Effect of Horizontal Resolution on a Meso‐β‐Scale Vortex Simulation in an Extreme Rainstorm on 22 May 2020 Over South China: A Contrastive Study Based on Different‐Resolution Ensembles.

A transient extreme rainstorm that occurred over Guangzhou city, China, on 22 May 2020, has been investigated based on two ensemble prediction systems (EPSs), one with 9‐km (TRAMS9km‐EPS) and the other with 3‐km (TRAMS3km‐EPS) grid spacings, respectively. The results show that the better performance in the rainstorm event of TRAMS3km‐EPS than that of TRAMS9km‐EPS has been attributed to the reasonable simulation of an eastward propagating meso‐β‐scale convective vortex (MβCV). The composite evolution of the good‐performing members for the two EPSs has verified the important role of the MβCV in modulating the structure of low‐level jet (LLJ) to converge toward the Guangzhou city, facilitating the formation of the rainstorm. In comparison to the gravity wave disturbance when LLJ passed through the mountains in TRAMS3km‐EPS, the topographic effect induced the moist air to ascend deeply in TRAMS9km‐EPS, which directly led to earlier outbreaks of convection and rainstorm processes and affected the structural intensity of the upstream MβCV. Quantitative PV diagnosis has demonstrated that the dispersedly strong high‐PV systems in TRAMS3km‐EPS were caused by positive feedback effect between meso‐scale convective systems and diabatic heating, and were favorable for the entire cyclonic development of MβCV. The unsymmetrical PV tendency around the moving MβCV under the combined effect of diabatic heating and advection process in TRAMS9km‐EPS resulted in a rapid propagation of this system. However, the symmetrically developed PV system within the MβCV in TRAMS3km‐EPS allowed the enhanced cyclonic circulation to persistently affect Guangzhou city, inducing extreme rainstorms similar to the observations. Plain Language Summary: An eastward propagating meso‐β‐scale convective vortex (MβCV) that triggered a transient extreme rainstorm over Guangzhou city, China, on 22 May 2020, has been examined and simulated using a same ensemble prediction system (EPS) model with two horizontal resolutions (3 and 9 km). In this study, we have analyzed the similarities and differences between the two EPSs. Our results show that the effect of horizontal resolution resulted in distinct MβCV structures in the two EPSs and led to different rainstorm processes in terms of intensity and duration. Some aspects, such as the low‐level jet, topographic effect, and small‐scale disturbances, are specifically compared between the two EPSs. The associated research highlights the effect of horizontal resolution on the MβCV simulation and a possible development mechanism of such a mesoscale system. Key Points: The topographic uplift and fore‐mountain convergence processes are amplified in low‐resolution ensemble simulationsThe unsymmetrical PV tendency around MβCV center determines the direction and velocity of the moving MβCVSmall‐scale convective systems enhance cyclonic structure of MβCV through positive feedback with diabatic heating [ABSTRACT FROM AUTHOR]