On The Characteristics Of The Summertime Extreme Hourly Precipitation Over The Yangtze River Delta And Its Relationship To Urbanization

The Yangtze River Delta(YRD)has undergone rapid urbanization since the mid-1990s,forming a“Z”-shaped densely populated and economically developed urban agglomeration along the Yangtze River and the Hangzhou Bay.Modifications of the underlying surface properties in urban regions have maken the surface runoff tougher.Thus extreme hourly precipitation often accounts for waterlogging and other disasters in the urban regions,resulting in huge losses of life and property.Researches on the relationship between the extreme hourly precipitation(EXHP)and the urbanization can not only deepen the understandings of the initiation and development of the convection which produces extreme rainfall in urban regions,but also help to improve the refined meteorological services and the ability of meteorological disaster prevention and mitigation.However,statistical analyses of the spatiotemporal characteristics and long-term changes of the summertime EXHP over the YRD region is limited,and the physical mechanism of how cities interact with each other and affect the convection as a whole is still unknown.This dissertation will focus on the following three aspects to answer the questions,i.e.,are there any relationships between the YRD urbanization and the local summertime EXHP?What's the relationship between the spatial distribution of the EXHP and the urban heat island(UHI)intensity?And how do cities affect the EXHP,i.e.,what's the physical mechanism?First,a climatology of the summertime EXHP(greater than the 90th percentile)is analyzed using the hourly rain gauge observations during the 1975-2018 summers.Results show that both occurrence frequency and amount of the EXHP over the YRD increase obviously during the rapid urbanization era of 1997-2018 as compared to those during pre-urbanization era of 1975-1996.Increasing trends in occurrence frequency and amount of both the TC-induced and non-TC EXHP are detected,with significant trends mainly concentrating in the urban key region.Statistically significant larger increasing trends in both the EXHP(including occurrence frequency,amount and intensity)and surface temperature are observed at urban stations than those at the nearby rural stations.Moreover,the mean C-C scaling rate at urban stations is larger than that at rural stations.It reveals that the EXHP changes over the YRD region,especially over the urban agglomerations,is related to the urbanization there.Second,an analysis about the thermodynamic conditions and rainfall distributions of the 113 locally developed non-TC extreme rainfall events(EXREs;greater than 23.7mm h^-1)with strong and weak urban heat island during the 2011-2018 summers is conducted,using hourly precipitation observations from the densely automatic weather stations(AWSs)and mosaic radar images.The EXREs is divided into strong-and weak-UHI events based on the UHI intensity,which is defined by the averaged surface temperature difference between stations within and without the urban key region.It is found that warm centers and prominent positive rainfall accumulations are observed over the band-shaped urban region of Shanghai-Suzhou-Wuxi-Changzhou during the strong-UHI events.In contrast,the warm centers and rainfall centers are located mostly over western Hangzhou,due likely to lifting of the unstable southwesterly flow by the complex terrain.The differences in thermal conditions and precipitation distributions indicate that urbanization over the YRD region could result in more occurrences of the EXHP with strong-UHI effects,especially within the urban key region.Third,a case study of a short-duration extreme rainfall event occurring on 26 July2018 is investigated to explore the physical mechanism of urban impacts on EXHP production using the high-resolution numerical model.The simulation results reveal that the presence of urban regions enhances upward motion with the UHI effects,accounting for convective iniation.In addition,the UHI effects over Suzhou-Wuxi-Changzhou propogate downstream,affecting convective development near a downwind city,i.e.,Nantong,with the help of lake-breezes from Taihu Lake,river-breezes from the Yangtze River and small towns downstream.Moreover,cold outflows play an important role on triggering a series of convective storms from Shanghai through Chongming Island to Nantong.The conclusion that UHI could trigger convection initiation and enhance precipitation is consistent with the former observational results of the linkage between strong UHI and more occurrences of EXHP,as well as the increases in EXHP over the urban regions.In summary,this dissertation finds that the spatial distribution pattern of the EXHP over the YRD has been changed due to the urbanization in the last several decades based on the statistical analyses.It reveals that urbanization could not only help trigger the convection over the cities by the UHI effects,but also influence downstream convective development with the propagation of the UHI effects and the spreading of cold outflows,with the help factors such as sea-breezes,lake-breezes,and river-breezes.