白银市夏季降水特征及其大气环流异常分析

Summer Precipitation Features and Atmospheric Circulation Anomaly in Baiyin

利用白银市1957～2009年夏季降水观测资料,采用线性趋势分析、最大熵谱、小波分析等统计诊断方法,分析了白银市夏季降水的变化特征,并利用NCEP/NCAR再分析资料对比分析了白银多、少雨年夏季大气环流的平均距平场特征。结果表明:白银市夏季降水量空间分布极为不均,降水量南多北少,中部降水变率高于南部和北部。近53年来,降水量总体呈下降趋势,近10年下降最为明显,降幅由北向南依次递增。降水变化存在3和8年左右的周期,且周期性不很稳定,目前无论在大、小尺度仍处在降水偏少的周期。白银市夏季多、少雨年,欧亚范围内200、500、700 hPa平均位势高度距平场存在明显的差异。多雨年在对流层高层辐散、中低层辐合加强;少雨年在对流层上层辐合增强,中下层辐散加强。在多雨年对流层中上层为强的上升运动,且对流层整层大气均为上升运动,有利于降水形成;而少雨年对流层上层上升运动偏弱,且在中层为下沉运动,不利于降水形成。

Based on the observation data of the summer precipitation in Baiyin from 1956 to 2009,the variation features of summer precipitation in Baiyin were analyzed by using linear trend analysis,maximum entropy spectrum and wavelet analysis,and then the average anomaly field features of the summer atmospheric circulation in wet and dry years in Baiyin were comparably analyzed by NCEP/NCAR reanalysis data.Results indicated that,the spatial distribution of summer precipitation was extremely uneven,more in south and less in north,and the rainfall variability was higher in central parts than in south and north.The summer precipitation presented an overall decreasing trend from north to south during the recent 53 years,especially in the recent 10 years.The precipitation showed an unstable periodic variation about every 3 and 8 years,and was at present in the less precipitation period from both the large and small scale.There were significant differences in the average geopotential height anomaly fields between the wet and dry years in summer in Baiyin.The upper troposphere divergence and middle and lower troposphere convergence intensified in the wet years;while the upper troposphere convergence and middle and lower troposphere divergence intensified in dry years.There was strong ascend motion in the upper troposphere and even the whole atmosphere at the wet years,which helped the formation of precipitation;while in the dry years,the ascend motion in the upper troposphere was weak,and the descend motion in middle troposphere was strong,which had negative impact on the formation of precipitation.