Climatology and Statistical Analysis of Unusually Developing Cyclones over Japan and its Surrounding Area

摘要

Recently, it frequently occurs that extratropical cyclones over Japanese islands and its surrounding area develop unusually in Spring and Autumn. It causes natural disaster and gives the damage to human activity with heavy precipitation and strong winds. For the reason of climatology, it is because North-South movement of jet streams which affect the formation of extratropical cyclones get intense by the expansion of subtropical high and advection of cold air in these seasons. Therefore, the main purpose of this study is to clarify the causation of these cyclones on the basis of fluctuation of the general circulation of atmosphere. In this paper, unusually developing cyclones are defined as the central pressure deepened to less than980 hPa, using synoptic weather chart provided by the Japan Meteorological Agency（JMA）. These cyclones are classified into three types, depending on the positions of the formation and cyclone tracks. In order to research the characteristics of the developing process respectively, upper troposphere pressure field, zonal wind field, and relative vorticity on the scale of East Asia were analyzed, using global reanalysis daily average dataset based on NCEP／NCAR. The results are summarized as follows, 1）According to the statistical analysis, unusually developing cyclones are more frequent recently. Especially, March is the highest in the probability where an extratropical cyclone develops unusually. 2）Nihon-Kai low type（N-type）has the smallest level of the deepening of a depression in three types. This type showed that a positive relative vorticity in upper troposphere（200hPa）contributes to the development of cyclones. 3）On the other hand, low passing along the south coast of Japan type（S-type）are formed by baroclinically unstable wave of the subtropical jet stream. In this type, subtropical jet stream and polar-front jet stream converge by the ridge（around105°E）in the west of Japanese islands. Consequently, vortices related cyclogenesis in the lower troposphere propagated to upper troposphere with developing cyclones. 4）Additionally, double-cyclone type（NS-type）develops by the coupling of the Nihon-Kai low and the low passing along south coast of Japan over Japanese islands. It has the largest level of the development and the most appearance days. As for this type, subtropical jet stream flowing at the high latitude side（around35°N） intensified by subtropical high. Further, polar-front jet stream meanders to the vicinity of Japan（around30°N） influenced by the trough at500hPa geopotential height field, and converge with subtropical jet stream. As a result, the lower tropospheric vortices with the low passing along south coast of Japan couple it with the upper tropospheric vortices with the Nihon-Kai low, and such a structure had an impact on developing cyclones unusually.