Synoptic Situations Of Extreme Hourly Precipitation Over China And Its Observed Link To Urbanization

This study examines the characteristics of extreme hourly precipitation?EXHP?in China and could be roughly divided into two parts.In the first part,synoptic situations associated with EXHP over China mainland and Taiwan,respectively,are investigated.All the EXHP records are classified into four types according to the synoptic situations under which they occur,i.e.,the tropical cyclone?TC?,surface front,low-level vortex/shear line,and weak-synoptic forcing types.In China mainland,the 99.9th percentile value during 1981-2015 is used as the threshold to define the EXHP for each station.The EXHP is most intense over the south coastal areas and the North China Plain(>60 mm h^-1).There are 5797 EXHP records in 2011-2015,among which 39.1%is related to a low-level vortex/shear line,13.9%is associated with a surface front,8.0%is caused by TCs,and 39.0%occurs under largely quiescent synoptic conditions.The TC-type occurs most frequently along the coasts and peaks in August;the frontal type is distributed evenly east of 104^oE;the vortex/shear line type shows a prominent center over the Sichuan Basin?June-September?with two high frequency bands extending from the center eastward and northeastward towards southeast China and North China Plain?May-September?,respectively;the weak-synoptic forcing type occurs more frequently in the southeast China?May-September?,southwest China?May-September?,the north China?July?,and east portion of the northeast China?June-August?.In Taiwan,the intensity thresholds of EXHP are estimated through GEV distribution with5-,10-and 20-yr return periods,while a fixed threshold of 100 mm h^-1 is also used.The TC type accounts for over three quarters of the total records,while the front-type and weak-synoptic forcing type are comparable?9-13%?.EXHP is mostly caused by Meiyu fronts during May to mid-June and by TCs during July to October.The TC type is further divided into seven subtypes according to the distance and location of the TC center relative to the island.When the TC center is over the island or near the coastline?distance<100 km?,the spatial distribution of subtypes I-IV is largely determined by the azimuth of the TC center relative to the island,i.e.,northwest,south,east,and northeast side of Taiwan.When the TC center is far away from the island?distance>100 km?,the spatial distribution of subtypes V-VII is determined by the interaction between the TC circulation and the environmental southwesterly or northeasterly winds,or by the impingement of TC circulation on the Central Mountain Range from its east side.As suggested by the analysis above,the EXHP is most intense over the south China coastal areas(>60 mm h^-1)and nearly half of the EXHP records occur under largely quiescent synoptic conditions,which poses a big challenge for both accurate prediction and disaster risk management planning.Besides,the Guangdong-Hong Kong-Macao Greater Bay Area?GBA?,one of the three major urban clusters in China,is also located in this area.Thus,understanding changes in sub-daily rainfall extremes in GBA is also critical for urban planners to build more sustainable and reliant cities.Therefore,in the second part of this study,both the hourly precipitation data in1971-2016 from 61 rain gauges and the historical land-use change data are used to investigate changes in EXHP over South China coast with a focus on its possible link to GBA urbanization.Statistically significant increase of hourly precipitation intensity leads to higher annual amounts of both total and extreme precipitation over the GBA urban cluster in the rapid urbanization period?1994-2016?than during the pre-urbanization era?1971-1993?,suggesting a possible link between the enhanced rainfall and the rapid urbanization.Those urbanization-related positive trends are more closely related to more frequent occurrence of the short-duration??6 h?,abrupt rainfall events,rather than the longer-duration,continuous or growing rainfall events,which suggest a possible link to the urban heat island?UHI?effect.Therefore,the relationship between the UHI effect of GBA and a total of 120 extreme rainfall events?EXREs?during 2011-16 is further analyzed using observations collected by densely-distributed automatic weather stations and radar network.The 120 EXREs are categorized into six types according to the originating location and movement of the extreme-rain-producing storms,i.e.,local/southwest wind type?31.7%?,local/shear line type?20.8%?,migratory-NW?28.4%?,-SW?10.8%?,-S?3.3%?and–NE?5.0%?types.Despite of the wide range of synoptic backgrounds and seasons,rainfall intensification by the strong UHI is a clear signal in all the six types,especially over the inland portion of the urban cluster with prominent UHIs.