Aerosol-induced Changes Inprecipitation: Observational Evidence

The aerosol-cloud-precipitation interactionremains one of the largest uncertainties in the weather and climate prediction models. Long-term three-dimensional space-borne aerosol-cloud-precipitation measurements,coincident ECMWF reanalysis data, and ground-based PM observations were used to investigate the aerosols effects on precipitation. Based on theanalyses concerning the spatio-temporal featuresofprecipitation and how the precipitation is associated with the forcing of dynamic and thermodynamic factors, we also attempted to find out the evidences of aerosol-induced changes in the vertical structure of precipitation. As such, our understanding of the aerosol indirect effects is expected to be improved further.The thesis is divided into three parts: first of all, the spatio-temporal characteristics of the liquid- and solid-phase precipitations over typical areas. The snow cover fraction(SCF) data during the period 2001 through 2012 from MODIS aboard Terra of NASA are used to derive the spatio-temporal distribution of snow, and to calculate its variation trend over the Qinghai-Xizang Plateau(QXP) using statistic analysis methods.The SCF during the months from October through the following April dominates compared with other months,whichreaches the maximum in February and hits to the bottom in July; The annual mean snow area over the QXP oscillates, but with a dinstinct three-year variation cycle; such terrain factors as altitude,slope, and aspects are found to be able to exert significant influence on SCF. Then, using six years(2007-2012) of TRMM precipitation data and coincident ECMWF reanalysis data, we analyzed the features of shallow rain, stratus rain and convective rain over the global tropical ocean regions(35°S-35°N).To be specific over tropical oceans, the spatio-temporal distribution of average rain rate is negatively associated with distribution of vertical velocity at 400 hPa(ω400hPa); In terms of the area with rainfall, stratus precipitating system dominates the contribution to regional precipitation, followed by convective precipitating system, and in contrast, convective precipitating system takes the lead in contributing to the accumulated precipitation amount in the Regions of Interest(ROI), followed by stratus precipitating system;As for the convective precipitating system, the rain top height,reflectivity center of gravity, and radar top height with reflectivity of 30 dBZ were elevated sharply with decreasing ω400hPa and increasing RH850 h Pa. The same holds for the increasing LTS but with a smaller magnitude in the elevated height. This implies thatω400hPa and RH850 h Pa most likely play a dominant role in dictating the vertical development of convective precipitation.Secondly, using the rain rate data from TRMM 3B42 combined with the aerosol optical depth(AOD) from MODIS aboard Aqua,how rain rate varies with AOD was analyzed over the global tropic ocean regions(35°S-35°N), in attempts to figure out the aerosol effects on precipitationunder different meteorological conditions. Likely due to the atmospheric transport, there is a hot spot characterized with high AOD values in the equatorial area neigbouring the continent, which is similar to the precipitation distribution. As AOD increased, the rain rate initially increased and then decreased(the turning point corresponds tothe aerosol loading where AOD≈0.4), irrespective of meteorological conditions. However, the peak in the precipitation tends to be affected by the constantly changing meteorological conditions. Overall, in terms of the changes in rain rate,both ω400hPa and RH850 hPa most likely play a dominant role, followed by aerosol effect, although the aerosol effect can not be ignored.Last but not least, in order to explore the effect of aerosol on microphysical process of precipitation, six years of ground-based measurements of daily mean PM10 and TRMM precipitation data over the Pearl River Delta(PRD, 22°N-24°N,113°E-115°E) of China were used, with a focus on rain rate(R) and vertical profiles of radar reflectivity(Z). An increasing trend in the highest mean 5‰ of Z was seen as the atmosphere variedfrom pristine to lightly polluted(PM10<45μg/m3)in the presence of stratus and convective rain, but not in thepresence of shallow-cloud rain. The 30 dBZ radar echo top heightincreased for convective rain under polluted conditions and decreased for stratus(shallow-cloud) rain. In general, the aerosol tends to exert a suppression(invigoration) effect on shallow-cloud and stratus(convective) rain. Also, we attempted to separate the impact of meteorology(i.e., vertical velocity, wind shear, LTS and vertically integrated of water moisture flux divergence) from aerosols, ending up with the discernible difference in precipitation under prestine and polluted atmospheric conditions. Thisfurther bore out the hypothesis that the phenomenon we have observed is largely caused by the aerosol effects on precipitating system.