近55 a中国西北地区夏季降水的时空演变特征

利用1961—2015年中国西北地区274个气象观测站点的日降水数据和再分析大气资料，采用EOF分析及累积距平等方法，研究了近55 a中国西北地区夏季降水的时空演变特征。结果表明：1）1961—2015年中国西北地区夏季降水的演变可分为三个时段，1961—1975年，该区域降水普遍偏少；1976—1996年，西北地区的东部降水偏多，西部降水偏少；1997—2015年，其东部降水偏少，而西部降水偏多。2）1976—1996年，西北地区东部降水偏多，是因为该地区夏季降水强度和降水频次明显增加，而西部降水偏少，则是该区域小雨与中雨的频次减少，降水强度偏弱造成的；1997—2015年，由于有效降水日数减少，降水强度偏弱等原因导致西北地区东部降水偏少，与此同时，西北地区的西部却因降水强度明显增强，持续降水日数和极端降水事件增加使得该区域降水呈现偏多的态势。3）降水区的转移，伴随着北半球对流层中层中纬度波列的演变，同时来自东欧与印度季风的水汽输送也对降水的异常起到了关键作用。

Spatial and temporal variations of summer precipitation in Northwest China over the past 55 years

Based on NCEP/NCAR reanalysis data and daily precipitation data from 274 meteorological observation stations in northwest China from 1961 to 2015, the spatial and temporal variations of summer precipitation in northwest China over the past 55 years are analyzed using EOF analysis and accumulative anomaly methods. The conclusion are as follows. 1) The evolution of summer precipitation in Northwest China from 1961 to 2015 can be divided into three periods. The precipitation in this region was generally low from 1961 to 1975; from 1976 to 1996, there was more precipitation in the east of this region and less in the west; however, precipitation in the east was less while it was more in the west from 1997 to 2015. 2) From 1976 to 1996, there was more precipitation in the east of Northwest China because of the obvious increase in summer precipitation intensity and frequency. However, the precipitation in the west was less because of the decrease of the frequency of light rain and moderate rain and weakened intensity of precipitation. From 1997 to 2015, due to the decrease of effective precipitation days and the weak rainfall intensity, the precipitation in the eastern part of Northwest China was lower. At the same time, the precipitation intensity in the western part of Northwest China obviously increased and the number of continuous precipitation days and extreme precipitation events was up, which led to more precipitation in this region. 3) The evolution of mid latitude wave train in the middle troposphere of the northern hemisphere led to the shift of precipitation area, while the water vapor transport from eastern Europe and the Indian monsoon played a key role to abnormal precipitation.