Effects of Aerosols on Low-Level Cloud Properties over Land and Ocean Using Ground-Based Observations

The aerosol indirect effect (AIE) involves a complex set of aerosol-cloud interactions, including the alteration of cloud microphysical properties such as cloud lifetime, droplet size distribution, liquid water content and path (LWC, LWP), optical depth (COD), and albedo. Aerosol-cloud interactions remain to be one of most important but very challenging research areas in atmospheric science. Over the ocean, marine boundary layer (MBL) stratiform clouds dominate the lower troposphere (<2 km MSL). The primary aerosol type is sea salt which is a hygroscopic, coarse mode aerosol that can readily activate as cloud condensation nuclei (CCN) to form cloud droplets. Over land, aerosols have similar effects on low-level cloud formation processes as those over ocean, depending on the types of air masses passing over continental regions. This study is a synergistic approach that uses the long-term ARM ground-based observations and retrievals from the Southern Great Plains (SGP) of the United States and Eastern North Atlantic Ocean (Azores) regions to investigate the seasonal and diurnal variations of aerosol and clouds under the continental and maritime conditions, as well as compare their similarities and differences. The retrieved cloud-base and –top heights, cloud optical depth, LWC/LWP, r_e and Nc, and surface CCN measurements, as well as meteorological variables, will be used to quantitatively estimate AIEs following the methods of Feingold et al. (2003, 2006, 2010) and Ghan et al. (2016), given by the partial log-derivative of the cloud properties with respect to the partial log-derivative of the aerosol properties.

This study will aid to improve aerosol and cloud parameterizations as well as provide an in-depth understanding of aerosol-cloud interactions in typical maritime and continental conditions. Through addressing the following two scientific questions, we will prove the hypotheses that different aerosol types can have different impacts on the cloud microphysical properties, thus different aerosol-cloud interactions over land and ocean.

SQ1: What are the similarities and differences of aerosol and cloud properties, as well as their interactions over ocean and land?

SQ2: What are the indirect effects of aerosols on cloud microphysical properties?