Quantitative Attribution of Vertical Motion Responsible for Summer Heavy Rainfall Over North China

Abstract

This study explores synoptic-scale circulation conditions that favor vertical motion on heavy rainfall days (HRDs) over North China (NC) and analyzes the quantitative contribution to vertical motion by different forcings using the quasi-geostrophic (QG) omega equation. On HRDs, moisture is brought from the South China Sea and East China seas by surface cyclonic vortex activity. When South Asian High is located anomalously northward, NC is on the south side of the upper-level jet (ULJ) stream entrance region. Diagnosis of the QG omega equation shows that vertical motion on HRDs is caused by dynamic forcing, diabatic forcing, and topographic forcing. The greatest contribution to vertical motion is from diabatic forcing, while topographic contributes to vertical motion at low levels. The dynamic forcing, which is manifested by the Q-vector, indicates that both the ULJ and baroclinic trough generate the vertical motion. Furthermore, the effect of the baroclinic trough is larger than that of the ULJ. On medium rainfall days, the contributions from dynamic forcing, diabatic forcing, and topographic forcing are clearly weaker than on HRDs, with reductions of 62.3%, 71.3%, and 73.8%, respectively.