  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 r_eff and the cloud droplet number concentration Nc from fast
cloud droplet probe (FCDP) in-situ measurements for both cases. Furthermore, we
show that aerosols affect cloud properties (Nc, r_eff, and LWC) via the
prescribed bulk hygroscopicity of aerosols and aerosol size distributions
characteristics. Nc, r_eff, and liquid water path (LWP) are positively
correlated to the bulk hygroscopicity of aerosols and aerosol number
concentration (Na) while cloud fractional cover (CFC) is insensitive to the
bulk hygroscopicity of aerosols 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.
