PNNL

Abstract

Aerosol effects on micro-/macro-physical properties of marine stratocumulus clouds over the Western North Atlantic Ocean (WNAO) are investigated using in-situ measurements and large-eddy simulations (LES) for two cold air outbreak (CAO) cases (February 28 and March 1, 2020) during the Aerosol Cloud meteorology Interactions over the western Atlantic Experiment (ACTIVATE). The LES is able to reproduce the vertical profiles of liquid water content (LWC), effective radius Aeff and the cloud droplet number concentration #2 from fast cloud droplet probe (FCDP) in-situ measurements for both the February 28 and March 1 cases. Furthermore, we show that aerosols affect cloud properties (#2 and Aeff , LWC) via prescribed ¯ ^ and aerosol size distributions characteristics. #2, Aeff , and liquid water path (LWP) are positively correlated to ¯ ^ and aerosol number concentration (#0) while cloud fractional cover (CFC) is insensitive to ¯ ^ and aerosol size distributions for the two cases. The changes to aerosol size distribution (number concentration, width, and the geometrical diameter) allow us to disentangle aerosol effects on cloud properties from the meteorological effects. We also use the LES results to evaluate cloud properties from two reanalysis products, ERA5 and MERRA-2. Comparing to LES, the ERA5 reanalysis is able to capture the time evolution of LWP and total cloud coverage within the study domain during both CAO cases while MERRA-2 underestimates them.