Study Of Urbanization Effects On Hourly Extreme Precipitation

Based on long term hourly precipitation observation data,this study firstly has analyzed the hourly-scale precipitation structure and change characteristics of three major city clusters in China,namely Beijing-Tianjin-Hebei(BTH),Yangtze River Delta(YZRD)and Pearl River Delta(PRD),and has found that the changes in hourly precipitation features of BTH are very unique.Subsequently,an in-depth study was conducted to investigate the linkage between changes in hourly extreme precipitation events and urban development in Beijing.The results show that the urbanization effects caused by city development have important effects on hourly extreme precipitation events.Further study on the differences between local and large-scale hourly extreme precipitation variations reveals the possible reasons for changes in hourly extreme precipitation events in Beijing.Finally,by using the WRF model,this paper analyzes the role of urbanization effects in the process of extreme precipitation,and the effects of urban underlying surface on hourly extreme precipitation events in Beijing are verified quantitatively through long-term numerical simulations.The main conclusions are as follows:(1)The results of the study on the hourly-scale precipitation variation characteristics of three major city clusters in China show that the rainfall duration,diurnal variation,and precipitation asymmetry have changed to some extent after the high urbanization development,but the most significant changes are in the changes of hourly extreme precipitation events.In the past decades,the hourly extreme precipitation events in the YZRD and PRD have shown a very significant increasing trend.Although the frequency of hourly extreme precipitation in BTH has a decreasing trend against the background of obvious decreasing summer precipitation,the percentage of extreme precipitation still shows a certain increasing trend.From the view of hourly extreme precipitation intensity,the hourly extreme precipitation intensities of the three city clusters have an overall increasing trend,and the higher the intensity level,the more obvious the increasing trend.(2)The study on the changes in hourly extreme precipitation events in Beijing and its linkage with urbanization development shows that although the overall decrease in summer precipitation in North China,the frequency of hourly extreme precipitation events and the contribution to summer total precipitation have increased significantly in Beijing since 2000.The changes in hourly extreme precipitation events in the urban areas of Beijing are consistent with the urban development,and the hourly extreme precipitation events in the urban areas increase significantly after the high urbanization compared to the suburban areas,especially during the night time.The significantly enhanced urban heat island effect has an important effect,leading to an amplified difference in surface air pressure between urban and suburban areas.This favors the formation of local circulation between urban and suburban areas,and thus will trigger more hourly extreme precipitation events.(3)The study on the changes in local-scale precipitation in Beijing indicates that localscale precipitation accounts for more than 40% of the total summer precipitation.Although the frequency of local-scale precipitation in North China as a whole decreased during the period of high urbanization influence(after 2000)compared with the period of low urbanization influence(before 2000),the frequency of local-scale precipitation within the urban area of Beijing increased to some extent,and the increase in the percentage of local precipitation is very obvious.Furthermore,the increase in hourly extreme precipitation events in urban areas of Beijing urban areas during the period of high urbanization impact is mainly caused by the increase in local-scale hourly extreme precipitation events at nighttime,while large-scale hourly extreme precipitation events have decreased instead.(4)Based on the Weather Research and Forecasting(WRF)model,the results show that WRF can well simulate the extreme precipitation process.The urban underlying surface makes the extreme precipitation increase within the urban area of Beijing,the temperature increases,the air pressure decreases,and the wind speed in the vertical direction increases.In the local precipitation process,the existence of urban underlying surface leads to more precipitation amount,larger rainfall intensity,and wider rainfall range.At the same time,urban areas have more favorable temperatures and dynamic conditions before local-scale rainfall occurs.Extreme precipitation is highly sensitive to air temperature,and higher air temperatures in urban areas can lead to further increases in extreme precipitation amounts.Through long-term numerical simulations,it is confirmed that expansions in the urban underlying surface of Beijing have an increasing effect on hourly extreme precipitation events and are mainly caused by local-scale precipitation processes.