Aerosol-cloud Parameterization And Its Impact On Precipitation In The Yangtze River Delta Region

Aerosol is a critical factor affecting the atmospheric hydrological cycle and climate change.Acting as cloud condensation nuclei(CCN)or ice nuclei(IN)for cloud formation,aerosols have a significant impact on regional precipitation.And the aerosol-cloud-precipitation interaction is a scientific issue that we urgently need to address to understand climate change and extreme weather.In the Yangtze River Delta,the accurate observation and parameterization of CCN against the background of high pollution concentrations is of great significance for the study of aerosol-cloud-precipitation interactions and assessment of aerosol effects on precipitation.In this paper,the spatio-temporal distribution of CCN concentration in the Yangtze River Delta is investigated based on the high-resolution visible infrared imaging radiometer(VIIRS)on board the Suomi-NPP(National Polar-orbiting Partnership)satellite;an aerosol-CCN parameterization scheme is established by combining the PM_2.5concentration data from ground-based observations and applied to the mesoscale WRF-Chem model;the effects of aerosols as CCN and IN on warm and cold cloud precipitation are further investigated using the WRF-Chem model.The main findings are as follows:1)An inversion study of CCN in the Yangtze River Delta was conducted.Based on the retrieved algorithm proposed by Rosenfeld,the retrieved results were filtered and optimized to give the spatial distribution characteristics of CCN in the Yangtze River Delta of eastern China.The average summer CCN concentration in the Yangtze River Delta was about 1030±324 cm^-3,and the CCN concentration in the Yangtze River Delta varied little at different wind directions,showing clear local characteristics.The cloud layer depth(Dc)at the onset of precipitation increased linearly with increasing CCN concentration,with Dc<1 km over the ocean and about2 km over the clean continent,compared to the Yangtze River Delta,where Dc tended to be above 4 km.2)An aerosol-CCN parameterization study was conducted.The main convective weather patterns in the Yangtze River Delta were identified by weather type classification.Further studies found that summer convective activity in the Yangtze River Delta was significantly enhanced under the PTT1 type(on the right side of the high value center of the difference field between 700 h Pa and ground temperature).The parameterized relationship between the CCN concentration and surface PM_2.5 was fitted using satellite results,with significant differences between regions.The Yangtze River Delta had the lowest CCN sensitivity to PM_2.5,and the activation rate of CCN still showed a significant oversaturation effect even under strong convective weather.CCN in Yangtze River Delta was mainly affected by three kinds of meteorological factors:surface,cloud bottom and cloud top,and the difference among them was the smallest under PTT1 state of the weather.The total amount of summer precipitation increased and the frequency of precipitation decreased year by year,leading to an annual increase in the precipitation concentration index in the most areas of the Yangtze River Delta.3)A numerical simulation study of the effect of aerosols on warm cloud precipitation was carried out through numerical simulations.The effects of different aerosol concentrations and different aerosol-CCN parameterization schemes on warm cloud precipitation were simulated using WRF-Chem mesoscale model.The effect of aerosols on precipitation was found to be non-linear,and only the appropriate concentration of CCN could further promote precipitation,otherwise it would inhibit precipitation.The effect of CCN>250 cm^-3 on precipitation in the OC50 case was significant,and aerosols could influence precipitation by changing the convective structure and micro-physical processes,especially the rate of the accretion of snow by rain.The aerosol-CCN parameterization scheme based on satellite retrievals improved the precipitation simulation in the Yangtze River Delta,and the OW and SCNR cases did not take into account the nucleation and radiative feedback of aerosols,resulting in very different precipitation results,suggesting that the direct and indirect role of aerosols in rainfall generation cannot be ignored.4)A numerical simulation study of the effect of aerosols on cold cloud precipitation was carried out through numerical simulations.The effect of IN on cold cloud precipitation was investigated using sensitivity experiments with different IN parameterizations.The ICAERO case,with aerosols taken into account,simulated higher precipitation efficiency than the ICC,ICM,ICH cases,which did not take into account aerosols,and simulated stronger and more concentrated precipitation.Most of the micro-physical processes in the ICAERO case had higher conversion rates than the other cases,and the formation and development of precipitation was promoted mainly by increasing the cumulative conversion of graupel particles.In summary,this study gave the spatio-temporal distribution characteristics of CCN in the Yangtze River Delta,established an aerosol-CCN parameterization scheme based on the localization of the Yangtze River Delta,and investigated the influence of aerosol-cloud interaction on warm and cold cloud precipitation through numerical simulation,and deepened the understanding of the mechanism of aerosol-cloud-precipitation interaction.