Aitken Mode Aerosols Buffer Decoupled Mid‐Latitude Boundary Layer Clouds Against Precipitation Depletion.

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 15 July 2017 during the Aerosol and Cloud Experiments in the Eastern North Atlantic 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 in the cumulus layer provides a reservoir for turbulence and convection to carry accumulation aerosols into the drizzling stratocumulus layer above. Further Aitken activation occurs aloft in the stratocumulus layer. Together, these activation events buffer this cloud regime 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, Aitken buffering sustains brighter, more homogeneous clouds for longer. Plain Language Summary: Aerosols, small particles in the atmosphere associated with ocean biology, sea spray, land, and human‐produced emissions, influence cloud brightness and, by suppressing precipitation and subsequent break up, cloud lifetime. Understanding aerosol‐cloud‐precipitation interactions is critical in understanding how aerosols influence the climate system. This study examines how the very smallest aerosol particles modify cloud formation, brightness, and lifetime over the North Atlantic ocean. We utilize a recent set of aircraft and satellite observations from a dedicated field campaign as well as a detailed model that resolves fine‐scale interactions important to cloud development. After comparing the model to real‐world observations, we test how modifying the amount of small particles impacts the cloud brightness and lifetime. We find that the small particles are able to offset precipitation removal of larger particles, helping clouds to last longer and stay brighter. Key Points: Observations of mid‐latitude decoupled low clouds constrain a large‐eddy simulation investigating aerosol‐cloud‐precipitation interactionsBoundary layer Aitken activation and turbulent and convective fluxes restore accumulation mode aerosols against precipitation lossesAitken buffering acts to sustain brighter, more homogeneous clouds for longer [ABSTRACT FROM AUTHOR]