Tropical cyclone Tracks and Motions characteristics over the western North Pacific

• Main Authors: Yen-Ta Fu, 傅彥達
• Other Authors: Ming-Cheng Yen
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/25843141789703618365

碩士 === 國立中央大學 === 大氣物理研究所 === 102 === Previous studies had shown that roughly more than 80% of tropical cyclones/ tropical depressions(TC/TDs) develop in the monsoon gyres over the western North Pacific and the activity of the monsoon gyres is associated with MJO (30-60 days oscillation) development. In addition, MJO also affects the track of TC/TDs, more recurving (straight moving) TC/TDs occur during MJO low (high). When the recurving TC/TDs go northward, they will be accelerated by the mid-latitude westerlies. Thus, one may wonder: Whether or not the fast moving TC/TDs are always recurving? Using the 1979-2012 JTWC TC/TD best track dataset, we found that the top 10% fast moving TC/TDs are recurving, while the last 10% TC/TDs which have slow moving speed cluster in the South China Sea. Based upon this preliminary result, two major issues are raised: (1)Why the slow moving TC/TDs do not go northward and eventually embed in the mid-latitude westerlies, but the fast moving TC/TDs do? (2)Why the initial speed is significantly higher for fast moving TC/TDs than those slow ones immediately after they are formed? In order to answer these two concerns, the NCEP-DOE Reanalysis-2 data are used for the composite analysis to examine the environmental differences between the fast and slow moving types of TC/TDs circulations. It appears that an anomaly low located between south of Japan and east of Taiwan is conducive to both the further development and northward migration of the fast moving TC/TDs. In contrast, the anomaly low is confined to the South China Sea and west of Philippines with an anomaly high toward its northeast side for the slow moving TC/TDs situation so that the slow moving TC/TDs are limited in this south region. To better understand the dynamic mechanism causing the initial speed differences between the fast and slow moving TC/TDs, the vorticity budget analysis is applied to evaluate the contribution of each term in the equation from two days before to two days after TC/TDs genesis. It is interesting that the time series of vorticity tendency for the fast moving TC/TDs show significant increase whereas the slow moving ones decrease gradually during these five-day initial stages. After careful investigation, it turns out that the advection term, not only the horizontal distribution but also the time variation, is similar to tendency term. Apparently, the differences in initial speed between fast and slow moving TC/TDs are dominated by the advection effect.