Impacts Of Urbanization On The Precipitation Associated With Tropical Cyclones Landfalling In Guangdong Province,China

Precipitation associated with landfalling tropical cyclones often causes serious casualties and huge economic losses in costal cities.Guangdong Province,which is located along the coast,has a complex underlying surface due to the existence of the Pearl River Delta mega-city cluster.This study is conducive to deepen the understanding of the impact of urbanization on precipitation and improve the precipitation forecast in urban areas during tropical cyclone periods.With the development of urbanization,whether precipitation characteristics in Guangdong Province,China,from 1981 to 2015 have changed are investigated using rain gauge data from 76 stations,ground radar observations and CMORPH data.These characteristics include annual precipitation,rainfall frequency,intense rainfall(defined as hourly precipitation ? 20 mm),light precipitation(defined as hourly precipitation ?2.5 mm),and extreme rainfall(defined as hourly rainfall exceeding the 99.9th percentile of the hourly rainfall distribution).Further,the influence of urban underlying surface on precipitation associated with tropical cyclones in Guangdong Province is investigated by using the high-resolution numerical model.The main results are as follows:(1)During these 35 years,the annual precipitation shows an increasing trend in the urban areas.While rainfall frequency and light precipitation have a decreasing trend,intense rainfall frequency shows an increasing trend.The heavy and extreme rainfall frequency both exhibit an increasing trend in the Pearl River Delta region,where urbanization is the most significant.These trends in both the warm seasons(May–October)and during the pre-flood season(April–June)appear to be more significant.On the contrary,the annual precipitation amount in rural areas has a decreasing trend.Although the heavy and extreme precipitation also show an increasing trend,it is not as strong and significant as that in the urban areas.During periods in which a tropical cyclone makes landfall along the South China Coast,the rainfall in urban areas has been consistently more than that in surrounding areas.The precipitation in the urban areas and to their west is higher after 1995,when the urbanization accelerated.These results suggest that urbanization has a significant impact on the precipitation characteristics of Guangdong Province.(2)The precipitation characteristics(distribution and intensity)of 9 typhoon cases in recent years are analyzed using data from several Doppler radars located in Guangdong Province.During TC landfall,the cumulative 3 km CAPPI reflectivity and the cumulative frequency of reflectivity that are greater than 20 d BZ of all precipitation samples both show a large value center in the urban area of the Pearl River Delta.Meanwhile,the first pattern of PCA(Principal Component Analysis)with a 29.8%variance contribution also shows the similar spatial distribution.Although the distribution of precipitation associated with tropical cyclones is mainly affected by the environmental vertical wind shear,the existence of urban agglomerations will also lead to some changes in the intensity and distribution of precipitation.The results show that there is always more rain in urban agglomeration than in surrounding rural areas during TC landfall.(3)In order to explore the role of urban effect in typhoon precipitation further,we use the WRF numerical model to simulate typhoon Nida.Sensitivity study is carried out by modifying the underlying surface information of the model.We perform two sets of experiments: with and without urban land-use type.It is found that the urban experiment captures reasonably well many aspects of the observed rainfall during TC landfall.However,in the non-urban experiment,the precipitation is obviously less than observation,and the area of heavy rainfall is smaller.The differences of this two sensitivity experiments results prove that the urbanization do has an effect on the distribution and intensity of precipitation associated with typhoon.By analyzing the physical mechanism,we found that the surface temperature and surface heat flux are both higher in the urban experiment than those in the non-urban experiment,which is due to the contribution of the urban heating effect.Also,the friction velocity of urban experiment is higher.Urban underlying surface enhances the low-level convergence,and then promotes the occurrence of convection and precipitation.The advection of the typhoon circulation brings vapor to the downwind area,thus enhancing the precipitation there.Based on the long-time ground observation data,high-resolution radar observation data and WRF numerical model,this paper reveals the precipitation characteristics during tropical cyclone periods in Guangdong Province.This study also discusses the impact mechanism of urbanization on the precipitation associated with landfalling tropical cyclones in Guangdong Province,deepening the understanding of urban effects on precipitation.