Development mechanisms of cyclonic vorticity under two distinct large-scale weather patterns associated with summer heavy rainfall over North China

Two types of weather patterns with distinct locations of low-level cyclonic vorticity anomaly, referred to as Type 1 and Type 2, associated with local heavy summer rainfall over North China have been identified (Li et al., J Clim 37:2655–2672, 2024). However, their evolutions and underlying mechanisms were not investigated. In this study, we find that the large positive value of vorticity anomalies (LPV) for both types exhibit eastward intensification from their early to developing stages. Nonetheless, they follow distinct trajectories. For Type 1, the LPV takes a northern path, which is primarily attributed to the northward shift of the Western North Pacific subtropical high (WNPSH). The eastward development of LPV for Type 1 is significantly linked to vertical motion stretching. Quantitative examinations reveal that the dynamic forcing dominated by temperature advection mainly contributes to vertical motion at the early stage for Type 1, contributing 66.4%. The underlying mechanism involves significant southerly wind anomalies transporting warm flows northwards, promoting strong warm advection and enhancing upward motion and LPV development. In contrast, Type 2 follows a southern trajectory, which is affected by both the southward shift of WNPSH and baroclinic trough. For the eastward propagation of LPV for Type 2, it is also closely related to vertical motion stretching. Diagnosis reveals the diabatic forcing plays a more significant role on vertical motion at the early stage for Type 2, accounting for 59.7%. The nature is abundant moisture near the LPV facilitates strong diabatic forcing-induced upward motion. By the developing stage, the diabatic forcing becomes the main driver of vertical motion for both types.