Abstract
A new Aitken mode aerosol microphysics scheme is developed for a large eddy simulation 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), https://doi.org/10.1029/2020jd033529. This scheme extends the single‐mode two‐moment prognostic aerosol scheme of Berner et al. (2013), https://doi.org/10.5194/acp-13-12549-2013. Seven prognostic variables represent accumulation and Aitken log‐normal aerosol modes in air and droplets as well as 3 gas species. Scavenging of interstitial and other unactivated aerosol by cloud and rain drops are treated using the scheme described in Berner et al. (2013), https://doi.org/10.5194/acp-13-12549-2013. The scheme includes coagulation of unactivated aerosol and 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. Nucleation of H2SO4 aerosol particles from gas‐phase H2SO4 is neglected. A deep, precipitating stratocumulus case (VAMOS Ocean Cloud Atmosphere Land Study 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 unactivated 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.
Plain Language Summary
Aerosols can have large effects on low‐level oceanic clouds, and cloud processes strongly affect aerosols as well. Both aerosols and clouds are a major source of uncertainty in climate projections. A standard tool for studying low altitude ocean clouds and their interactions with aerosols is large‐eddy‐simulation (LES), but this tool can be very computationally expensive when aerosols of many different sizes are included, which can restrict the geographic coverage, the model resolution, and the time extent of model simulations. We present a new numerical framework for LES that allows treatment of two size categories of aerosols and their interactions with clouds in a relatively simple and inexpensive manner. We use this framework and perform a wide range of numerical simulations where aerosol number, properties, and physics are varied in a 10‐day simulation of low clouds in a small region over the Southeast Pacific Ocean. Our simulations show that small aerosols can have large effects on low cloud survival in some circumstances.
Key Points
A simple two‐mode aerosol scheme allows exploration of Aitken effects on aerosol and cloud evolution
Aitken aerosols supplement the accumulation mode and can prevent boundary‐layer collapse in some cases
Scavenging of unactivated aerosol particles by cloud droplets has a strong effect on aerosol evolution
