;(3)作为登陆后长久维持热带气旋 ,Winnie和Tim均从次天气环流中获得正涡度 ,有助于维持其气旋性环流 ,但Winnie获得的正涡度比Tim明显的大。此外 ,Winnie从次天气尺度环流中获得动能补充 ,Tim则只有次天气尺度环流对其动能的耗散。因此 。

Both of typhoon Winnie(1997) and Tim(1994) sustained over land for a long period (2-3 days) and caused severe rainfall after they made landfall. The former developed distinctly with its extratropical transition(ET)process while the latter dissipated gradually after it entered into the mid-latitude region. Scale discretion of T BB 1-degree grid data and NCEP reanalysis 2.5-degree grid data analysis was performed with filtering scheme. Then the sub-synoptic scale features of Winnie and Tim were compared based on movable coordinate. Besides, the conversion of kinetic energy and vorticity between tropical cyclone (TC) and sub-synoptic scale systems (SSS) were calculated on TC area mean basis. Results showed that the intensity change of TC over land was closely related to the SSS in TC circulation. T BB disturbances displayed a clear spiral structure of cloud bands in TC circulation which was favorable to sustain TC intensity. After landfall, Winnie's spiral structure was asymmetric at beginning but returned to symmetric structure when it was re-intensified. Tim's cloud bands were broken after landfall, and its spiral structure of the cloud bands loosed gradually. The sub-synoptic scale stream fields exhibited a kind of wave structure (WS) in TC circulation. It was found that the interaction between TC and westerly trough could stir strong disturbances on the periphery of TC. For Winnie, the disturbances located northwest quadrant and led to strong mesoscale wave trains through TC that strengthened its WS. As to Tim, the strong wave trains located its east area that destroyed its WS. The results of the kinetic energy and vorticity conversion between TC and SSS demonstrated that SSS provided kinetic energy for the re-intensification of Winnie but consumed kinetic energy from Tim. Besides,Winnie obtained positive vorticity from SSS more than Tim did.