黄土高原夏季极端降水及其成因分析

利用1961-2016年夏季黄土高原地区64个气象监测站的逐日降水资料及同期NCEP/NCAR再分析数据,分析了近56年黄土高原夏季极端降水的时空变化特征,并对比了极端降水强弱年以及不同年代黄土高原地区夏季大气环流形势的异同。结果表明,黄土高原夏季极端降水占夏季总降水量的54%左右,总体上呈现出东多西少的空间分布特征。在近年来黄土高原夏季降水整体下降的背景下,极端降水占比在将近70%的站点表现出增长趋势。对比极端降水强弱年环流形势发现,当贝加尔湖低压增强,西北太平洋副热带高压偏强偏北时,有利于西北太平洋及南海暖湿气流向北输送,与北方干冷空气在黄土高原上空交绥,从而导致更多的极端降水。在强极端降水年的夏季,存在着异常强的水汽净收入。6月,北边界和南边界的水汽输入加强。北边界在7月和8月转为水汽输出边界,同时西边界和南边界的输入加强,且8月的增加更为明显。在强极端降水年,夏季黄土高原上还可以观察到更加明显的冷暖空气交绥,这有利于研究区域不稳定能量的释放和极端降水的增加。此外,研究发现黄土高原夏季极端降水在1980年代经历了从偏多到偏少的转变,近些年来又逐渐增加。对比不同年代夏季大气环流可见,当水汽净收入为正,同时冷暖空气交汇明显时,对应年代的黄土高原夏季极端降水偏多,反之则降水偏少。

Characteristics and Possible Causes for Extreme Precipitation in Summer over the Loess Plateau

Using the daily precipitation datasets from 64 meteorological monitoring stations in the Loess Plateau and NCEP/NCAR reanalysis data during the period of 1961-2016,the spatial-temporal variations of the extreme precipitation were investigated in the Loess Plateau.In addition,the similarities and differences in atmospheric circulation situation corresponding to strong and weak years of extreme precipitation,and in different ages were compared.Results showed that extreme precipitation was the major type in summer in the Loess Plateau,which accounted for about 70%of the total summer precipitation,and presented spatial decreases from east to west of the plateau.Under the background of overall decline of summer precipitation in the Loess Plateau from 1961 to2016,the proportion of extreme precipitation showed an increasing trend among almost 70% of the stations.Comparison of atmospheric circulation between strong and weak year indicated that when Baikal low strengthened and the subtropical high extended northward in summer,the warm and humid northward air flow from the Northwest Pacific Ocean and South China Sea intensified,leading to the convergence of cold and warm air over the Loess Plateau and consequently more extreme precipitation in summer.In strong extreme precipitation years,there were positive water vapor income anomalies in summer in the Loess Plateau.The positive contribution to water vapor mainly originated from the northern and southern boundaries in June.The water vapor at northern boundary was converted to an output part in July and August,while the water vapor input at the western and southern boundaries increased and more pronounced in August.The more obviously convergence of cold and warm air in summer was detected in strong extreme precipitation years,which was beneficial to the release of unstable energy and the increase of extreme precipitation in study area.Moreover,extreme precipitation in summer was found to have experienced a transition from wet condition to dry condition in the Loess Plateau in the 1980 s,and had begun to increase in recent years.Based on the analyses of the characteristics of summer atmospheric circulation in different ages,it can be observed that positive net income of water vapor and obvious convergence of cold and warm air in summer typically resulted in more extreme precipitation in the Loess Plateau in the corresponding year,and vice versa.