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华北地区两次暴雨过程低涡结构和发展演变机制对比分析

Comparative analysis of the evolution mechanism of low vortex during two heavy rainfall processes in North China

  • 摘要: 本文基于地面常规观测资料和ERA5再分析数据对比分析了2016年7月19-21日(个例1)和2021年7月11-13日(个例2)两次华北低涡强降水过程的低涡垂直结构及其发展演变机制。结果表明:两个低涡系统均在西低东高的相似环流背景下生成,随后沿太行山东麓东移北上并给华北地区带来强降水,个例1高空急流明显强于个例2,这是大尺度环流背景的显著区别。两个低涡的垂直结构存在差异,个例1低涡在垂直方向发展深厚,分别在高层(300 hPa)和低层(850 hPa)存在正涡度中心,低层正涡度中心随低涡的移动强度逐渐减弱,高层正涡度中心强度逐渐增强,且呈现上暖下冷的温度分布,中层400 hPa存在弱暖心结构,其下为冷气团,低涡中心轴线西侧有干冷空气;个例2低涡发展较为浅薄,仅存在低层一个正涡度中心,且始终位于850 hPa附近,对流层中层700~300 hPa之间呈现明显的暖心结构,冷心较强,位于近地面。两个低涡系统的发展均与高空位涡下传、低层暖湿气流的输送有关,但个例1有干冷空气侵入,高低空存在明显锋生和对流不稳定,有利于上升运动的产生和正涡度的增强;个例2对流层整层以暖平流为主,且位涡中心和暖平流中心均位于850 hPa以下,两者近乎重合,产生上升运动,使低涡系统发展。涡度收支分析表明,两个低涡在各演变阶段低涡中心北侧和东侧收支为正,表明低涡有向东、向北移动的趋势,水平散度项和扭转项在发展阶段对两个华北低涡低层涡度增长起到了主要的正贡献,之后平流项和散度项呈显著正贡献,后期个例1垂直项在低涡中心及其东侧为正贡献,而个例2垂直项始终为负贡献。

     

    Abstract: Based on the conventional ground observation data and ERA5 reanalysis data, the vertical structure and evolution mechanism of the low vortex during the heavy precipitation process in North China during July 19-21, 2016 (case 1) and July 11-13, 2021 (case 2) were compared and analyzed in this paper. The results show that the two vortex systems are generated in a similar circulation background of low west and high east, and the upper-level jet stream in case 1 is significantly stronger than that in case 2, which is the obvious difference between the two vortex structures in large-scale circulation background. The vertical structure of the two low vortices is different. In case 1, the low vortices develop deeply in the vertical direction, and there are two positive vorticity centers in the upper layer (300 hPa) and the lower layer (850 hPa), respectively. With the movement of the low vortex, the positive vorticity center in the lower layer gradually weakens, while the positive vorticity center in the upper layer gradually increases, and the temperature distribution is warm at the top and cold at the bottom. There is a weak warm core structure at 400 hPa, under which there is a cold air mass, and the dry and cold air on the west side of the central axis of the vortex. In case 2, the development of low vorticity is relatively shallow, with only one positive vorticity center in the lower layer, and it is always located at 850 hPa. The middle troposphere presents an obvious warm core structure between 700 and 300 hPa, and the cold core near the ground. The development of the two low vortex systems is related to the downward transmission of the upper potential vortex and the transport of warm and humid air at the lower level. However, in case 1, there is the intrusion of dry cold air, and there is obvious frontogenesis and convective instability in the upper and lower levels, which is conducive to the generation of upward motion and the enhancement of positive vorticity. In case 2, the whole troposphere is dominated by warm advection, and the center of the potential vortex and the center of the warm advection are both below 850 hPa, and they almost overlap, resulting in upward motion and the development of the low vortex system. The vorticity budget analysis reveals that during various evolution stages of the two vortices, the budget on the northern and eastern sides of the vortex centers is positive, indicating a tendency for the vortices to move eastward and northward. The horizontal divergence term and the tilting term play a primary positive role in the vorticity growth at the lower levels of the two North China vortices during their development stages. The advection term and the divergence term contribute significantly and positively during the strength and attenuation stages. The difference between the two cases lies in the fact that, for Case 1, the vertical term contributes positively in the strength and attenuation stages, particularly in the vortex center and its eastern side, whereas for Case 2, the vertical term remains negative throughout the entire evolution process.

     

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