Study Of Extratropical Transition Of Tropical Cyclones Over The Western North Pacific

The process of extratropical transition (ET) of tropical cyclones (TCs) overwestern North Pacific often produce intense rainfall, strong winds and hazardous seas,which cause significant financial damage to marine and land activities. Explosivedevelopment of precipitation and wind patterns associated the complex physicalprocesses during ET poses a significant challenge to both researchers and forecasters.Thus figuring out the evolutions of TC structures during ET, determining the ET onsetand completion times have important values in both academic and forecasting field.In this paper, the ET process of Typhoon Mindulle (2004) is analyzed. Mindulleinteracts with a northwestern trough in the upper level, dry and cold air from thenorthwest intrudes the cyclone center with the cyclonic flow, and gradually decays thewarm core. The thermal structure of Mindulle becomes hybrid, and warm-occludedlike frontogenesis patterns presented. The divergence in the upper level and thepositive vorticity advection contribute to the development and maintenance of theupdraft. The developed extratropical cyclone re-intensifies due to the upper-levelpositive potential vorticity advection to its west.Four methods, i.e., Open Wave (OW), scalar frontogenesis parameter (SFP),cyclone phase space (CPS) and isentropic potential vorticity (IPV) method, fordetermining the extratropical transition (ET) onset and completion times of TyphoonsMindulle (2004) and Yagi (2006) based on four datasets, i.e., JRA-25, FNL, RSM-20km, and WRF simulated results, which differ in sources and have different horizontalresolutions were examined in this study. The OW and SFP methods failed to smoothlyand consistently determine the ET completion by using the four data sources.Although the CPS technique succeeded in determining the ET onset and completiontimes, it appeared to be sensitive to the data sources in the case of Yagi. The IPVmethod also suggested the ET onset times of Mindulle and Yagi using all the datasets,whereas the determined ET onset time of Yagi strikingly differed from that by theCPS method, which may creates forecast uncertainty. And IPV method failed indetermining the ET completion time based on four datasets. For the slight-reintensifi-cation case of Mindulle, all the methods are not sensitive to different data sources.However, for the rapid-reintensification case of Yagi, the methods are seemingly ofsignificant sensitivity.The features of ET TCs over western North Pacific during2001to2007areanalyzed based on the RSM-20km datasets. According to the revolution of925-hPamaximum wind, ET TCs are divided into four types as intensified, weakened, rapidly weakened and the others. Based on IPV method, the track, intensity, spatial andtemporal characteristics, along with the IPV features are discussed. It is found thatintensified, weakened and rapidly weakened types have different locations of IPVsensitive zone in the upper level (at330K,335K and340K, respectively). The ETcompletion time failed to be determined since the averaged330-K IPV within500-kmradius cannot reach1.6PVU in each type. And the315-K IPV demonstrated aconsistent increase thus cannot indicate the development of cyclones. As the averagedfield reduced, it is found that the330-K IPV averaged within500-or400-km radiuscan be used to determine the ET onset time, and the315-K IPV averaged within300-km radius is more likely to forecast the intensity evolution of cyclones.