The Initiation and Organization of a Severe Hail‐Producing Mesoscale Convective System in East China: A Numerical Study

The initiation and organization of a long‐duration hail‐producing mesoscale convective system (MCS) in eastern China are investigated using convection‐allowing simulations at 3‐km grid spacing with the Advanced Regional Prediction System. The lifecycle of this MCS is characterized by two stages. In the first stage, a series of convective storms are initiated along the northwest border of Jiangsu Provinces. These storms organize into a northwest southeast line as they moved southeastward, eventually organizing into an eastward moving bow‐echo structure during the second stage. Our analyses show that the storms initiate along a northwest‐southeast oriented convergence boundary set up between two low‐level mesoscale vortices, one to the northwest of Jiangsu Province and one over the East China Sea. Comparisons between the fake‐dry and control simulations show that the rearward (westward) spreading of a cold pool from another MCS proceeding the hail‐producing MCS plays a key role in generating and enhancing the vortex over Eastern China Sea. The tilting of baroclinically generated horizontal vorticity along the edge of the cold outflow creates a cyclonic and anticyclonic vortex couplet. The cyclonic vortex becomes the dominant East China Sea vortex as it is superposed onto the background cyclonic circulation. This cold outflow‐induced vortex also has strong impacts on the later organization of the hail‐producing MCS, leading to the eventual establishment of a bow‐echo structure. Finally, a three‐stage conceptual model for the initiation and organization of the long‐lasting multicellular MCS is proposed, and the understanding on the complex interactions between two MCSs will be helpful to operational forecasters.