东北冷涡背景下中尺度低涡的演变成因及其对MCS的影响分析

Analysis of the evolution reason of the mesoscale vortex and its influence on MCS under the background of the northeast cold vortex

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2015年8月3日发生在山东地区附近的一次东北冷涡背景下中尺度低涡伴随MCS的过程进行数值模拟和分析,研究了东北冷涡影响下中尺度低涡发生发展的原因及其对MCS发展的影响。结果表明:(1)在天气尺度的东北冷涡槽前,高层高空急流入口区右侧存在强辐散场,低层有不稳定能量释放为中尺度低涡以及对流发生提供有利背景场,低层中尺度低涡的生成促进了MCS的形成;(2)中尺度低涡形成和发展时期,低层正涡度主要受到水平散度项和垂直输送项的影响,水平散度项作用更大,中层正涡度主要受到垂直输送项的影响,由于两项的正贡献作用,中低层正涡度增加,低涡发展;中尺度低涡减弱时期,中低层水平散度项减弱,垂直输送项变为负值,正涡度制造减弱,负涡度制造增加,低涡消亡;(3)在MCS发展过程中,扰动有效位能的增加主要由于潜热释放作用和中尺度低涡纬向平均有效位能向扰动有效位能转化,之后扰动有效位能通过垂直运动向扰动动能转化,扰动动能的增加促进强对流发展。中尺度低涡不仅为强对流的发展提供能量,其中风场变化也是影响强对流演变的重要原因。

An event of mesoscale vortex with MCS occurred in Shandong region on 3 August 2015 was investigated and analyzed using the WRF mesoscale numerical model based on NCEP/NCAR analysis data and Doppler radar observation data.The study revealed the reason for the occurrence and development of the mesoscale vortex and its effect on the mesoscale convection system under the environment of the northeast cold vortex.The results are as follows.(1)The upper-level jet stream’s strong divergence in front of the trough of the synoptic-scale northeast cold vortex and the low-level instability energy release provided favorable condition for the lower mesoscale vortex and convection occurrence.The generation of the low-level mesoscale vortex promoted the occurrence of the MCS.(2)When the mesoscale vortex forms and develops,the positive vorticity of the lower layer is mainly affected by the horizontal divergence term and the vertical transport term,with the former having a greater effect.The positive vorticity of the middle layer is mainly affected by the vertical transport term.Due to positive contributions of the two terms,the positive vorticity increases,and the vortex develops.When the mesoscale vortex weakens,the horizontal divergence term in the middle and lower layers weakens and the vertical transport term becomes negative,leading to the decrease of positive vorticity generation and the increase of negative vorticity generation,and therefore the vortex decreases.(3)When the MCS develops,the increase of perturbation effective potential energy mainly comes from the latent heat release and the conversion from zonal mean effective potential energy to disturbance effective potential energy,and then the perturbation effective potential energy is transformed into the perturbation kinetic energy through the vertical motion.The increase of the perturbation kinetic energy promotes the development of convections.The mesoscale vortex not only provides energy for the severe convection,but also influences the evolution of the strong convection.