Impact of Aerosol on Precipitation in Stratocumulus Clouds: Insights from Long-Term Measurements in the Eastern North Atlantic

MAIQI ZHANG, Shengqian Zhou, Virendra Ghate, Maria Cadeddu, Christine Chiu, Jian Wang, Washington University in St. Louis

     Abstract Number: 213
     Working Group: Aerosols, Clouds and Climate

Abstract
Aerosols can strongly influence climate by modifying the properties of clouds. An increase in cloud condensation nuclei (CCN) concentration can reduce cloud droplet size, and thus lower coalescence efficiency among smaller cloud droplets, leading to precipitation suppression and enhanced cloudiness. However, quantifying this precipitation suppression effect is challenging because of the confounding effects of meteorological and thermodynamic factors. Additionally, the size of CCN is critical to precipitation formation. In particular, giant cloud condensation nuclei (GCCN) can instead promote the formation of large drops and expedite precipitation formation, but their impact on precipitation has not been quantified using long-term observations.

To isolate the impact of aerosol on precipitation, we employed precipitation rate susceptibility (S_R) as a metric, which quantifies the variation of precipitation with changes in CCN concentration under fixed cloud liquid water path (LWP). Earlier studies of S_R were mostly based on short-term measurements, and very few of them examined the impact of GCCN. In this study, we analyzed over 6 years measurements of CCN, cloud, and precipitation from the Graciosa Island in the Eastern North Atlantic to investigate how the S_R varies with LWP, and how it compares with previous studies. We will discuss how these variations in the trend of S_R -LWP are attributed to the transition from autoconversion to accretion in precipitation formation. Using supermicron aerosol scattering coefficient as a proxy for GCCN concentration, we found that higher GCCN concentrations reduce the sensitivity of precipitation to aerosol perturbations (i.e., lower S_R value) by “short-circuiting” the coalescence bottleneck and formation of drizzle embryos. By controlling the CCN concentration and LWP, we showed that higher GCCN concentrations promote precipitation rates. These results provide new insights into the complex relationship between aerosols, clouds, and precipitation, contributing to our understanding of the aerosol effects on clouds.