利用地面观测资料、探空资料、NCEP再分析资料、多普勒雷达资料及WRF输出资料，研究了2014年6月20—21日发生在江南地区的一次持续性暴雨过程中中尺度对流系统（mesoscale convective system，MCS）的演变过程、结构特征及模态转变机理。此次暴雨发生在500 hPa东移发展的短波槽前、850 hPa切变线与低空暖湿急流之间的不稳定区。MCS演变过程中组织模态发生转变，对流东移发展形成拖尾型（tailing stratiform，TS）MCS，后逐渐转变为平行型（parallel stratiform，PS）MCS。环境风场上，TS型MCS表现为垂直对流线的相对入流，弓状回波北侧有气旋生成；PS型MCS低层表现为垂直对流线的相对入流，中高层表现为平行向相对入流。TS型MCS中，强对流区位于正扰动气压带，形成垂直作用于对流线的气压梯度力，导致相对入流垂直对流线；而在平行对流线方向上，扰动气压变化小，平行相对气流弱，整体呈现为较强的垂直向相对气流；PS型MCS中，气压扰动整体表现为西南正气压扰动与东北负气压扰动的分布特征，在西南-东北向气压梯度力作用下，形成平行对流线向后的相对入流，导致MCS模态的转变。
In this study,based on intensive surface observational data,sounding data,NCEP reanalysis data,Doppler radar data,and resolution data produced by the WRF model,the evolution process,organizational structure and modal transformation mechanism of the mesoscale convective system during the process in a continuous rainstorm in the Jiangnan region from 20 to 21 June 2014 are studied.The rainstorm process which formed in the region was ahead of the eastward trough at 500 hPa,between the shear line and the wet and warm low-level jet at 850 hPa.An organizational modal transformation occurred during the MCS evolution.In the eastward development,the convection formed TS MCS,then gradually changed to PS MCS.
In order to analyze the mechanisms of the transition,the WRF model was used to simulate the process.The results of the simulation were nearly in accordance with the observation facts in the isochrones of the squall line,the 24 h simulated cumulative precipitation,and the transition process from TS mode to PS mode.The main results could be summarized as follows:
1)Blocked by Fengshan,Hunan Province,the airflow of the climbing slope forms scattered convection in the leeward slope of the mountain,while the convection moved eastward and continuously developed.Under the combination of terrain blocking and wind field convergence,the convection developed into TS MCS,then rapidly into a bow-shaped echo.Under the push of the low vortex,the arcuate echo quickly moved toward the northeast.Blocked by the front,the bow-shaped echo moved at a slower speed,and the direction of movement changed,while a new convection at the front side of the north-south convective zone was constantly forming.Combined with the initial convection,the new convection gradually transformed into PS MCS.
2)In the environmental wind field,the relative inflow of TS MCS was a vertical convection line,and the cyclone was generated on the north side of the arcuate echo.The circulation distribution of PS MCS is such that the lower level is the relative inflow of the vertical convection line and the middle and upper layers exhibit the relative inflow of the parallel convection line.A smaller scale cyclonic circulation formed at the tail of the convection zone.
3)In TS MCS,the strong convection zone was located in the positive disturbance zone,forming vertical force acting on the convection line,and thus resulting in a relative inflow perpendicular to the convection line.However,in the parallel convection line direction,the disturbance pressure change was small,thus the parallel relative airflow was weak,and the overall presentation of the relative airflow was a vertical convection line.In the PS MCS,the overall performance of the pressure distribution characteristics included the southwest positive pressure disturbance and northeast negative pressure disturbance.The pressure gradient force in the northeast and northwest formed a relative inflow of the parallel convection line,which led to a transformation of the MCS microstructure.
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