[Objective] The research aimed to analyze the disastrous weather of serious drought in the northwest of Hunan in summer and au tumn of 2009. [ Method] According to the meteorological data in Zhangjiajie in the northwest of Hunan during the drought period from June to September in 2009, the disaster characteristics of continuous drought in summer and autumn were analyzed. Based on NCEP/NCAR 2.5°× 2.5° reanalysis data, by using the climatic diagnostic method, the formation reason of serious drought was analyzed initially from die circula tion characteristics in the middle and high latitudes, Western Pacific subtropical high, the abnormal characteristics of sea surface temperature in the equatorial mid-eastern Pacific Ocean and the tropical system activity. [ Result] The characteristics of serious drought in the northwest of Hunan in summer and autumn of 2009 were the quick developed speed, wide influence range, long duration, big disaster loss and long high temperature time. The influence range, duration and harm degree were rare in the history. During the arid period (June-September), the at mospheric circulation was abnormal. The polar vortex in the northern hemisphere was weak, and the center was by north. It was two-trough one-ridge type in the middle and high latitudes of Eurasia. The long-wave trough existed respectively from Lake Toba to the east coast in China and from the Sea of Okhotsk to the east coast in China. The long-wave ridge maintained from Lake Baikal to Central Asia and stabilized in 90° - 110° E in Central Asia. From the middle dekad of June to the middle dekad of September, the westerly index increased. The zonal circula tion was the main one in the middle and high latitudes of Eurasia. The cold air in the high-latitude frontal zone spread eastward with the small amplitude fluctuation form along the latitude circle direction and was difficult to pass the westerly barrier near 45° N to reach the low latitude. Meanwhile, Western Pacific subtropical high jumped northward to control Jiangnan and South China for a long time. The down airflow was prevalent. It was hot and rainless. The drought developed quickly. The sea surface temperature in the equatorial mid-eastern Pacific Ocean started to rise in June, and it entered into El Nino state. When the obvious temperature increasing of El Nino event started to appear in spring and summer, the plum rain amount was less in the middle and low reaches of Yangtze River in the year when El Nino broke out or in the next year. The probability was 80%. In El Nino year, the typhoon was less. In addition, for the influence of strong Western Pacific subtropical high, the landing pathway of typhoon was by east or south. The kind of typhoon had the small role for easing the drought in the northwest of Hunan. [ Conclusion ] The research provided the theory basis for improving the prediction level of short-term climate and the understanding of extreme climate event.%[目的]分析2009年湘西北夏秋严重干旱灾害天气.[方法]根据2009年6～9月湘西北张家界干旱期间的气象资料,对这次夏秋连旱的灾害特点进行了分析；并利用NCEP/NCAR 2.5°×2.5°再分析资料,采用气候诊断方法,从中高纬环流特征、西太平洋副热带高压及赤道中东太平洋海袁温度异常特征、热带系统活动等方面,对这次严重干旱的成因进行初步分析.[结果] 2009年湘西北夏秋严重干旱的特点是发展速度快、影响范围广、持续时间长、受灾损失大、高温酷暑时间长,其影响范围、持续时间及危害程度历史上罕见.干旱期间(6～9月)的大气环流异常,北半球极涡偏弱,中心偏北,欧亚中高纬为两槽一脊型,巴湖附近和鄂霍次克海到我国东部沿海各有一长波槽,而贝加尔湖至中亚地区有长波脊维持；长波脊稳定在90°～110°E中亚地区.6月中旬～9月中旬,西风指数向高指数调整,欧亚中高纬以纬向环流为主,在高纬度锋区上的冷空气是沿纬圈方向以小振幅波动形式向东传播,很难越过45° N附近这一西风屏障到达较低纬度；与此同时,西太平洋副热带高压北跳长期控制江南和华南地区,盛行下沉气流,酷热少雨,导致干旱迅速发展.6月开始赤道中东太平洋海温升高进入厄尔尼诺状态,凡是在春、夏季开始出现明显增温的厄尔尼诺事件,在厄尔尼诺爆发的当年或次年长江中下游梅雨量一般以偏少为主,偏少概率80％.厄尔尼诺年台风生成个数偏少,再加上受强大的西太平洋副热带高压影响,台风登陆的路径偏东或偏南,此类台风对缓解湘西北的干旱作用不大.[结论]该研究为短期气候预测水平和极端气候事件认识的提高提供理论依据.
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