2011年秋季强台风“纳沙”导致广西灾害成因分析

Analysis on Disaster Causes of Strong Typhoon Nesat in the Fall of 2011

2011年第17号强台风"纳沙"于9月30日20时在越南北部减弱为热带低压后,其中心并没有深入到广西内陆却给广西带来持续3d的暴雨天气,导致全区受灾人口323.15万,因灾死亡7人;农作物受灾面积38.265万hm2,直接经济损失高达26.88亿元。利用MICAPS常规观测资料、自动站资料、卫星云图以及ECMWF等资料,对1117号强台风"纳沙"减弱后导致广西严重灾害的成因进行了分析。结果表明:①"纳沙"进入北部湾并在越南北部沿海登陆减弱为热带低压后,青藏高原东移的冷空气从低层渗透进入华南西部,并与"纳沙"东部的偏南气流在桂南汇合,形成中尺度辐合线,使得广西南部的动力辐合和上升运动得以加强和维持;②"纳沙"东部强盛的东南风急流将南海大量的水汽输送到广西上空,为持续性暴雨提供充足的水汽;③"纳沙"减弱消亡后,其外围环流位于广西南部上空的垂直风切变小,在这种环境条件下,有利于热量、水汽的积聚和辐合上升运动,有利于对流云团的发生发展并释放大量的凝结潜热,对暴雨的增幅也有重要的正反馈作用。

After making landfall over the northern part of Vietnam and weakening to be the tropical low pressure at 20 BT 30 September 2011,the strong typhoon Nesat,brought about 3-days continuing torrential rain to Guangxi area,with its center not reaching the inner of Guangxi province,and caused 3 231 500 of disaster population,7 deaths,382.65 hm2 of disaster crops,and 2.688 billion RMB of directed economic losses.By using traditional observation data on MICAPS,meteorological auto-station data,satellite images and data from ECMWF,analysis was made to the severe disaster causes of the No1117 typhoon Nesat after weakened.It shows that ① Nesat entered into the Beibu Gulf and then weakened to be the tropical low pressure after making landfall over the northern part of Vietnam.The eastward cool mass from Qing-Zang Plateau penetrated the western area of south China in the low layer,and met with the south airflow on the east side of Nesat in the southern part of Guangxi,thus the mesoscale convergence line was formed,promoting and keeping the dynamic convergence and rising flow in the southern part of China.②The strong southeast stream on the east side of Nesat transferred large amount of moisture to the upper of Guangxi,providing sufficient vapor for the continuing torrential rain.③As Nesat weakened and dissipated,the vertical windshear grew small over the southern Guangxi on the outer circulation of Nesat,which favored for the accumulation of heat and vapor and the converging rising,as well as developing of the convective cloud cluster and the releasing of large amount of latent heat from water condensation,resulting in the important positive feedback of the amplification of heavy rain.