The extreme short‐term rainfall rate as caused by a convective storm under a beneficial dynamic pattern

Abstract Within a meso‐ γ ‐scale convective storm, dynamic processes play a pivotal role in extreme rainfall production. However, there are still large unexplained gaps in understanding the effects of dynamic processes on the generation of extreme short‐term rainfalls. In this study, a nocturnal rainfall event with an extreme hourly rainfall (EHR) of 184 mm on 7 May 2017 over the coastal city of Guangzhou is examined based on cloud‐permitting simulations, focusing on the generation of the EHR. Results reveal that the EHR is featured by obvious horizontally delivered rainwater ( q r ) from the front to the rear within a meso‐ γ ‐scale convective storm. The horizontally delivered q r from the front of the storm overlayed on the q r produced by cloud microphysical processes locally overhead in the rear of the storm, leading to a deep q r layer with values over 4 g·kg −1 at the lowest 0–4 km levels above the ground. Thus, huge q r poured down in a short time, resulting in the EHR. According to statistical results, at least 80 mm q r was provided by horizontal delivery for the majority of grid points with hourly rainfall over 120 mm. This dynamic delivery mechanism is further confirmed by a trajectory analysis of raindrops. We argue that this mechanism may play a decisive role in EHR formation in particular scenarios while admitting that EHR can also be produced sometimes mainly via cloud microphysical processes. The formation mechanism of EHR proposed herein may help further understand and forecast localized extreme short‐term rainfall.