Organized Variations in MBL Cloud Microphysical Properties Observed by Aircraft and Satellite and Simulated by Model

Abstract. Marine boundary layer (MBL) clouds in subtropical regions strongly impact global energy balance, but complete understanding of the processes that control their microphysical properties remain elusive. We analyze aircraft in-situ measurements of MBL clouds for two selected cases from the ACE-ENA field campaign that contain mesoscale convective cells (MCCs) on the order of tens of kilometers embedded in the large-scale overcast cloud field. The aircraft flight tracks aligned with the MCC organization, such that vertically-stacked, horizontal flight legs alternated between sampling clouds along organized MCCs and sampling clouds between MCCs. This alignment is well-suited to study the distinctly different microphysical properties for the two cloud regimes. Clouds within organized MCCs had lower droplet concentrations, but larger droplet sizes and liquid water contents with enhanced drizzle relative to clouds between MCCs. While observed aerosol properties below these two cloud regimes are generally consistent with their corresponding cloud microphysical properties, preexisting organization of the aerosol field was probably not required in the development of the MCC organization. In contrast, the lower aerosol and CCN concentrations observed below the MCC cloud layer most likely developed from precipitation and coalescence scavenging. A cloud-resolving WRF model simulation with realistic large-scale forcing reproduces the MCC organization of the cloud field suggesting that updraft velocity is the key to explain the differences in cloud microphysics. Both observations and model simulations indicate that under moderate-heavy drizzling conditions, precipitation and coalescence scavenging dominates and drives spatial gradients of cloud droplets, aerosols and CCN concentrations rather than local sources.