中纬度瞬变涡旋活动对东亚夏季平均环流和降水的影响

首先利用CFSR再分析数据,分析了东亚夏季平均环流结构及瞬变涡旋活动特征,再通过WRF模式设计控制性试验和敏感性试验分别模拟受到/不受到来自北边界中纬度瞬变涡旋活动影响的东亚夏季环流和降水,通过两组试验对比揭示了瞬变涡旋活动对东亚夏季平均环流和降水的贡献。结果表明,中纬度瞬变涡旋活动可以通过系统性的输送动量、热量、水汽和涡度来改变背景场,从而影响夏季平均环流和降水。当中纬度瞬变涡旋活动大幅度减弱时,其产生的向极动量、热量和水汽输送也显著减弱。一方面向北热量和水汽输送的减少使东亚东部大陆上低层的平均温度和水汽增加,为降水提供了环流不稳定条件和水汽条件,另一方面瞬变涡旋的动力输送变化在环流场上形成相当正压结构响应,整个中国东部地区受强西南风控制,东亚夏季风增强并推进到更北的区域。这两个因素的共同作用使我国北部地区和华南地区产生了水汽辐合和对流增强,导致该地区降水的增强。

The influence of mid-latitude transient eddy activities on the summer mean circulation and precipitation in East Asia

Based on the CFSR reanalysis data, the structure of summertime mean circulation and the characteristics of transient eddy activities in East Asia were firstly analyzed. Then by using the WRF model, a control run and a sensitivity run were designed to simulate the East Asian mean circulation and precipitation in summer that with/without the influence of the mid-latitude transient eddy activities from the northern boundary, respectively. The comparison between the two sets of runs reveals the contribution of transient eddy activities in the mid-latitudes to the East Asia summertime mean circulation and precipitation. Results show that the mid-latitude transient eddy activities can change the background field by systematically transporting momentum, heat, moisture and vorticity, thereby affecting the mean circulation and precipitation in summer. When the mid-latitude transient eddy activities are greatly weakened, their induced poleward momentum, heat and water vapor flux transportation are also significantly weakened. On one hand, the decreasing of poleward heat and water vapor flux transportation increases the mean temperature and water vapor in the lower layers in the eastern continent of East Asia, which provides unstable conditions and moisture conditions for precipitation. On the other hand, the change of transient eddy dynamic transportation causes an equivalent-barotropic structure response in the circulation field. The entire eastern part of China is controlled by stronger southwesterly, thus the East Asian summer monsoon strengthens and extends to further north. The combined effect of these two factors results in moisture convergence and enhanced convection in the northern and southern parts of eastern China, leading to increasing of precipitation in this area.