Numerical Simulations Of Extreme Summer Precipitation In Northern China

Extreme precipitation is one of the hazardous weather systems causing great property damage and casualties.Global warming and rapid urbanization have seen the more frequent occurrence of extreme precipitation in the urban area.The investigation of extreme precipitation thus attracts great attention from weather and climate communities.Many studies have been dedicated to improving the numerical prediction of extreme precipitation events in urban areas.However,as the development of heavy precipitation in the urban area is complicated,our knowledge on controlling factors and physical mechanisms of extreme precipitation in the urban area is still limited.In particular,the role of urbanization and multiple aerosols influence in the formation of extreme precipitation events has not been well understood.This dissertation employed the Weather Research and Forecasting model(WRF)to model three extreme precipitation events in the urban areas,and attempted to gain an improved understanding on the model sensitivity and prediction of extreme precipitation events in the urban area with nudging methods,as well as the impact of urbanization and aerosols on the formation of extreme precipitation in the urban area by using WRF-Chem model.The main findings are as follows:First,the sensitivity experiments conducted with different atmospheric driving data,model initial time,domain size and nudging method indicate that WRF can well simulate the extreme precipitation event under weak background conditions.The low-resolution FNL reanalysis outperforms the high-resolution ERA reanalysis in the simulations as the former produced higher forecast skills of the extreme precipitation events.Using spectral nudging in the outside domain of WRF simulation is beneficial to improve the forecast skills of 500-h Pa Geopotential height,leading to a better simulation of the spatial distribution and temporal evolution of the extreme event.We also found that a larger size of the outside domain for WRF simulations can result in better capture of the evolution of precipitation area,which is attributed to the improved simulation of Western Pacific Subtropical High(WPSH)and tropical cyclones.The simulation results show that under reasonable Settings,reasonably good forecast skill was still achieved for the extreme precipitation event that lasts over 48 hours by WRF.In addition,the extreme precipitation in WRF simulations is sensitive to the model initial time because of the sensitivity of the low-level moisture convergence over Henan.The analyses hints that the forecast skills of extreme precipitation are more likely dependent on the accuracy of the low-level atmospheric conditions in the model simulations.Second,the simulation results of the heavy rain process on July 21,2012 in Beijing by WRF model show that,current models have difficulty to simulate the spatial and temporal distribution characteristics of such events.In this study,nudging methods,including grid nudging(GN)and spectral nudging(SN),and more accurate surface type data retrieved from remote sensing were used in the Weather Research and Forecasting(WRF)model to simulate this extreme precipitation case.When the default city underlay surface of the WRF model was replaced by a more accurate urban surface(NU),the precipitation intensity could be better simulated,but the peak moment of precipitation seriously lagged.Although the peak precipitation intensity simulated by the GN experiment was weak,the simulated precipitation time was basically consistent with the observations.Using GN in only the outside domain could better simulate precipitation peaks,while using GN in both the inside and outside domains could better simulate the spatial distribution characteristics of precipitation.Additionally,the precipitation from GN could be better simulated than that from SN.Overall,the two nudging methods could contribute to better simulations of this case because the nudging methods could improve the simulations of 500-h Pa geopotential height,850-h Pa water vapor transport,and low-level weather systems,which are the key factors in adjusting the spatial and temporal distributions of precipitation.Third,the impact of aerosols and surface types on the extreme precipitation event in urban areas are investigated.We found that the Aerosol cloud interaction(ACI)can cause a dramatic increase of droplet number concentration in the warm cloud when there is sufficient water vapor transportation to the precipitation area,which promotes the development of the warm clouds.In the period of extreme precipitation that is dominated by the frontal systems,the ACI leads to the increase of the number concentration of ice crystals,which is accompanied by the latent heat release and the enhancement of convection and results in the increase of convective precipitation.The absorption properties of black carbon aerosols and the terrain effects in the west and northern mountainous areas of Beijing can trigger the"aerosol-enhanced conditional instability” to enhance the precipitation.In addition,the urban land effects affect the spatial distribution of the precipitation,and lead to an40% increase(?80mm)in the total precipitation in the urban area,which also plays a key role in the formation of the extreme precipitation.Overall,our analyses suggest that the aerosols and urbanization can trigger different mechanisms associated with the precipitation in one extreme precipitation event.The extreme precipitation events can be easily affected by the higher concentration of aerosols and suitable terrains in the urban area,increasing the risks of property damage and casualties in the city.Fourth,the uncertainties of extreme precipitation events in the urban area caused by the dust aerosols were also discussed.The WRF-Chem model simulation was compared with the satellite observations and reanalysis to understand the impact of the dust aerosol transportation on an extreme precipitation event over North China.We found that urbanization inhibits the development of precipitation in this case,while the long-distance transportation of dust aerosols to the city increases the precipitation and provides favorable conditions for the formation of extreme precipitation events.Specifically,the long-distance transportation of dust aerosols triggers the conversion of supper-cooled water cloud to ice cloud,which leads to the increase of ice water path(IWP)and decrease of ice particle radius(IPR)and facilitate the formation of ice cloud.with the coordination of plenty of water vapor transportation and strong large-scale convergence,the dust aerosol transported to the higher troposphere(over 10 km)can promote the formation of the ice nuclei and the associated latent heat release.This will further enhance the development of the convection and precipitation,contributed to a 40 percent increase in heavy rainfall south of Beijing.Overall,the dust aerosols play a key role in the development of ice cloud,convection and the formation of extreme precipitation.