“7.20”华北特大暴雨过程中低涡发展演变机制研究

利用中国地面加密自动站观测资料、北京地区雷达探测资料、NCEP （1°×1°） FNL资料、ECMWF ERA Interim （0.125°×0.125°）逐日再分析资料等,对造成2016年7月19-20日华北极端暴雨中的低涡系统发展演变的结构特征和加强机制进行了研究。华北地区这次特大暴雨过程出现了3个阶段降水,其中与低涡系统强烈发展对应的第2阶段降水是本次华北暴雨过程的主要降水阶段。针对该低涡的分析表明：（1）850 hPa以西南低涡为中心的低压带中,在河南西北部新生低涡系统,并且其在向华北地区移动过程中显著加强,该低涡系统在空间结构上,从倾斜涡柱逐渐发展成近乎直立的、贯穿整个对流层的深厚低涡系统;（2）中低层低涡系统快速发展过程与高低空系统构成耦合作用有关：低层低涡系统显著加强之前,对流层上层（300-200 hPa）首先出现高空槽异常加深并向南发展,该高空槽发展的开始阶段与其本身冷暖平流造成的斜压发展过程对应;而后,随着高纬度平流层高位涡沿等熵面向南运动,造成华北地区对流层上层涡度增强,形成正位涡异常区;当这一正位涡异常区叠加在对流层中低层锋区上空时,造成对流层中低层气旋快速发展并向下伸展,诱发河南西北部的新生气旋;低涡系统的发展进一步强化了低空暖平流,促使低空气旋向东北方向发展"移动"（本质上是暖平流前端造成的气旋发展）,这一动力学过程反过来使高层的涡度增强;这一正反馈过程形成的耦合环流不仅造成了整个涡度柱强度增强,而且垂直结构上逐渐由倾斜涡柱演变为近乎于直立的涡柱;（3）随着低涡系统增强,极大地加强了垂直上升运动并触发了对流,形成大范围的强降水,大量的凝结潜热释放,造成了低层低涡系统在强降水开始阶段的快速发展和增强;20日00时（世界时）以后,虽然对流活动显著减弱,但低涡系统的加深维持了大范围强降水过程的持续。强降水与低涡发展的正反馈过程是这次华北暴雨得以长时间维持的重要机制之一,这一过程形成的持续性潜热释放也是对流层中上层低涡系统热力结构发生改变的重要原因。

A study on the mechanism for the vortex system evolution and development during the torrential rain event in North China on 20 July 2016

Using intensive auto weather station (AWS) data and Doppler radar data in Beijing, and the NCEP finally reanalysis data (1°×1°) and ERA-Interim (0.125°×0.125°) data, the structure characteristics and evolution mechanisms for the vortex that triggered the torrential rain over North China on 19-20 July 2016 are analyzed. Among the three stages of the torrential rain event, the second stage was the major one corresponding to rapid development of the vortex and heavy precipitation. Analysis of the vortex system shows that a newly formed vortex first appeared in northwestern Henan province, which was located at the northeastern area of a low pressure zone associated with the Southwest vortex in Sichun basin. The intensity of the vortex at 850 hPa and the cyclone in the surface level gradually strengthened following their movement toward North China. Meanwhile, the vortex axis tilted seriously at the beginning and gradually evolved into a nearly vertical vortex system throughout the entire troposphere. The rapid development of the low-level Vortex system was associated with the coupling of high and low level synoptic systems. Prior to the rapid development of the vortex in lower levels, the upper trough at 300-200 hPa had deepened and developed abnormally toward the south. At the initial stage, its development corresponded to the baroclinic structure caused by cold advection in its rear and warm advection in front of the trough. As the high potential vorticity (PV) in stratosphere propagated southward from higher latitude along the isentropic surface, anomalously positive PV appeared in the upper levels of the troposphere. When the abnormally positive PV overlapped with the frontal zone in the middle and lower troposphere, the vortex developed and stretched downward rapidly, and the new cyclone was induced in northwestern Henan province. The developing vortex enhanced temperature advection, and the dynamical process was favorable for the vortex or cyclone to move northeastward (the essence was the development and propagation of newborn vortex because of warm advection forcing). Vorticity in upper levels in turn strengthened. The positive feedback not only enhanced the intensity of the vortex, but also promoted its vertical structural evolution, and the vortex column evolved from obviously tilt to almost erect. The vertical velocity greatly intensified and convective activity was triggered by the strengthened vortex system. Large latent heat release accompanied with the heavy rain made the vortex to develop rapidly in the lower troposphere during the initial stage. Although convection weakened obviously after 00:00 UTC 20 July, the intensified vortex system maintained heavy rain in a large area. The positive feedback between the heavy rainfall and vortex development is not only an important mechanism for the torrential rainstorm that lasted for a long time in North China, but also a major reason for the thermal dynamic structural changes in the vortex system in the mid-troposphere with sustained latent heat release.