Impacts of aerosol scavenging and processing on the transition of a stratocumulus cloud system to open cells: A comparison of Lagrangian and bin microphysics schemes in LES

Stratocumulus clouds, a key component of global climate, are sensitive to aerosol properties. Aerosol-cloud-precipitation interactions in these clouds influence their closed-to-open cell dynamical transition and, hence, cloud cover and radiative forcing. This study uses large-eddy simulations with Lagrangian super-particle and bin microphysics schemes to investigate the impacts of aerosol scavenging and physical processing by clouds on drizzle initiation and the cellular transition process. The Lagrangian microphysics simulation with explicit representation of cloud-borne aerosol and scavenging shows significant aerosol processing. Sensitivity simulations using the bin scheme and their comparison with the Lagrangian microphysics simulations suggest that reduced aerosol concentration due to scavenging is a primary microphysical driver for enhanced precipitation and earlier transition to open cells using the Lagrangian scheme. Changes in the aerosol distribution shape through processing do not contribute appreciably to the differences in precipitation rate in a clean condition. A lower droplet activation rate, larger mean radius, and higher cloud water mixing ratios are the initial triggers for faster rain development using the Lagrangian scheme compared to the bin scheme. A positive feedback then accelerates aerosol removal and further rain production using the Lagrangian scheme and, consequently, reduces cloud droplet number, increases mean size, and increases droplet spectral width, leading to a rapid transition to open cellular convection. These results highlight how enhanced droplet activation, likely from the equilibrium assumption for droplet activation, and higher cloud droplet number initially using the Eulerian bin scheme delay open cell formation compared to simulations using the Lagrangian scheme.