北京“7·21”特大暴雨中的干侵入分析研究

Research and Analysis of Dry Intrusion during Beijing ‘7·21’ Extreme Torrential Rain

利用中尺度数值模式WRF V3.2模拟分析2012年7月21日发生在北京特大暴雨过程的天气形势与中尺度系统特征,并结合干侵入理论分析了暴雨过程中的干冷空气活动及其对暴雨的影响。结果表明,此次暴雨过程发生在高空槽引导冷空气南下与强盛的西南暖湿气流在华北一带剧烈交汇的天气形势下,西太平洋副热带高压阻碍了高空槽东移,使北京地区的降水过程维持较长时间。暴雨过程伴随着明显的中尺度对流复合体MCC活动,MCC的持续活动与降水中心在时空上具有一致性。WRF模式对暴雨过程有较好的模拟能力,降水发生之前的24 h内不断有来自35°N对流层顶附近的高位涡、低湿的干冷空气,沿着倾斜向北向下的路径侵入大气中低层39°N附近的700 h Pa高度。干侵入在降水开始前24 h到降水前10 h强度变化不大,随后略有减弱,在降水开始之后迅速减弱消失。干侵入对暴雨的影响主要通过在降水开始前及降水初期影响北京地区的大气热力与动力环境来完成。干侵入可以增大暴雨落区大气的位势不稳定,为对流发展储备充沛的对流有效位能,为MCC的发生、发展提供有利的环境条件。同时,干侵入增大了大气中低层的气旋性涡度,有利于中低层空气辐合上升运动,是引发北京地区局地的强对流天气,如MCC及其伴随的暴雨过程可能的触发机制。

The weather situation and mesoscale systems characteristics of the extreme torrential rain occurred in Beijing on 21 July 2012 w as analyzed. With the mesoscale numerical mode WRFV3. 2 the extreme heavy precipitation is simulated. And based on the dry intrusion theory,the process of dry cold air activities during the heavy rain and its impact of the torrential rain are analyzed. The results show that: The torrential rain occurred in weather situation of the violent intersection of the southw ard cold air guided by upper trough and strong southw est w arm w et air in North China area. WPSA prevented the upper-level trough from moving eastw ard and thus the precipitation process in Beijing kept a long time. The convective development process is along with the obvious mesoscale convective complex( M CC) activities,and M CC 's ongoing activities and the rainfall centers are temporal and spatial consistent. The WRF model could accurately simulate this torrential rain process. Within 24 h before the precipitation occurs there is constant high potential vorticity and low humidity dry cold air from 35°N near the tropopause being transported along northw ard and dow nw ard path to 39°N near 700 h Pa at the low er-level atmosphere in Beijing area. Dry intrusion changed little from 24 h to 10 h before the start of the precipitation,soon afterw ards had a slight w eakening and w eakened rapidly after the precipitation started. The dry intrusion changed atmospheric thermal and dynamic environment of the Beijing area to impact the torrential rain before the precipitation started. Dry intrusion could increase the atmospheric potential instability in rainfall area and reserve CAPE abundant for the development of convection,providing the environment conducive for the occurrence and development of M CC. At the meantime,it increased the low-level cyclonic vorticity w hich is conducive to low-level air convergence and upw ard motion and this may be the trigger mechanism of severe convective weather,such as the M CC and its accompanying torrential rain in Beijing area.