High free-tropospheric Aitken-mode aerosol concentrations buffer cloud
droplet concentrations in large-eddy simulations of precipitating
A new Aitken mode aerosol scheme is developed for a large eddy
simulation (LES) model in order to better investigate cloud-aerosol
interactions in the marine boundary layer and to study the Aitken
buffering hypothesis of McCoy et al. (2021). This scheme extends the
single-mode two-moment prognostic aerosol scheme of Berner et al.
(2013). Nine prognostic variables represent accumulation and Aitken
log-normal aerosol modes in air and droplets as well as 3 gas species.
Scavenging of interstitial aerosol by cloud and rain drops and
coagulation of dry aerosol are treated using the scheme described in
B13. The scheme includes a simple chemistry model with gas phase H2SO4,
SO2, and DMS as prognostic variables to capture basic influences of
sulfur chemistry on the model aerosols. Primary nucleation of H2SO4
aerosol particles from gas-phase H2SO4 is neglected. A deep,
precipitating stratocumulus case (VOCALS RF06) is used to test the new
scheme. The presence of the Aitken mode aerosol increases the cloud
droplet concentration through activation of the larger Aitken particles
and delays the creation of an ultraclean, strongly precipitating cumulus
state. Scavenging of dry accumulation and Aitken particles by cloud and
precipitation droplets accelerates the collapse. Increasing either the
above-inversion Aitken concentration or the surface Aitken flux
increases the Aitken population in the boundary layer and prevents the
transition to an ultraclean state.