Challenges in understanding the aerosol–cloud interactions and their impacts
on global climate highlight the need for improved knowledge of the underlying
physical processes and feedbacks as well as their interactions with cloud and
boundary layer dynamics. To pursue this goal, increasingly sophisticated
cloud-scale models are needed to complement the limited supply of
observations of the interactions between aerosols and clouds. For this
purpose, a new large-eddy simulation (LES) model, coupled with an interactive
sectional description for aerosols and clouds, is introduced. The new model
builds and extends upon the well-characterized UCLA Large-Eddy Simulation
Code (UCLALES) and the Sectional Aerosol module for Large-Scale Applications
(SALSA), hereafter denoted as UCLALES-SALSA. Novel strategies for the
aerosol, cloud and precipitation bin discretisation are presented. These
enable tracking the effects of cloud processing and wet scavenging on the
aerosol size distribution as accurately as possible, while keeping the
computational cost of the model as low as possible. The model is tested with
two different simulation set-ups: a marine stratocumulus case in the
DYCOMS-II campaign and another case focusing on the formation and evolution
of a nocturnal radiation fog. It is shown that, in both cases, the
size-resolved interactions between aerosols and clouds have a critical
influence on the dynamics of the boundary layer. The results demonstrate the
importance of accurately representing the wet scavenging of aerosol in the
model. Specifically, in a case with marine stratocumulus, precipitation and
the subsequent removal of cloud activating particles lead to thinning of the
cloud deck and the formation of a decoupled boundary layer structure. In
radiation fog, the growth and sedimentation of droplets strongly affect their
radiative properties, which in turn drive new droplet formation. The
size-resolved diagnostics provided by the model enable investigations of
these issues with high detail. It is also shown that the results remain
consistent with UCLALES (without SALSA) in cases where the dominating
physical processes remain well represented by both models