A Study on Typhoon-Induced Rainfalls over Beijing:Statistics and Case Analysis

Based on the Shanghai Typhoon Institute （STI） Typhoon Year t3ook and typhoon precipitation data, Japan Meteorological Agency satellite TBB data, and National Centers for Environmental Prediction （NCEP） reanalysis data, the climatic characteristics of rainfalls in Beijing associated with typhoons were analyzed for the period 1949-2006, and two typhoon cases with remarkable differences in rainfall intensity over Bei- jing were compared and diagnosed. The 58-yr statistical results show that rainfall events associated with typhoons occurred in Beijing about once every three years during June-September. These typhoons were mainly active in the region 20°- 50°N, 109°- 128°E and most of them moved northwestward while the others turned to the northeast. The typhoon rainfall over Beijing in general sustained for 2 5 days. Typhoon centers were usually located in the areas from Jiangxi to Anhui, the Yellow Sea, or near Beijing, when rain- storms occurred over Beijing. Case study indicates that the 2-day torrential rainfall event that happened in Beijing in 1984 was due to the interaction between Typhoon Freda （8407） and a westerly trough, while only a medium-strength rainfall event occurred in Beijing in 2005 in spite of the dominating cyclonic circulation of Typhoon Matsa （0509） directly over Beijing. It is found that both Preda and Matsa underwent extra- tropical transition and possessed an asymmetric structure. The rainfall difference was caused by the fact that Beijing was located in different convective development areas of the two typhoons. On the other hand, the lifting conditions were different although plentiful atmospheric moisture and convective unstable energy existed over Beijing during both events. The ascending motion of warm southerly in Beijing was stronger and deeper and a larger vertical wind shear was associated with Typhoon Freda. However, the lifting of water vapor was restrained by the descending motion of northerly cold airflow in Beijing under the impact of Typhoon Matsa. Besides, it is also found that topography played an important role in the typhoon rainfall over Beijing.

北京地区的台风降水特征研究

采用上海台风研究所1949—2006年台风降水和台风年鉴资料、中国FY-2卫星和日本气象厅TBB资料以及NCEP再分析资料,首先分析58年台风影响北京降水的气候特征,然后对8407号台风Freda和0509号台风Matsa影响下北京两次强度差异显著的降水过程进行对比研究。结果表明:(1)台风影响北京降水年均0.33次,出现在6—9月。降水过程多为大雨以上天气过程,持续时间一般2 d,最长可达5 d。(2)影响台风的活动范围大致为(20°—50°N,109°—128°E)。路径主要包括西北行和转向东北行两类,并以前者居多。北京发生暴雨时,台风中心主要出现在江西一安徽一带、黄海或北京附近。(3)Freda对北京的影响发生在台风与西风槽相互作用过程中,而Matsa的影响表现为台风低压环流直接控制北京。两个台风均受冷空气影响而变性,具有明显的非对称结构,两次过程中北京位于台风不同的对流运动发展区域是造成降水强度差异的原因之一。(4)两次台风过程中北京均具有较好的水汽条件和对流不稳定层结,但动力抬升条件差异明显。Freda影响下北京具有较强的水平风垂直切变,偏南暖湿气流上升运动深厚。而Matsa影响下北京盛行偏北气流,中低层下沉运动显著,水汽抬升受到抑制。另外,北京西北高东南低的地形也是其台风降水产生差异的原因之一。在台风东侧,地形作为偏南气流的迎风坡可加强上升运动,而在台风西侧则作为偏北气流的背风坡增强下沉运动。

我国秋季台风与相邻气团相互作用的天气气候特征分析

利用中国气象局1949—2007年台风年鉴、我国大陆和海上相关气象测站资料,以及密集性和连续性程度较高的民用机场气象资料,通过气候统计方法和重点个例分析,从气候学和天气学2个方面对我国秋季台风及其与相邻气团相互作用的天气气候特征进行了分析。结果表明,受到季节进程影响,我国的秋季台风中强台风和超强台风的发生频次较多,但登陆频次较少,且台风越强,其登陆概率越低,在近海发生转向的可能性也越大;除了登陆频次的变化,秋季台风登陆点的位置也随时间有明显的变化,其登陆纬度随着时间推移逐渐南移,而未登陆台风的转向点随时间变化也逐渐南移。不同强度的台风在靠近我国东南沿海时对大陆气团的影响有显著不同,大陆气团、海上气团与台风自身环流的相互作用最终影响到台风的移动方向。