Ensemble Sensitivity Analysis of Heavy Rainfall Associated With Three MCSs Coexisting Over Southern China

Abstract

AbstractUsing ensemble‐based sensitivity analysis, a heavy rainfall event over southern China was analyzed to investigate the key controlling factors of rainfall amount and relevant predictability. The heavy rainfall event was caused by three coexisting mesoscale convective systems (MCSs) over southwestern China (MCS‐A), the south coast of China (MCS‐B), and northern Taiwan (MCS‐C). The three MCSs exhibited differences and similarities with respect to their forecast accuracy and relevant key factors contributing to the rainfall centers, such as low‐level vortex/shear lines and low‐level jets. The strength and shape of the southwest vortex, a mesoscale vortex usually located over southwestern China with a horizontal scale of 200–600 km, exerted a significant effect on MCS‐A and MCS‐B, whereas the stronger shear line near the east coast of China was accompanied by increased precipitation in MCS‐C. In addition, the low‐level jets (LLJs) and their associated moisture transports had a strong influence on the three MCSs according to the LLJ location, strength, height, and direction. A synoptic LLJ located to the southeast of the southwest vortex with a stronger meridional (zonal) wind component and an axis more to the north (south) was related to increased precipitation in the corresponding region of MCS‐A (MCS‐B). In addition to the synoptic LLJ, the rainfall of MCS‐B was also influenced by a boundary layer jet over the ocean. Uncertainties in the aforementioned key factors might further influence the predictability of the three MCSs and thus lead to the differences in their forecast accuracy.