A study on rainfall features of Beijing assoc iated with typhoons.

Based on “Typhoon Year Book” data, typhoon rainfall data from 1949 to 20 06, satellite TBB and NCEP reanalysis data, the climatic characteristics of Beij ing rainfall associated with typhoons were analyzed. Two typhoon cases with remarkable difference in rainfall rate over Beijing were compared and diagnosed . Statistical results show that a rainfall event associated with typhoon occurre d in Beijing about every three year during June to September averaged on 58 years. Those typhoons mainly occurred in the region of （20°-50°N, 109-128°E）and most of them moved northwestward while the others turn to the northeast. The typhoon rainfall in Beijing sustained 2 days in general and 5 days the longest duration is. The typhoon centers usually appeared in the areas of Jiangxi and An hui, Huanghai sea or near Beijing when rainstorms occurred over Beijing. Compari son study indicates that 2day torrential rainfall event happened in Beijing un der the interaction between typhoon Freda (8407) and westerly trough, while only general rainfall occurred in Beijing though dominated by the low circulation of typhoon Masta (0509). It is found that both of Freda and Masta underwent extrat ropical transition process, which resulted in typhoon asymmetric structure with the heavy rainfall developing in the northern and eastern sides of typhoon center. That contributed to the distinct rainfall deference in Beijing, which located in different areas of the two typhoons. On the other hand, the lifting conditio ns for precipitation were different in spite of plenty water moisture and convec tive unstable in Beijing. The ascending motion of warm southerly was strong and deep under the strong wind vertical shear associated with typhoon Freda. However , the lifting of water vapor was restrained due to the descending motion of nort herly cold airflow under the impact of typhoon Matsa. Besides, the Beijing topog raphy also played an important role in the rainfall difference between the two t yphoons. The southerly (northerly) would be lifted (sunk) in typhoon eastern (western) part under the topographic effect of Beijing with mountainous area in its northwest and plain in its southeast.