中文摘要：

用气旋相空间方法（Cyclone Phase Space,简称CPS）对58个经历变性过程的西北太平洋热带气旋的2 021个样本（时次间隔为6 h）资料进行分析,得到该区域热带气旋在变性阶段的气旋结构及其环境特征。聚类分析结果显示,强的热带气旋更易发生变性,变性开始于气旋非对称性增加,随之伴有高层冷心出现和加强,最后完成于低层出现冷心,变性维持平均时间约28 h。利用美国国家环境预报中心（NCEP）再分析资料对各聚类环境场的动态合成分析表明,发生变性的热带气旋在北移过程中逐渐嵌入西风带,在中高纬斜压系统影响下变性为温带气旋。对于发生并完成变性的热带气旋,其西北侧始终有明显的位涡梯度存在,而且在环境中有明显的水汽输送通道。

英文摘要：

58 extratropical transition（ET） cases, including 2 021 observations（at 6-hour intervals）, over the western North Pacific are analyzed using cyclone phase space（CPS）, in an effort to get the characteristics of the structure evolution and environmental conditions of tropical cyclones（TCs） during ET over this area. Cluster analysis shows that strong TCs are more likely to experience ET. ET begins with the increment of thermal asymmetry in TCs, along with the generation and intensification of an upper-level cold core, and ends with the generation of a low-level cold core. ET lasts an average of about 28 hours. The tracks in the CPS of TCs which experience different structure evolution show that the CPS has good discriminating effect. Dynamic composite analysis of the environmental field of different clusters, carried out using National Centers for Environmental Prediction（NCEP） reanalysis data, shows that when TCs move northward, they are gradually embedded in the westerlies and gradually transform into extratropical cyclones under the influence of the mid- and higher-latitude baroclinic systems. As for those TCs which complete ET, there is always potential vorticity gradient in the northwest of them and obvious water vapor transport channels in the environment..