Long-Term Trends in Pre-Summer Daytime and Nocturnal Extreme Hourly Rainfall in a Coastal City of South China

The long-term trends in the occurrence frequency of pre-summer daytime and nocturnal extreme hourly rainfall(EXHR) during 1988-2018 in Hong Kong and their spatial distributions are examined and analyzed. Despite a significant increasing trend observed in the occurrence frequency of pre-summer EXHRs during the investigated period,the increase in daytime and nocturnal EXHRs show distinct spatial patterns. Nocturnal EXHRs show uniform increasing trends over the entire Hong Kong. However, the increase in daytime EXHRs is concentrated over the northern or eastern areas of Hong Kong, indicating a downstream shift of pre-summer EXHRs in Hong Kong with regard to the prevailing southwesterly monsoonal flows in south China. The clustering of weather types associated with daytime and nocturnal EXHRs further reveals that the increase in EXHRs over Hong Kong are mainly contributed by the increase of the events associated with southwesterly monsoonal flows with relatively high speeds. During the past few decades, the southwesterly monsoonal flows at coastal south China have undergone a substantial weakening due to the increased surface roughness induced by the urbanization over the Guangdong-Hong Kong-Macao Greater Bay Area since 1990s,leading to enhanced low-level convergence and thus significant increase in EXHRs at coastal south China. Meanwhile,daytime sea-wind circulation at coastal south China is markedly enhanced during the investigated period, which is the main reason for the northward shift of daytime EXHRs in Hong Kong. In addition, the blocked southwesterly monsoonal flows at coastal south China are detoured eastward, leading to stronger convergence and increase in EXHRs at eastern coast of Hong Kong, especially during daytime, when the easterly sea winds prevail at the region.

A Possible Dynamic Mechanism for Rapid Production of the Extreme Hourly Rainfall in Zhengzhou City on 20 July 2021

In this study,the unprecedented extreme rainfall event during 19-20 July 2021,which caused devastating flooding in Zhengzhou City and its nearby areas,is examined based on observational data analysis and WRF model 40-h simulations on 1-km horizontal resolution.The results show that the model successfully reproduces(i)major synopticscale weather systems(i.e.,the western Pacific subtropical high,the Tibetan high,two typhoons,and the Huang-Huai cyclone),(ii)convective initiation along the east to north edge of the Songshan Mountain,where orographic lifting is obvious,and(iii)subsequent formation of the convective storm producing the extreme rainfall in Zhengzhou.In particular,the model generates the maximum rainfall rate of 233 mm h^(-1)and 40-h accumulated rainfall of 704 mm,corresponding well to the observed extreme values of 201.9 mm h^(-1)and 818 mm,at nearly observed timing and location.Importantly,the model reproduces an intense quasi-stationary,well-organized meso-γ-scale convective system,surrounded by an arc-shaped convergence zone,allowing the development of convective updrafts in a three-quarter circle around the convective system,in a way similar to“multidirectional pumping,”attracting all associated precipitation overlaid and concentrated into the same trailing region to generate the extreme hourly rainfall over Zhengzhou.Our study emphasizes the significant contribution of the unique dynamic structure of the well-organized meso-γ-scale convective system to the record-high hourly rainfall.A possible dynamic mechanism for short-time extreme rainfall production is proposed.That is,the arc-shaped convergence zone of the mesoscale convective system,acting like multidirectional lifting pumps,transports precipitation from different directions into the same region,and thus produces the extreme rainfall.The results gained herein may shed new light on better understanding and forecasting of short-time extreme rainfall.