Characterization of Particle Interactions with Clouds and Precipitation Using Diverse Datasets

Abstract

The largest uncertainty in quantifying global anthropogenic radiative forcing is linked to interactions of aerosol particles with clouds (IPCC, 2013), which is partly driven by the difficulty of conducting the required measurements and separating the influence of meteorology and aerosol pollution on clouds. The impact of aerosol particles on cloud macro/microphysical properties is challenging to address but is important for reasons extending from improving the model representation of clouds and precipitation to identifying the impacts of wet deposition on aquatic and terrestrial ecosystems due to inputs of nutrients and contaminants that may have originated from particles serving as cloud condensation nuclei (CCN) or ice nuclei (IN). In the present study, particle interactions with clouds and precipitation were investigated using diverse data sets. The results of this work are presented in the form of four studies. In the first study, sources of particles in a rural California site were identified. The interrelationships between aerosol and precipitations were investigated using data collected from co-located sampling sites. In the second study, chloride concentrations were used as a proxy for giant CCN (GCCN) concentration in low-level clouds. Using the proposed proxy, the influence of GCCN on stratocumulus cloud properties such as droplet size distribution was examined. It was shown that fixing other variables such as liquid water path, high values of sea salt led to an increase in the number of droplets with a diameter greater than 20 μm. In addition, the potential influence of giant sea salt particles on drizzle intensity was studied. In the third study, aerosol characteristics in the entrainment interface layer (EIL) were examined. The results suggest that EIL has favorable conditions for the formation of new particles. Also, it was shown that particles in the EIL have the characteristics of aerosols present in the adjacent layers including free troposphere and sub-cloud layer. In the last study, stratocumulus clearings off the U.S. West Coast were characterized. The data for the summers between 2009 and 2018 were analyzed to identify the days that clearing occurs. The average daily probability of clearing events over the summer months was about 0.3. This result demonstrates that clearings were not rare events over the Northeast Pacific. Most clearings initiated and developed near coastal topographic features such as Cape Blanco and Cape Mendocino. The spatial characteristics of clearings exhibited diurnal variability. The median lengths and area of clearings were smallest in the morning at 09:00, with an increase between 09:00 and 12:00, and then a leveling off in expansion until 18:00. Clearings have distinguishing features such as an enhanced Pacific high shifted more towards northern California, offshore air that is warm and dry, stronger coastal surface winds, enhanced lower tropospheric static stability, and increased subsidence. Gradient Boosted Regression Tree (GBRT) modeling was successfully used to investigate the influence of environmental parameters on the evolution of clearing events.