东北冷涡下一次飑线和MCV的形成与水平涡度的关系

利用WRF中尺度数值模式，NCEP/NCAR分析资料，多普勒雷达观测资料等，对2016年7月25日一次东北冷涡下的飑线过程进行数值模拟，研究了飑线形成和维持与水平涡度的关系及飑线过程中中尺度对流涡旋(MCV)的形成机制，分析发现，高低层水平涡度逆时针旋转对本次飑线的形成和维持有很好的指示意义。(1)飑线发生前，高层渤海湾西侧出现水平涡度的逆时针旋转中心，并有较强的辐散配合，低层水平涡度为逆时针弯曲，为飑线产生提供了有利的上升运动条件。随后高层多个对流单体的水平涡度气旋式涡旋合并形成较大范围的气旋式涡旋结构，触发低层的上升运动，同时低层对流区前部形成一致的气旋式弯曲使得对流单体组织成带状结构，形成飑线。(2)飑线成熟时期高层水平涡度表现为统一大范围气旋式涡旋结构，低层则呈现典型的S型弯曲结构，水平涡度x方向的分量沿对流带从南至北表现为正负正，y方向的分量始终为正，并由对流带的中心向两侧减小，显示出水平涡度矢量旋转的方向对飑线影响的重要性。(3)由垂直涡度方程的分析得出，在飑线发展中期，MCV形成前，雷达反射率回波在500 hPa左右表现出明显的旋转，此时主要与500 hPa以上强的正涡度水平平流项及中层倾侧项和水平散度项有关，之后，在这几项的作用下使得中层风场产生气旋式旋转，形成MCV。 

THE RELATIONSHIP BETWEEN THE FORMATION OF A SQUALL LINE AND MCV AND THE HORIZONTAL VORTICITY UNDER COLD VORTEX IN NORTHEAST CHINA

A squall line process in Bohai Bay on July 25, 2016 was simulated using WRF mesoscale numerical model, NCEP/NCAR reanalysis data and Doppler radar observation data, The relationship between the formation and maintenance of squall line and horizontal vorticity, and the formation mechanism of mesoscale convective vortex (MCV) in the process of squall line were studied. It is found that the anticlockwise bending of the horizontal vorticity vectors at the upper and lower level is of great significance to the formation and maintenance of squall line: (1) Before the squall line occurs, a counter-clockwise rotating center of horizontal vorticity at 200 hPa appears in the west side of the Bohai Bay together with a strong divergence at 900 hPa. The horizontal vorticity in the lower layer bends counterclockwise, which provides a favorable upward motion condition for the formation of the squall line. Then the horizontal vorticity cyclonic vortices of several convection cells in the upper layer merge to form a unified large-scale cyclonic vorticity structure, which triggers the ascending motion of the lower layer. At the same time, the uniform cyclonic curvature at 900hPa in front of the convective region makes the convection cells organize into a banded structure, forming the squall line; (2) During the mature period of squall line, the horizontal vorticity in the upper layer presents a unified large-scale cyclonic vortex structure, while in the lower layer it presents a typical S-shaped curved structure. The horizontal vorticity x component is positive, negative and positive along the convective belt from south to north, and the y component is always positive and decreases from the center of the convective belt to both sides, which shows the importance of the rotation direction of the horizontal vorticity vector to the squall line; (3) From the analysis of the vertical vorticity equation, it can be concluded that the radar reflectivity echo exhibits obvious rotation around 500 hPa before the formation of MCV in the middle period of squall line development, which is mainly related to the strong positive vorticity horizontal advection term over 500 hPa, the mid-level tilting term and the horizontal divergence term, which makes the middle wind field produce cyclonic rotation and form MCV.