1949—2007年登陆我国变性热带气旋的特征统计及合成分析

A STATISTICAL STUDY AND COMPOSITE ANALYSIS ON THE CHARACTERISTICS OF THE EXTRATROPICAL TRANSITION OF LANDFALL TYPHOONS DURING 1949—2007

利用1949—2007年热带气旋年鉴等资料,统计分析登陆中国并在我国或附近变性的热带气旋的特征,着重分析不同区域及路径的变性气旋特征。分析结果得出,登陆我国的热带气旋很多,但在我国及附近沿海变性的却较少,大多数热带气旋变性后不发展或迅速衰减,只有约17%的变性热带气旋继续发展。热带气旋变性大多发生在114°E以东,移动方向为东北;其登陆地点与路径有一定关系,在台湾登陆后又在福建或浙江登陆的变性热带气旋往往会到达我国北方地区,在广东登陆的变性热带气旋移不过32°N。由合成分析得出,热带气旋变性前24小时,处在南亚高压脊线上,所有变性热带气旋均处在200 hPa的西南气流中,其北部有西南风高空急流,移向一般随南亚高压的中心移动。热带气旋变性后南亚高压一般迅速减弱。在广东登陆的陆地变性的热带气旋在变性前往往已经大大减弱。北方登陆的热带气旋的涡度场与其他类热带气旋的涡度场有较大差异,在热带气旋中心西北侧有一个正涡度中心,在500 hPa热带气旋中心并不与涡度中心对应,可见这类热带气旋变性前较弱。与正涡度中心对应的低涡的吸引作用可能是北方沿海热带气旋登陆的主要原因,热带气旋变性前24小时较弱可能也是大陆涡旋能吸引热带气旋登陆的主要原因。从垂直结构上看,一般情况下西部的θse比东部的密集,北部的θse比南部的密集。在热带气旋东侧受副高影响,东部风速大于西部,南部风速大大小于北部。

Using the data of typhoons yearbooks for 1949-2007,this paper statistically studies a number of features of tropical cyclones which have landed in China and made transition in China or near it. Different areas and tracks of the tropical cyclones are analyzed. The results showed that among the many typhoons landing in China, few experienced extratropical transition near its coast. Most of them stopped developing or weakened rapidly while 17% of them continued to develop after transition. The majority of typhoons experienced extratropical transition to the east of 114 ~E, and then moved to the northeast. The landing location had much to do with the tracks of typhoons. The extratropical transition（ET） of typhoons, which first landed in Taiwan, then in Fujian or Zhejiang, always went far to the north, and those landed in Guangdong cannot move more northward than 32 °N. From the composite analysis, we found that the typhoons were always on the ridge of South Asia High 24 hours before transition. All of them were in the Southwest jet on 200 hPa, while the southwest upper jet was to their north. The typhoons were always moving with the center of South Asia High. Generally, South Asia High reduced quickly after the transition. TCs which landed in Guangdong and made transition in land always greatly weakened before transition. There is a big gap in the vorticity field between TCs landing in the north of China and those landing elsewhere. There is a strong positive vorticity in the northwest of the TC but it is not corresponding to TC＇s center on 500 hPa.That means, this kind of TCs are weak before transition. The main cause of TCs landing in the north of China is the attraction mechanisms of vortexes. Its weakness 24 hours before transition may be the major reason that land vortexes attract these TCs to land. From vertical structures, it is known that θse in the west and north of the TC is denser than in the east and south. The wind speed in the east of TCs is faster than that of the west in part due to the effect of the Subtropical High.