Mesoscale and microphysical processes leading to extreme hourly rainfall prior to the merger of two mesoscale convective systems in Central China

Abstract The merger of convective systems has been considered as an important process triggering or intensifying convection, but it is still elusive how this process can produce extreme rainfall. Based on multisource observations and quasi‐idealised numerical simulation, this study investigates physical processes leading to the production of extreme hourly rainfall (EHR; 153.2 mm) prior to the merger of two mesoscale convective systems (MCSs), in central China on 21 July 2021. The EHR was produced by the northern MCS (denoted as MCS‐N) that developed with the convective ‘back‐building’ mechanism while the southern MCS (denoted as MCS‐S) was approaching rapidly. The numerical simulation can well capture the location and amount of the EHR, as well as the morphology and evolution of both MCSs. Analysis of the vertical momentum budget indicates that the low‐level upward acceleration in MCS‐N was mainly contributed by the enhanced convergence of low‐level horizontal winds, which was caused by the MCS‐S‐generated strong outflows. Such southerly outflows pushed the environmental warm moist air between the two MCSs into MCS‐N, enhancing its convective intensity before its merger with MCS‐S. The updrafts invigorated the microphysical production of rainwater, which was transported northwestward inside MCS‐N. This storm‐internal advection and local microphysical processes aloft the EHR location contributed approximately 70% and approximately 30%, respectively, to the EHR. These results emphasise the collective roles played by the MCS‐S‐generated outflows, the microphysical production and dynamic delivery of rainwater inside the MCS‐N in producing the EHR.