Aerosol Effects On Summer Precipitation Over East China: Observational Evidence And Model Simulations

East China is a domain where is characterized by dense population, rapid-developed economy and huge emission of various pollutants. A variety of observations and model simulations show that East China is a hot spot in terms of aerosol pollution. At the same time, constantly affected by the East Asian summer monsoon, large-scale rainfall tends to take place in this region in summer. Especially the heavy rain events occurred frequently in recent years. Aerosol particles participate in the formation of cloud and precipitation by acting as cloud condensation nuclei(CCN) or ice nucleus(IN). In this thesis, two subregions, Pearl River Delta(PRD) and North China Plain(NCP), were selected for their high aerosol loadings. By means of observations and numerical model simulations, aerosol-induced changes in precipitation on hourly and daily time scale, respectively, and their corresponding possible physical mechanism is investigated.Over the PRD subregion, how aerosols affect heavy rain and lightning has been explored mainly based on the diurnal cycles of aerosol, rain and lightning. We firstly studied the high spatiotemporal resolution hourly data of PM10, precipitation and lightning over the PRD. Their corresponding diurnal variations during the summertime of the time period 2008~2012 have been reported over PRD, showing coherent variations in PM10, precipitation and lightning. To examine any potential effect of aerosol, we sorted the daily PM10 data into clean and polluted subsets, and examined diurnal variations of summertime precipitation and lightning under differentaerosol pollution conditions. The heavy precipitation and lightning occurred more frequently and later during a day under polluted conditions than cleaner ones. In addition, we also studied the diurnal variations of several meteorological factors including air temperature, vertical velocity and wind shear. At 14:00BJT, the three factors can invigorate heavy rain. Consequently, standard multivariable linear regression was employed to figure out to what degree various factors separately account for the diurnal variation of heavy precipitation. The results showed that lightning and aerosols are the most optimal-related factors for heavy rainfall compared with any other factors. The invigorative effects of aerosol on hourly heavy precipitation can’t be ignored. Meanwhile, in order to bear out the observational results, high-resolution model simulations(ARW) were conducted for different settings of aerosol loading and inclusion of different aerosol effects(with or without radiative effect of aerosol). Consistent findings were reached between the observed and simulated relationships concerning the delay impact of aerosols on the diurnal variations of severe storms, in support of the above arguments in regard to the important effects of aerosols on diurnal variation of heavy rainfall.Over the NPC, based on three stations situated in Taihang Mountain and four city plain stations located to the east of the mountain, we analyzed 40 year(1966~2005) data sets of daily average visibility(a proxy for surface aerosol concentration) and hourly precipitation during summertime(MJJAS), in an attempt to determine the effects of aerosols(emitted mostly from cities in plain areas) on precipitation from orographic clouds. Results indicated that light rain decreased, whereas heavy rain increased as visibility decreased over the period studied. The decrease in light rain was seen in both orographic-forced shallow clouds and mesoscale stratiform clouds. The consistent trends in observed changes in visibility, precipitation, and orographic factor(Ro) appear to be a testimony to the effects of aerosols. Meteorological factors, such as precipitable water, convective available potential energy(CAPE), and vertical wind shear has little effect on precipitation induced by orographic clouds in the mountainous area. To validate our observational hypothesis about aerosol effects,Weather Research and Forecasting model simulations with spectral-bin microphysics at the cloud-resolving scale were conducted. Model results confirmed the role of aerosol indirect effects in reducing the light rain amount and frequency in the mountainous area for both orographic-forced shallow clouds and mesoscale stratiform clouds and in eliciting a different response in the neighboring plains. The opposite response of light rain to the increase in pollution when there is no terrain included in the model suggests that orography is likely a significant factor contributing to the opposite trends in light rain seen in mountainous and plain areas.