Converting AOT to CCN for Studying the Impact of Aerosol on Cloud and Precipitation

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Aerosols affect the climate system by changing cloud characteristics in many ways and with a high degree of uncertainty. Using 10 years' worth of Atmospheric Radiation Measurement (ARM) ground-based measurements of a variety of aerosol and meteorological quantities at the Southern Great Plains (SGP) site, robust relationships were found between aerosol concentration, cloud height and rain frequency. To extend the study from a single location with limited types of aerosols, we will analyze world-wide surface Aerosol Robotic Network (AERONET) aerosol data, together with matched satellite-retrieved cloud and precipitation products, to further investigate the impact of all kinds of aerosols and meteorological conditions on clouds and precipitation. While the AERONET provides the most reliable and consistent retrievals of aerosol optical thickness (AOT), it is cloud concentration nuclei (CCN) that is more directly related to cloud formation, development and precipitation. Many previous studies used AOT as a proxy for CCN, because there are much more observations of AOT than those of CCN. These two quantities describe different aerosol attributes so a thorough understanding of their relationship is warranted, which is lacking at present.The objectives of this research are to:1) investigate factors that influence the relationship between AOT and CCN, such as ambient relative humidity (RH), the particle size distribution and hygroscopic properties of aerosol particles;2) develop more robust AOT-CCN relationships by accounting for major influencing factors and apply them to AERONET AOT data to obtain better estimates of CCN at AERONET sites;3) match estimated CCN data with satellite cloud and precipitation data from CloudSat to study the impact of aerosols on cloud and precipitation.The study will make use of extensive measurements from AERONET, field experiments, and satellite missions. Tasks 1 and 2 will rely on measurements of aerosol properties, ambient meteorological conditions and CCN collected in field campaigns conducted in many parts of the world. Numerous factors will be examined, such as air mass, particle size distribution, soluble fraction, etc., and their influences on both optical and hygroscopic properties of aerosols will be assessed. Intellectual merit:1) A comprehensive analysis of how AOT and CCN are affected by aerosol properties and the ambient environment that have been most widely employed for studying aerosol radiative and microphysical effects;2) The improved relationship between AOT and CCN will enable the extension of widely available AOT from ground-based and space-borne sensors to study the impact of aerosols on cloud and precipitation, as many have done but without rigorous investigation of their relationship;3) Further investigation and understanding of the impact of aerosols on cloud and precipitation on global scales for different types of aerosols under diverse meteorological conditions.Broad impacts:1) With an improved knowledge and understanding of the AOT-CCN relationship, modelers would be better informed and motivated to explore the use of global satellite AOT products to fill the void currently present in studying the impacts of aerosols on global climate;2) The research will provide interdisciplinary training opportunities in measuring aerosols, data analysis, and modeling for undergraduate and graduate students and postdoctoral fellows;3) This study will have an immediate impact on the teaching of cloud physics, atmospheric chemistry, climate and environmental courses.