With the use of data from the National Centers for Environmental Prediction Climate Forecast System Reanalysis,the environment and structure of typhoon Toraji(2001)are investigated during the re-intensification(RI)stage of its extratropical transition(ET),a process in which a tropical cyclone transforms into an extratropical or mid-latitude cyclone.The results provide detailed insight into the ET system and identify the specific features of the system,including wind field,a cold and dry intrusion,and a frontal structure in the RI stage.The irrotational wind provides the values of upper-and lower-level jets within the transitioning tropical cyclone and the cyclone over Shandong Peninsula,accompanied with the reduced radius of maximum surface winds around the cyclone center in the lower troposphere.Simultaneously,dry air intrusion enhances the formation of fronts and leads to strong potential instability in the southwest and northeast quadrants.The distribution of frontogenesis shows that the tilting term associated with vertical motion dominates the positive frontogenesis surrounding the cyclone center,especially in the RI stage.The diagnostics of the kinetic energy budget suggest that the divergent kinetic energy generation whose time evolution corresponds well to that of cyclone center pressure is the primary factor for the development of Toraji in the lower troposphere.The ET of Toraji is a compound pattern that contains a development similar to that of a B-type extratropical cyclone within the maintaining phase and an A-type extratropical cyclone within the strengthening period,which corresponds to the distribution of the E-P fluxes with vertically downward propagation in the maintaining stage and upwards momentum in the strengthening phase.
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