Aitken Mode Aerosols Buffer Decoupled Mid-latitude Boundary Layer Clouds
Against Precipitation Depletion
Abstract
Aerosol-cloud-precipitation interactions are a leading source of
uncertainty in estimating climate sensitivity. Remote marine boundary
layers where accumulation mode (~100-400 nm diameter)
aerosol concentrations are relatively low are very susceptible to
aerosol changes. These regions also experience heightened Aitken mode
aerosol (~10-100 nm) concentrations associated with
ocean biology. Aitken aerosols may significantly influence cloud
properties and evolution by replenishing cloud condensation nuclei and
droplet number lost through precipitation (i.e., Aitken buffering). We
use a large-eddy simulation with an Aitken-mode enabled microphysics
scheme to examine the role of Aitken buffering in a mid-latitude
decoupled boundary layer cloud regime observed on July 15, 2017 during
the ACE-ENA flight campaign: cumulus rising into stratocumulus under
elevated Aitken concentrations (~100-200 mg-1). In situ
measurements are used to constrain and evaluate this case study. Our
simulation accurately captures observed aerosol-cloud-precipitation
interactions and reveals time-evolving processes driving regime
development and evolution. Aitken activation into the accumulation mode
occurs primarily in the cumulus layer, providing a reservoir for
turbulence and convection to carry accumulation aerosols into the
drizzling stratocumulus layer above. Thus, the cloud regime is buffered
against precipitation removal, reducing cloud break-up and associated
increases in heterogeneity. We examine cloud evolution sensitivity to
initial aerosol conditions. With halved accumulation number, Aitken
aerosols restore accumulation concentrations, maintain droplet number
similar to original values, and prevent cloud break-up. Without Aitken
aerosols, precipitation-driven cloud break-up occurs rapidly. In this
regime, mesoscale and synoptic-scale uplift enhance cloud condensate and
brightness, but Aitken buffering sustains brighter, more homogeneous
clouds for longer.