中文摘要：

选用陕西51个气象站1962-2008年3-5月降水量数据和NCEP/NCAR再分析资料,根据综合干旱指数确定了陕西春季干旱等级和持续时间,利用合成分析的方法分别对陕西春季干旱和春末夏初季干旱年的大气环流进行研究,并与多雨无旱年的大气环流特征作对比分析。结果表明,陕西春季干旱持续时间长,呈北强南弱的不均匀分布模态。陕北长城沿线风沙区、关中平原和秦巴山区东部是春季易发生干旱的关键区。陕西春季干旱年500 hPa环流形势表现为欧亚中高纬度距平场纬向分布呈现＂-＋-＂,中亚高压脊偏强,东亚大槽明显偏深,表现出冷暖空气交汇区偏南的冬季风环流特征;相反春季多雨年中纬度盛行纬向气流,东亚大槽偏浅且位置偏东,咸海、里海附近多低压系统活动,南支西风带中孟加拉湾长波槽偏深。春末夏初季影响陕西干旱的环流表现为＂-＋-＋＂的距平分布,中亚高压脊偏强,中纬度盛行纬向气流,冷空气多偏北路径向东南移动,西太平洋副高偏弱,位置偏东偏南;相反春末夏初季多雨年乌拉尔山高脊偏强,贝加尔湖多长波槽活动,同时西太平洋副高偏强,对流层低层青藏高原东侧南风较常年同期偏强。最后相关分析表明陕西春季干旱指数与前期冬季乌拉尔山附近高度场有显著负相关,前期冬季乌拉尔山高脊偏强（弱）,陕西春季易发生干旱（多雨）。

英文摘要：

Using the observed precipitation data from March to May during the past 47 years in 51 stations in Shaanxi,as well as the NCEP/NCAR reanalysis data,this paper determines the criterion of years with spring drought and rainy years in Shaanxi by the Synthetical Drought Index,and analyses compositely different atmospheric circulations in years with spring and early summer droughts and rainy years in Shaanxi.The results display that the Shaanxi spring droughts are long-lasting and more droughts happen in northern part than in south,showing inconsistent distribution.There are some key areas with more spring droughts happen,which are the soil erosion areas along the Great Wall over the northern part of Shaanxi,Guanzhong Plain and Qinba mountainous areas.During the years with spring drought,＇-＋-＇ height anomaly distribution in mid-high latitudinal of Eurasia at 500 hPa field reveals that mid-Asian ridge and east-Asian trough are much intense,which results in the significant winter monsoon circulation with southern area of warm and cold air;On the contrary in the years with rainy spring,the atmospheric circulation is latitudinal,and the east-Asian trough is weaker and lies in more east area.At the same time,there are more lows in Caspian Sea and Aral Sea,with intense trough in the Bay of Bengal.During late spring and early summer,the height anomaly is ＇-＋-＋＇,which means that more latitudinal circulation with stronger ridge in mid-Asia.As a result,the cold air can move toward southeast by the northern way and the west Pacific high is less strong and lies in its south-east;Oppositely,during the rainy years in late spring of Shaanxi,the Ural ridge is more intense,and more troughs active in the Lake Baikal area.At the same time,the west Pacific high is much stronger,and stronger south wind at the lower troposphere is more prevail than in normal times.At last,the negative correlation between spring droughts and the preceeding-winter height distribution displays that if the Ural ridge is more intense（weaker）,S