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利用西北地区中部55个站点1961-2015年夏季降水资料、NCEP/NCAR再分析资料及统计诊断方法,分析了西北地区中部夏季降水异常模态及同期大气环流特征和水汽输送条件.结果表明:西北中部夏季降水存在三类主要空间模态:全区一致型、东西反相型及类似“三明治”型.进一步分析表明,造成降水不同空间模态的环流和水汽条件不同:全区一致降水正异常时,副热带西风急流中心偏东,乌拉尔山阻塞高压偏强,水汽主要来源于孟加拉湾、南海和热带太平洋北上的水汽输送,同时在西北中部大范围地区有明显的水汽辐合;东多西少型降水正异常对应“丝绸之路”遥相关型环流,中亚增强的反气旋性环流提供了较强的冷空气,西北太平洋地区反气旋性环流南侧的偏东风输送水汽,在甘肃东部地区形成较明显的水汽辐合;类似“三明治”型降水正异常对应欧亚中高纬环流为北高南低型分布,贝加尔湖南侧低槽加强提供了西风异常,而水汽主要源自孟加拉湾向高原东侧的输送,在青海南部形成明显的水汽辐合,以致青南牧区降水偏多.综合分析表明,三种主要模态的冷空气条件和水汽来源具有明显差异.“,”Based on the 55 stations data in the middle of Northwest China and NCEP/NCAR reanalysis dataset,the dominant modes of summer precipitation anomalies over the middle of Northwest China and corresponded circulation characteristic,water vapor transportation conditions were analyzed by diagnostic methods.There are three dominant EOF modes about summer rainfall.The first mode showed the same anomaly in whole region.The second showed an inverse pattern between the east and west.The third showed the opposite anomaly between the south and north.Further analysis were shown that the dominant precipitation EOF modes had different circulation and water vapor condition features.For EOF1 positive phase,the center of subtropical westerly wind was eastern to its normal position,Ural blocking high usually stronger than normal,water vapor transportation originates from Bay of Bengal,South China sea and west tropical Pacific.The water vapor convergence was significant in the middle of Northwest China.The EOF2 mode was connected with ‘ silk road’ teleconnection pattern circulation.For the more rainfall in east and less in west pattern,the enhanced anticyclone circulation over central Asia provided strong cold air.And the anticyclone circulation anomaly in Northwest Pacific provided strong transport water vapor to the middle-west regions of China.A significant convergence was formed in the east part of Gansu province.The EOF3 mode was deeply affected by the middle-high geopotential height pattern.When rainfall anomaly was positive in south and negative in north,the circulation showed ‘North-high and South-low’ pattern over Eurasia.This mean a strengthened trough over Baikal which provides the strong westerly anomalies to the south of Baikal.At the same time,the vapor transformation originated from the Bay of Bengal,and further move to the east side of Tibet.A clear convergence formed in the southern of Qinghai province.From the above analysis,it showed significant differences about cold air conditions and the source of water vapor of the three dominant modes of summer precipitation in the middle of Northwest China.